Stationary ramjet

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sevenperforce
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Stationary ramjet

Postby sevenperforce » Tue Sep 08, 2015 5:45 pm UTC

The primary difficulty in achieving a fully-reusable launch vehicle capable of reaching orbit in a single stage (SSTO) has been engine design. Engines, it seems, may either be high-efficiency but low-thrust (airbreathing) or low-efficiency and high-thrust (rocket). Several solutions for a hybrid or combined-cycle engine have been proposed, but to date none of these have achieved success.

One of the most promising designs in combining the high performance of an airbreathing engine with the high thrust and reliability of a rocket engine is the air-augmented rocket, also known as a ramrocket, ducted rocket, bypass rocket, or ejector jet. In such a design, a conventional rocket engine is placed within a duct or shroud which serves to direct airflow around the engine so that it mixes with the exhaust. This promises to greatly increase the working mass and, in some designs, allows secondary combustion (similar in principle to an afterburner) if the rocket is operated at a fuel-rich mixture:

ramrocket.png
A typical ram-rocket design is depicted, showing how shrouded airflow may be added to a high-temperature, fuel-rich rocket exhaust plume to increase working mass, reduce thermal losses and produce a secondary combustion zone.
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Theoretically, the increase in working mass more than makes up for the added drag, while secondary combustion can decrease the amount of required oxidizer and represents an additional advantage. This allows a ramjet-like operation even at subsonic speeds and can transition smoothly to full ramjet operation at supersonic and hypersonic speeds.

However, air-augmented rockets have not yet been successfully employed in orbital launch vehicles, for a simple reason. Even though the shrouding increases both thrust and efficiency, this effect only takes place once sufficient (albeit subsonic) airspeed is reached. Thus, the integral rocket engine must be sufficiently large to lift the entire vehicle without the benefits of air augmentation. And if you already have a rocket engine large enough to get you into space, it makes more sense to simply add some extra propellant and not bother with the heavy shrouding. Rocket fuel is very cheap. Since you're probably going to be throwing the rocket away (the most reusable launch system ever deployed, the Space Shuttle, only flew a few dozen missions per orbiter), it's not economical to increase cost and complexity just for a meager increase in efficiency. More complexity means more failure points; adding efficiency at the cost of reliability is never a good trade-off.

The requirement for a full-static-thrust engine presents the biggest hurdle in launch vehicle design. It is possible to envision a horizontal-to-vertical launch, where the launch vehicle with a smaller rocket engine accelerates horizontally under rocket power until it can sustain enough airflow for ram-augmentation; it would then angle up and take off vertically:

transition flight example.png

However, such a launch system would obviously require the inclusion of landing gear suitable for takeoff as well as aerodynamic control surfaces, increasing launch weight significantly and countering any gains from decreasing the size of the rocket engine. It would also add additional failure modes and increase launch complexity. Other possibilities, such as the use of a ground-powered launch ramp, introduce similar problems.

It's unfortunate that we need a large rocket engine for liftoff, because ideally we would only need a rocket engine large enough for orbital insertion. Air augmentation via ramjet or scramjet can conceivably take your vehicle up to more than half of orbital speed, at which point you've burned enough fuel that it only takes a small rocket engine and a small amount of fuel for that final boost. But since you need large rocket engines for liftoff, it makes much more sense to go with a tried-and-tested two-stage-to-orbit (TSTO) launch vehicle, where large first-stage rocket engines take you part of the way up and you can use a second stage with a single small engine for the final leg. That's what SpaceX has done quite well. An added advantage is that the first stage can use engine bells optimized for sea-level operation while the second stage can use engine bells optimized for vacuum operation.

A quick note: a rocket engine bell essentially converts the thermal energy of the hot supersonic exhaust to kinetic energy, by allowing expansion in the axial direction while restricting expansion in normal plane. At sea level, however, this expansion must be halted before the internal pressure of the stream drops too low, or atmospheric pressure will "pinch" the sides of the flow and produce significant pressure drag. Thus, an engine bell designed for use at sea level must be shorter and/or wider than an engine bell for use in a vacuum:

expansion.png
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Again, that's why it makes more sense to use a separate set of engines at launch and at altitude, which fits the whole staging idea nicely.

It's unfortunate, though, because you end up using massive rocket engines at the altitude where airbreathing engines would be most advantageous. The only way to fix this is if we can come up with an engine that can somehow use airbreathing thrust from a standstill, but without using the heavy turbines that lower thrust/weight ratios and prevent vertical takeoff.

But maybe there's a solution.

High-test hydrogen peroxide (80% and up) can be used as a monopropellant because it decomposes catalytically, producing supersonic steam exhaust with an impulse that's about half as high as solid-fueled rockets. However, because it is nearly 50% oxygen by weight, it can also be used as an oxidizer. That was the route taken by the highly-reliable Gamma engines of the British Black Arrow boosters, which used the decomposition of the hydrogen peroxide to operate the turbopumps that fueled the main engine. Moreover, decomposed peroxide is hypergolic with most rocket fuels, conveniently removing the need for an ignition source.

Recall that an over-extended rocket bell designed for optimal vacuum efficiency will drop below atmospheric pressure at sea level, causing a negative pressure that sucks in air. Ordinarily, this is a problem.

However, if an over-extended peroxide rocket bell was placed inside an air-augmentation shroud...

low presure.png
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...then a region of turbulent low pressure would form around the slender exhaust plume.

This would cause higher-pressure air surrounding the plume to flow into the low-pressure region, where it would become entrained with the supersonic exhaust plume and accelerate back:

entrainment.png
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We now have a weakly-supersonic flow of oxidizer-enriched air. Adjust your shroud and inlet, add a secondary spike and suddenly you have the inlet conditions of a conventional ramjet:

ramjet.png
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This design would use a small amount of monopropellant oxidizer to accelerate and compress a much larger mass of air into the inlet of a ramjet engine. This would, in principle, enable a relatively small "rocket" engine to produce sufficient thrust for vertical takeoff without a penalty, using air as the primary working mass from the very beginning.

As the engine gained speed, the flow rate and expansion ratio of the peroxide engine could be adjusted to maintain the same ramjet inlet conditions independent of forward airspeed. This could allow a seamless transition from zero airspeed all the way up to typical ramjet velocities, where the peroxide engine could be throttled down to idle and ram effect would take over completely. The risk of unstart would be minimal, as the residual flow from the peroxide engine would serve to maintain pressure and ensure sustained ignition.

Once above airbreathing altitude, the inlet could be closed and the peroxide engine throttled up. Hydrogen peroxide and ordinary rocket fuel have a respectable specific impulse, allowing an efficient orbital insertion under pure rocket power.

All in all, this engine design promises a high thrust-to-weight ratio from the start, complete control over oxidizer ratio throughout ramjet operation, and a highly efficient reaction at any altitude or airspeed. Hopefully enough to make VTVL SSTO a possibility.

I guess you would call this a monopropellant-pumped oxidizer-assisted ramjet?
Last edited by sevenperforce on Tue Sep 08, 2015 8:28 pm UTC, edited 1 time in total.

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Re: Stationary ramjet

Postby sevenperforce » Tue Sep 08, 2015 7:55 pm UTC

A more likely configuration, in which the oxidizing monopropellant is released from around the rim of the inlet:

likely config.png

When stationary, the monopropellant bell is fully opened and the inlet spike is set far back. Specific impulse is on the order of 600 seconds.

As speed increases, engaging a mild ram effect, the monopropellant bell is trimmed (as incoming pressure has increased) but the inlet spike remains far back. Specific impulse is on the order of 800 seconds.

At transonic and low-supersonic speeds, the inlet spike is brought forward to direct the shockwave around the inlet, but the flow speed of the decomposed peroxide remains greater than the air inlet speed. Specific impulse is on the order of 1800 seconds.

At high-supersonic speeds, full ramjet operation engages, and the flow of peroxide is idled, with only enough remaining to ensure hypergolic ignition. Specific impulse is on the order of 2100 seconds.

At hypersonic speeds, scramjet operation engages, with the idled flow of peroxide now serving to slow the incoming air stream slightly to allow greater combustion time. This allows the combustion chamber to remain at high impulse and prevents any need for shockwave-induced combustion. Specific impulse is on the order of 1500 seconds.

Finally, as ultrasonic speeds are reached and the atmosphere fades away, the spike is brought all the way forward to close the inlet and pure rocket operation commences. Specific impulse drops to around 320 seconds.

Notably, the three variables (inlet position, peroxide flow, and monopropellant bell position) can be operated in concert to allow smoother transitions between operating modes as well as deviations from the constant-mass-flow-trajectory requirement usually imposed on dual-mode sc/ramjets.

A quick-and-dirty analysis of specific impulse and Δv suggests that the ideal achievable mass ratio would be around 19:81 (19% vehicle, 81% fuel). If the scramjet mode can, with the assistance of the monopropellant flow, function up to a velocity of Mach 21, then the mass ratio increases to 25:75.

I think the minimum required mass ratio to make SSTO feasible is something like 1:10? So that's not bad.

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Re: Stationary ramjet

Postby wumpus » Wed Sep 09, 2015 2:55 pm UTC

I missed the whole point about being "stationary".

SSTO is inherently less efficient than staged launching. The whole reason that people ever latched on to SSTO is basically a logical fallacy: since magically efficient spacecraft would use SSTO, then any spacecraft capable of SSTO would then be magically efficient. In truth they are still less efficient than staged spacecraft. SpaceX has also made it very clear that early (and thus sub-orbital) stages should be far easier to recover (early shuttle designs had pilots landing the early stages), thus reducing even more of the desire for SSTO.

The two biggest issues seem to be missing here: as far as I know, NASA's record for SCRAMJET engines is mach 6.8 and that had to start at mach 4 (the Chinese WU-14 may have done mach 10). Going past mach 5 is strictly bleeding edge, and going from mach 10-orbital velocity is strictly theoretical (of course, you could always dump the scramjet and use rockets). Then there is the heating issue (note, this isn't from friction but from compression of air once it hits the air/spacecraft), these things get HOT.

Finally, for high-volume efficiency (i.e. for putting up enough stuff that the manufacturing costs override R&D, pretty much required for forum readers to get into space), don't underestimate what [sc]ramjets can do to reduce spacecraft size/weigh/cost. I've noticed that as a rule of thumb, for each "3 machs" a standard rocket will burn roughly half its weight. Getting to mach 6 should reduce the size of the rest of the ship to 1/4 the size, and getting to mach 9 should reduce it to 1/8. Note that at extreme speeds the ISP falls off to close to rocket levels while the issues with heat and air resistance only increase, making conventional rockets much more appropriate. [Sc]ramjets may have their place, but they will [hopefully] have to race a space elevator to get there.

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Re: Stationary ramjet

Postby sevenperforce » Wed Sep 09, 2015 4:16 pm UTC

wumpus wrote:I missed the whole point about being "stationary".

Haha, apparently.

If you build a ducted rocket with a forward injection nozzle/ring which burns an oxidizing monopropellant to produce a low-pressure region at the inlet, and the low-pressure region causes air to flow into the inlet and be accelerated to supersonic speeds by the monopropellant exhaust, then you have an engine capable of producing supersonic inlet conditions without moving. Thus, you have a stationary ramjet.

An ducted rocket engine produces significantly more thrust than the bare rocket engine inside it. If you have a ducted rocket engine which is capable of ramjet thrust at zero forward airspeed, then you can achieve vertical liftoff with a much smaller and lighter rocket engine than would otherwise be required. This is what makes a ducted rocket a worthwhile investment; without augmented thrust at zero airspeed, your bare rocket engine needs to be big enough to lift the whole vehicle by itself, at which point you're better off adding extra fuel and dispensing with the duct system entirely.

The two biggest issues seem to be missing here: as far as I know, NASA's record for SCRAMJET engines is mach 6.8 and that had to start at mach 4 (the Chinese WU-14 may have done mach 10).

You're working with old info, I think. The X-43 managed Mach 9.68 under its own power in November 2004. The more recent X-51 was not quite so fast but focused more on control and duration; it had a 210-second-long scramjet burn in May 2013.

The Chinese WU-14 is a hypersonic glide vehicle, not a scramjet; it has demonstrated hypersonic maneuvering but it was boosted to Mach 10 by a conventional missile. There are rumors that the Chinese are working on a scramjet version but that's unconfirmed.

Going past mach 5 is strictly bleeding edge, and going from mach 10-orbital velocity is strictly theoretical (of course, you could always dump the scramjet and use rockets).

I think you missed something here. This IS a rocket. See the image in this post? By moving the inlet spike forward gradually, you transition evenly and smoothly from injection-pumped ejector jet to ordinary ejector jet to transonic ramjet to conventional ramjet to scramjet to rocket. This engine would provide significant advantages in the scramjet operation window for two reasons: first, the flow of gas from the forward fuel injectors can be used to slow down the incoming air without heating the engine, enabling better combustion; second, variable oxidizer injection allows the vehicle to deviate from the depressed trajectory that a conventional scramjet must follow, reducing the amount of compression heating it must deal with.

SSTO is inherently less efficient than staged launching. SpaceX has also made it very clear that early (and thus sub-orbital) stages should be far easier to recover (early shuttle designs had pilots landing the early stages).

Yes, they should be far easier to recover. But so far, they haven't been. I have no doubts that Musk et al will eventually achieve full reusability of their first-stage boosters, but it's still something that is "bleeding edge" (more so than Mach 5+ anyway). We have yet to successfully land an orbital takeoff vehicle under its own power, ever. In contrast, the Shuttle program gave us TONS of experience bringing spaceplanes down from orbit, so if we can manage an SSTO launch then recovery is trivial by comparison.

The idea here is to have an extremely simple and lightweight engine which is nonetheless capable of all modes of operation (jet, ramjet, scramjet, and pure rocket) with seamless transitions between each, plus hybridizations as required/desired.

As it turns out, such an engine was already proposed and tested once: the ejector jet engine designed by Space Access in the 90s. It used a secondary gas generator burning a small amount of rocket fuel and oxidizer to produce a negative pressure region across the inlet of a cylindrical ramjet engine, allowing high thrust from a standstill. However, it lacked the geometry for seamless transition between flight modes, and it also used LOX rather than H2O2. As far as I can tell, they were so focused on high efficiency that they never considered the idea of using an oxidizing monopropellant. Forward injection of an oxidizing monopropellant is what really makes my design work, because it's practically zero weight cost compared to a turbine or a secondary gas generator, but it still enables air-augmented thrust from a standstill so it is suitable for vertical takeoff. It also enables full use of the oxidizer rather than wasting it.

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Re: Stationary ramjet

Postby Wolfkeeper » Thu Sep 17, 2015 2:39 am UTC

wumpus wrote:I missed the whole point about being "stationary".

SSTO is inherently less efficient than staged launching. The whole reason that people ever latched on to SSTO is basically a logical fallacy: since magically efficient spacecraft would use SSTO, then any spacecraft capable of SSTO would then be magically efficient. In truth they are still less efficient than staged spacecraft. SpaceX has also made it very clear that early (and thus sub-orbital) stages should be far easier to recover (early shuttle designs had pilots landing the early stages), thus reducing even more of the desire for SSTO.

And they've been very unsuccessful at getting their kit back in one piece so far! They'll get it eventually, but performance is still not fantastic; and they lose a lot of payload when they do reusability, so one hand gives, but the other takes some of it back again.
The two biggest issues seem to be missing here: as far as I know, NASA's record for SCRAMJET engines is mach 6.8 and that had to start at mach 4 (the Chinese WU-14 may have done mach 10). Going past mach 5 is strictly bleeding edge, and going from mach 10-orbital velocity is strictly theoretical (of course, you could always dump the scramjet and use rockets). Then there is the heating issue (note, this isn't from friction but from compression of air once it hits the air/spacecraft), these things get HOT.

Finally, for high-volume efficiency (i.e. for putting up enough stuff that the manufacturing costs override R&D, pretty much required for forum readers to get into space), don't underestimate what [sc]ramjets can do to reduce spacecraft size/weigh/cost. I've noticed that as a rule of thumb, for each "3 machs" a standard rocket will burn roughly half its weight. Getting to mach 6 should reduce the size of the rest of the ship to 1/4 the size, and getting to mach 9 should reduce it to 1/8. Note that at extreme speeds the ISP falls off to close to rocket levels while the issues with heat and air resistance only increase, making conventional rockets much more appropriate. [Sc]ramjets may have their place, but they will [hopefully] have to race a space elevator to get there.

Scramjets are very heavy; the vehicle/engine thrust/weight ratio is about 2. And they don't work below about mach 4.

Skylon/SABRe on the other hand seems to work from 0 to Mach 5.5 in airbreathing mode, and then goes rockety to reach orbit. And it looks like it's going to be fully reusable; they've got performance in hand to add that (which pure rocket approaches don't really have- see Space X's troubles, everything has to be so very lightweight).

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Re: Stationary ramjet

Postby sevenperforce » Thu Sep 17, 2015 3:40 pm UTC

The poor T/W ratio of scramjets is definitely a limiting factor, and that's one reason I wanted to design a combined-cycle airbreathing rocket engine with an injection/ejector monopropellant stream. You can make the scramjet a lot simpler and less heavy and make up for the reduced performance by enabling smoother transitions and a broader operating range.

Although, if simplicity and weight reduction is our ultimate goal, why don't we just switch to a dual-mono bipropellant design and dispense with the traditional rocket/jet engine altogether, using the monopropellant streams to create virtual compression, combustion, and expansion chambers?

Hydrazine/H2O2 burns with a vacuum specific impulse rivaling the sea level specific impulse of LH2/LOX. Moreover, hydrazine is exceedingly dense, denser than kerosene and far denser than LH2, which enables us to design a much more compact vehicle.

Instead of the traditional design of a rocket or jet engine, let's just go with a pure "flying stovepipe". We can line the interior with small vectorable monopropellant thrusters. Vectoring would likely come from some sort of pressure differential rather than actual gimbals, just for the sake of simplicity.

flying stovepipe.png
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By vectoring the thrusters forward and backward and varying their intensity, complete control over compression, combustion, and expansion could be realized at any forward airspeed.

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Re: Stationary ramjet

Postby Wolfkeeper » Thu Sep 17, 2015 4:32 pm UTC

Honestly, the SABRE stuff completely destroys stovepipes in every dimension.

They're running a very high pressure cycle jet engine, and they can do that because the incoming air is always at about -150C so, when they compress it very hard, it doesn't get uselessly hot. So they're able to get very high overall pressure ratios indeed; like 90 or something.

Meanwhile a ramjet has a pressure ratio of ~8 or something.

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Re: Stationary ramjet

Postby sevenperforce » Thu Sep 17, 2015 5:12 pm UTC

Wolfkeeper wrote:Honestly, the SABRE stuff completely destroys stovepipes in every dimension.

Except for the dimensions of weight, thrust/weight ratio, airframe integration, high-efficiency operating range, and proven reliability. And propellant volume.

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Re: Stationary ramjet

Postby Qaanol » Thu Sep 17, 2015 6:02 pm UTC

I still don’t understand why launches from the ground are done with rockets rather than, say, a train track that ends in a ramp, to send a large jet up which climbs and accelerates further before decoupling from the rocket at high altitude and velocity. The jet could even land on the ocean if it’s too big for a runway.

Basically, this would give the rocket a huge boost from ground-based and jet-based stages, to reduce the amount of rocket fuel that has to be carried and thus also reduce the size of the rocket. I know there have been proposals like this, such as maglev launch rings, but I don’t get why no-one has actually done it. It seems pretty achievable to me, especially at the scale where you’re already, you know, launching rockets.
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Re: Stationary ramjet

Postby sevenperforce » Thu Sep 17, 2015 6:45 pm UTC

It probably has a lot to do with startup cost. You can design, build, and test VTO rockets with (comparatively) minimal infrastructure. Once you've been doing that for long enough to launch commercially, it doesn't make sense to invest millions and millions more in building a launch ramp and a jet booster stage, especially when you deal with the complexity of trying to re-use the jet booster stage. Rockets are cheap; jet engines are less cheap. You expect to throw away rockets but not so much jet engines.

EDIT:

Another issue is that turbofans just don't make very good first stages. Even an afterburning turbofan has a limited ceiling and speed. When you factor in the weight of an entire aerodynamically controllable lift-based jet first stage, then whatever weight savings you originally had will be lost several times over fighting your way through dense sea-level air at drag-inducing but minimal (from an orbital standpoint) speed. The fastest afterburning-turbofan-powered aircraft ever to fly, the SR-71, topped out at barely 10% of LEO orbital velocity.

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Re: Stationary ramjet

Postby Wolfkeeper » Thu Sep 17, 2015 8:34 pm UTC

sevenperforce wrote:
Wolfkeeper wrote:Honestly, the SABRE stuff completely destroys stovepipes in every dimension.

Except for the dimensions of weight, thrust/weight ratio, airframe integration, high-efficiency operating range, and proven reliability. And propellant volume.

Thrust/weight of SABRE is about 14, which is pretty good, and it integrates as well as a ramjet; the high-efficiency operating range is M0-M5 much wider than a ramjet.

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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 18, 2015 5:03 pm UTC

Well, when I said "stovepipe" I didn't mean just a ramjet; I meant the entire range of air-augmented free-flow combined-cycle propulsion technologies. SABRE's T/W ratio of 14 is definitely not bad; in comparison, the T/W ratio of the latest-generation Boeing 747 engine from GE is 5.64. The T/W ratio of a scramjet is a little trickier to come up with; the X-43's vehicle T/W ratio was around 4 but the vehicle was mostly engine so it's difficult to quantify.

Unfortunately, that T/W of 14 is less than half that of a basic ducted or air-augmented rocket, which is why Skylon wouldn't be able to manage VTO, which means it needs wings. Wings mean more weight and more drag. Moreover, the angle of attack required to get lift and the depressed flight profile bleed off fuel savings to drag while requiring the odd curved-engine-pod profile, which is what makes airframe integration challenging. In contrast, the airframe integration of a ramjet or scramjet serves to provide compressive lift, which makes for a simpler, sleeker design which works together with the airframe rather than working against it.

A combined-cycle air-augmented scramrocket has an operating range of M0-M18 or so since the ducting can continue to add reaction mass to the exhaust flow at speeds far greater than SABRE's airbreathing capacity.

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Re: Stationary ramjet

Postby Wolfkeeper » Fri Sep 18, 2015 6:20 pm UTC

Actually Skylon has a T/W ratio well over 1 at zero speed, so in principle it could do VTOL just fine. The nacelle is that funny shape, not because of the engine, but because they decided that it would give optimum efficiency for the vehicle; they have the exhaust angled downwards slightly for some of the lift.

Air augmented rockets have been extensively investigated over the years; but the Isp varies between rocket and ramjets; and the thrust isn't spectacular; they also have notable cooling issues. One report I saw stated that the 25% or so increase of Isp from the duct around the rocket seemed to be simply because the rocket exhaust was fuel-rich; the rest of the fuel was burning off in the duct. But the duct was fairly heavy, so it was not giving a significant gain in payload.

But none of this solves the basic problem with all of these stovepipe-type designs: when running in airbreathing mode; they have lousy Isp due to their poor compression ratios.

The point about Skylon is that it's MASSIVELY more efficient all the way up to Mach 5+; the Isp is around 2000-3000 seconds. You just can't get that from any kind of stovepipe without turbomachinery; the overall pressure ratio just isn't there.

Scramjets are particularly problematic, because they don't work below Mach 4; but most of the gains in performance are to be had below about Mach 5.5. As the speed goes up, the Isps of all jet engines start to converge with rockets, so above that speed you're not gaining a lot, and the heat loads on the vehicle become appallingly large.

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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 18, 2015 9:04 pm UTC

Wolfkeeper wrote:Actually Skylon has a T/W ratio well over 1 at zero speed, so in principle it could do VTOL just fine.

The figures cited by the company claim a total loaded liftoff weight of 325 metric tonnes and an airbreathing engine thrust of 0-2 kN for each engine, but I'm not sure whether 100% of engine thrust is obtainable from a stationary start.

The nacelle is that funny shape, not because of the engine, but because they decided that it would give optimum efficiency for the vehicle; they have the exhaust angled downwards slightly for some of the lift.

It's because the short, stubby wings (required for HTOL) aren't large enough to provide sufficient lift at a 0 angle of attack and thus it must fly at a high angle of attack. However, because the engine intake requirements are so specific, the intake must be pointing directly into the airflow without deviation. But, as you pointed out, the thrust needs to be vectored down, below the COG. Hence the curvy nacelle.

One report I saw stated that the 25% or so increase of Isp from the duct around the rocket seemed to be simply because the rocket exhaust was fuel-rich; the rest of the fuel was burning off in the duct. But the duct was fairly heavy, so it was not giving a significant gain in payload.

Air augmentation has actually been widely implemented in solid-fueled/hybrid missiles which function as something between a ramrocket and a ramjet. With a solid fuel, the gain in Isp is quite high, around 85% or more. I'm not sure what they do for pure-liquid-fueled rockets.

But none of this solves the basic problem with all of these stovepipe-type designs: when running in airbreathing mode; they have lousy Isp due to their poor compression ratios.

The point about Skylon is that it's MASSIVELY more efficient all the way up to Mach 5+; the Isp is around 2000-3000 seconds. You just can't get that from any kind of stovepipe without turbomachinery; the overall pressure ratio just isn't there.

Actually, it's not too terribly difficult to get a suitably high compression ratio from a standstill if you use the exhaust gases of a preburn to entrain and compress the inlet airflow. Of course, preburning fuel to compress your air is a waste of fuel, though it's arguably better than carrying a heavy turbine along to do the compression mechanically. However, if you were using two different monopropellants as your bipropellant fuel, then you wouldn't be wasting a single drop of fuel and you get full compression at every airspeed, along with complete control over combustion pressure.

Scramjets are particularly problematic, because they don't work below Mach 4; but most of the gains in performance are to be had below about Mach 5.5. As the speed goes up, the Isps of all jet engines start to converge with rockets, so above that speed you're not gaining a lot, and the heat loads on the vehicle become appallingly large.

Right; the very geometry which you need to induce ram compression at high-supersonic and hypersonic speeds becomes a massive heat reservoir at the upper envelope of the speeds you're looking for.

Above Mach 15, it's unlikely that you'll be able to do very much oxidization of your flow...BUT if there was a way for you to entrain the flow without significantly compressing it, then you could still use it for reaction mass in an air-augmented arrangement. Which is why a truly-cylindrical stovepipe might have a better chance of continuing to partially breathe air right up to orbital speeds.

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Re: Stationary ramjet

Postby Wolfkeeper » Fri Sep 18, 2015 9:41 pm UTC

Reaction Engines Limited talk about sc/ramjets here:

http://www.reactionengines.co.uk/tech_d ... 08-117.pdf

There's also an equation called the airbreather's burden:

http://www.islandone.org/Propulsion/SCRAM-Spencer1.html

basically you need high acceleration; also the L/D ratio at high mach is super important. Usually the L/D ratio hits 1, perhaps even as low as Mach 5, so the behaviour of a scramjet at Mach 15 can be pretty moot.

The NASP was also interesting; they found that the heat of the aeroshell actually had to be recycled; they needed to put the compression heat back into the propellant so that they could return it to the exhaust and turn back it into propulsion to cancel out most of the drag, otherwise the drag was far too great. That meant the entire outside of the vehicle had to be actively cooled, and piped to the engine, which meant that the vehicle ended up ridiculously heavy. It all ended up rather comical and the program was cancelled in the end. So scramjets at Mach 15. Sorry, not going to happen.

I'm sure scramjets work fine for missiles, but if you're trying to get to orbit, you need it all to be super-efficient. The term 'scamjet' has frequently been applied for launch systems.

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Re: Stationary ramjet

Postby sevenperforce » Tue Sep 22, 2015 3:17 pm UTC

Read both of those links; definitely a lot of interesting and important information.

I did notice that the claimed T/W ratio of the SABRE engine varies heavily based on speed. At a static start, the T/W ratio is just under 9 rather than the peak T/W ratio of 14 (which is obtained at around Mach 2). This suggests that static thrust is on the order of just 2.6 kN, a far cry from the 3.2 kN necessary for VTO.

Wolfkeeper wrote:basically you need high acceleration; also the L/D ratio at high mach is super important. Usually the L/D ratio hits 1, perhaps even as low as Mach 5, so the behaviour of a scramjet at Mach 15 can be pretty moot.

Of course, if you can manage sufficiently high acceleration via comically large T/W ratio, then L/D becomes less of an issue.

The NASP was also interesting; they found that the heat of the aeroshell actually had to be recycled; they needed to put the compression heat back into the propellant so that they could return it to the exhaust and turn back it into propulsion to cancel out most of the drag, otherwise the drag was far too great. That meant the entire outside of the vehicle had to be actively cooled, and piped to the engine, which meant that the vehicle ended up ridiculously heavy. It all ended up rather comical and the program was cancelled in the end.

Yikes!

It makes sense, though, if you think about it. It's a basic momentum exchange thing; the airstream needs to leave your vehicle moving faster than when it arrived, or you're going to be losing speed. The faster you're moving, the less you can afford to slow down the incoming airstream, because it will become harder and harder to make up the difference. Since basic momentum exchange dictates that you won't be able to get an airbreathing exhaust velocity anywhere close to the pure H2/O2 rocket exhaust velocity of ~4 km/s, supersonic combustion over Mach 10-11 is a pipe dream (no pun intended).

To derive useful thrust from an airbreather at hypersonic airspeeds, then, you need to be able to add energy to the flow without slowing it down at all. In other words, you need a cylindrical intake, without any ram compression. If you can add energy to the flow which causes it to expand toward the back of your engine, then it will accelerate backward and thus provide air-augmented thrust.

In the hypersonic regime, an airbreather cannot effectively supplement its oxidizer requirements and so operation should focus on increasing its reaction mass. Very low-molecular-mass exhaust is desirable in a pure rocket application because it decreases mass fraction for a given amount of energy, but if the reaction mass is borrowed from the surrounding air then this becomes far more efficient. One kilogram of H2/O2 exhaust traveling at 4 km/s has a momentum of 4000 kg*m/s and a kinetic energy of 8 MJ; mixing this with four kilograms of air increases the total momentum to 8944 kg*m/s, more than doubling the impulse.

From the second link:

Henry Spencer wrote:V_final = Isp_half * g * ln(mass_ratio) * 1/(1 + 1/(L/D * A/g))

The really fun part is that last term. The second term in its denominator, 1/(L/D * A/g), is what you might call the Air Breather's Burden. L/D is average lift/drag, a familiar basic measure of aerodynamic performance for winged vehicles. And A/g is just average forward acceleration in Gs. If either of these goes to infinity -- you have either miraculous aerodynamics or tremendous acceleration -- the value of the ABB goes to 0, the value of the whole last term goes to 1, and you get the familiar rocket equation.

But if they don't go to infinity, what you get is trouble. Hypersonic L/D ratios typically are not good. Zubrin guesses L/D of 5 for NASP and 7 for his design (better because methane tanks are more compact than hydrogen tanks, permitting a slimmer, less draggy shape). If NASP's average acceleration is 0.2G -- nothing for rockets, but fairly impressive for airbreathers at high speeds -- the ABB equals 1.0, and NASP gets only about half the V_final you would predict from the bare rocket equation. This reduces its effective Isp to about 500s, which means it needs nearly the same mass ratio as a good oxyhydrogen rocket SSTO, despite not having to carry any oxidizer. (Worse, that means it needs *seven times* as much hydrogen, since the LOX/LH2 ratio is 6:1 for the rocket.)

So it turns out that T/W is not only a limiting factor on the takeoff, but also on the boost phase. We can conclude that in order to be viable, an airbreather must have higher thrust to compensate for increased drag that comes from its higher efficiency.

An orbit-capable airbreathing rocket engine would have to have an inlet large enough that the air-augmentation of the thrust completely dwarfs the increased drag of the larger inlet. In other words, utterly unlike any launch vehicle ever designed.

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Re: Stationary ramjet

Postby Wolfkeeper » Wed Sep 23, 2015 10:24 pm UTC

Another amusing thing I observed is that there's a neat symmetry, missile people seem to think that scramjets wouldn't make very good missiles, but might make a really good launch vehicle, whereas the launch vehicle guys think it wouldn't make a very good launch vehicle but might make a good missile. :D

In other words, the more you know about the requirements for any given application, the less likely you think scramjets can help.

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Re: Stationary ramjet

Postby sevenperforce » Thu Sep 24, 2015 3:30 pm UTC

Wolfkeeper wrote:Another amusing thing I observed is that there's a neat symmetry, missile people seem to think that scramjets wouldn't make very good missiles, but might make a really good launch vehicle, whereas the launch vehicle guys think it wouldn't make a very good launch vehicle but might make a good missile. :D

In other words, the more you know about the requirements for any given application, the less likely you think scramjets can help.

I don't know about that; missile people seem to be pretty psyched about using scramjets. They've been ramjetting/ramrocketing missiles for ages now, and making a maneuverable hypersonic missile seems to be a popular idea.

Unrelated, but scary cool: during the Cold War, one of the most ridiculously unsettling designs was the Nuclear Thermal Ramjet, known as Project Pluto (alternately as SLAM, the Supersonic Low Altitude Missile). The idea was simple: build a flying stovepipe, but instead of combusting fuel using the airstream, stuff an open-cycle nuclear reactor into the middle of it. Load it with nuclear bombs, boost it to supersonic speeds with conventional rockets, and then let it fly. Because atmospheric air was the sole working mass and no fuel was expelled, the thermal ramjet had something no other missile has ever boasted: infinite range and infinite persistence. Once launched, the missile could fly in a holding pattern for hours or days or weeks or even months before diving and roaring over enemy territory at extremely low altitudes. Its speed and low altitude meant nothing could engage it. It could drop bombs wherever it wanted while its supersonic radioactive wake leveled and irradiated everything in its path, then dive kamikaze-style into a final target. The final dive would slam hundreds of tonnes of superheated metal, along with the unshielded reactor, into the ground at near-hypersonic speeds with the same basic effects as a dirty tactical nuke.

They built a couple of test engines but decided not to go further...partly because they weren't quite sure how to test it safely, and partly because they were scared of what would happen if the Soviets decided to reciprocate in kind.

Anyway, it really does look like the perils of supersonic combustion make a scramjet a bit too tricky to serve as a good launch vehicle. The need for a depressed trajectory to maintain constant mass flow is a big part of this; keeping a constant mass flow means keeping ρ*v constant, resulting in constant thrust, but drag is proportional to ρ*v2 and so you end up with net acceleration dropping off precipitously as velocity increases, even if you are able to miraculously maintain optimal burn.

I daresay it would be better to reduce dependence on the atmosphere as an oxidizer and focus primarily on optimization of air augmentation. Historically, air-augmentation designs have focused on increasing the efficiency of a conventional rocket arrangement (i.e., adding a shroud to an existing engine), but that's going about it the wrong way. Better to build a chemical-thermal ramjet from the ground up.

In such a system, high-impulse bipropellants would become less important; the more important factor would be the total energy density of the bipropellant. Of course, the really high-energy-density stuff is likely to be super toxic, but that's a separate issue. A tetrafluorohydrazine-pentaborane rocket has a combustion temperature of 4800 C and a combined density of 1.34 g/cc. If such a rocket were used primarily to compress and heat a large airflow, a very small amount of fuel could be used to produce an incredibly high amount of impulse. Because the airflow would more than compensate for increased drag, a winged vehicle could be designed which flew at relatively low altitudes to gain tremendous impulse from denser air, gaining altitude only as needed to reduce airframe stresses; there is no need to maintain stoichiometric mass flow. The mass fraction would be unbelievably low.

The challenge would be to make the ramjet work from a standstill in order to have high thrust from the very beginning. To that end, I imagine that an annular injector design that injected supersonic superheated rocket exhaust into the inlet would entrain and compress air like a bladeless turbine. I'm just not sure whether it would work well enough.

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Re: Stationary ramjet

Postby Wolfkeeper » Thu Sep 24, 2015 4:35 pm UTC

tetrafluorohydrazine-pentaborane

LOL. Pentaborane is considered unworkable as a propellant. The USA destroyed their stocks in 2000, it's incredibly toxic, above 1 ppm it's considered deadly, and 0.005 ppm is the 8 hour limit. All the fun of nerve gas trying to be used as a rocket fuel. And if you manage to burn it, the exhaust is toxic too.

tetrafluorohydrazine appears in Clarke's classic text 'Ignition'. I haven't actually go it to hand, but this classic work is basically a long description of his work developing new propellants, and to summarise it if you try anything exotic in regards rocket propellants, it probably won't work, it's going to be extremely expensive, explosive and you're probably going to die.

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Re: Stationary ramjet

Postby sevenperforce » Thu Sep 24, 2015 9:00 pm UTC

Wolfkeeper wrote:Pentaborane is considered unworkable as a propellant. The USA destroyed their stocks in 2000, it's incredibly toxic, above 1 ppm it's considered deadly, and 0.005 ppm is the 8 hour limit. All the fun of nerve gas trying to be used as a rocket fuel. And if you manage to burn it, the exhaust is toxic too.

If you try anything exotic in regards rocket propellants, it probably won't work, it's going to be extremely expensive, explosive and you're probably going to die.

Oh, I'm well aware that this is nasty, nasty stuff. I'm not suggesting we use anything that horrible. *shudder*

Rather, I'm pointing out some of the potential advantages of switching to an engine which is designed to use air as its primary working mass. With a conventional rocket, you're pretty much forced to use either liquid hydrogen or methane as your reductant because it's also serving as your working mass, and you need lightweight exhaust products to keep your mass fraction low. Airbreathers, too, are forced to use liquid hydrogen because supersonic combustion is impossible with heavier fuels. However, if your primary focus changes to using fuel to heat and compress an airstream, then the door is open to investigate denser, hotter-burning, higher-specific-energy fuel combinations...hopefully ones less toxic than pentaborane et al.

I wonder...in lieu of a purely pressure-entrained flow, would it be possible to build a central-flow annular compressor/turbine to accelerate air into the inlet at high speeds? There'd still be a weight cost, of course, but it would be considerably less than a central-axial turbine, and it could remain in place without impeding supersonic flow once the vehicle was traveling a lot faster.

central flow turbine 0.png
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This would be mounted at the front of a very basic stovepipe design with small, lightweight monopropellant thrusters mounted along the inside walls. The monopropellant thrusters would alternate between a reductant fuel (shown in green) and an oxidizing fuel (shown in yellow), and would start out angled back but move toward the normal further down the shaft.

central flow turbine.png
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At a standstill or at low speeds, the inside-out turbine would be spun up, drawing air in through the front inlet and forcing it back. The boundary airflow would entrain a larger volume of air. The first three sets of thrusters, firing a reducing monopropellant, would compress the incoming airflow. The next three sets of thrusters, firing an oxidizing monopropellant, would ignite on contact with the fuel-air mixture, causing the entire flow to undergo controlled supersonic combustion. The primary problem with supersonic combustion, insufficient diffusion, is not an issue here because both flows are supersonic or near-supersonic; moreover, the inward pressure of the oxidizing thrusters serves to compress the combusting flow. The thrusters at the very end would be fired as well, combusting in a toroidal shape and producing a virtual "throat" to produce a choke effect.

central flow turbine start.png
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The central-flow turbine would probably be spun up using exhaust gases from decomposition of the reducing monopropellant, with small exhaust ports drilled through it. This way, there would be no additional mechanical apparatus required.

As speed increases, less power is routed to the turbine, as the inflow increases. The flow of oxidizer can also be decreased slightly, as ram compression (using the inward momentum of the reductant monopropellant) begins to increase the compression of incoming air, but the primary source of energy is still the burning of the two propellants.

Once the hypersonic regime is reached, the choke thrusters are shut off and all of the front thrusters are engaged. Because of the greater force of the flow, very little ram compression takes place at this stage, which reduces drag and prevents overheating of the incoming airstream. The leading thrusters are bent back into the flow and combust annularly around the central flow of air, which is compressed radially and heated, causing it to accelerate backward:

central flow turbine hyper.png
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In this stage, there is little or no combustion of atmospheric air, because the airflow does not slow down appreciably. Rather, it is radially compressed and heated, with the (comparably low-mass) rocket exhaust added to the flow. Thus, full thrust and impulse augmentation can be maintained at all speeds without taking combustion into consideration.

Because this allows for dramatically greater thrust in higher-density air, the vehicle would fly a flight path as low as its airframe was able to handle. However, due to the consistently high thrust, the duration of time at maximum aerodynamic stress would be far lower than with a scramjet. Such an engine could climb gradually and operate within the atmosphere all the way up to orbital speeds, using only a small burn of the remaining fuel for orbital insertion.

With such a design, the air inlet could be made quite large. Because the cross-sectional area goes up with the square of the inlet diameter while the exposed drag area scales linearly with the inlet diameter, the diameter can increase freely up to the limits of airframe stability.

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Re: Stationary ramjet

Postby Wolfkeeper » Fri Sep 25, 2015 3:27 pm UTC

Which bit of my long winded explanation that 'scramjets are actually pretty rubbish' didn't you understand? ;p

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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 25, 2015 3:38 pm UTC

Wolfkeeper wrote:Which bit of my long winded explanation that 'scramjets are actually pretty rubbish' didn't you understand? ;p

That's not a scramjet; it's a high-bypass turborocket that functions as an airbreathing ramrocket at ramjet speeds and a supersonic thermal ramjet at hypersonic and ultrasonic speeds.

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Re: Stationary ramjet

Postby Neil_Boekend » Fri Sep 25, 2015 3:54 pm UTC

Sevenperfoce, I am curious: how big do you expect the advantage of not-carrying-as-much-oxidizer over not-accelerating-too-much-in-dense-atmosphere is? Because all ramjet acceleration is going to be in dense atmo, resulting in more drag and thus higher fuel requirements. I wonder how weight savings by not taking as much oxidizer weigh up to that.
BTW: This is not meant as a "this is a bad idea and you should feel bad". This is meant as an "I'm curious" comment. I just don't know how to word my question better.
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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 25, 2015 5:55 pm UTC

Neil_Boekend wrote:Sevenperfoce, I am curious: how big do you expect the advantage of not-carrying-as-much-oxidizer over not-accelerating-too-much-in-dense-atmosphere is? Because all ramjet acceleration is going to be in dense atmo, resulting in more drag and thus higher fuel requirements. I wonder how weight savings by not taking as much oxidizer weigh up to that.
BTW: This is not meant as a "this is a bad idea and you should feel bad". This is meant as an "I'm curious" comment. I just don't know how to word my question better.

You're fine; I could tell it was a genuine/curious question.

I think the reduction in oxidizer mass is actually going to be fairly slight. The oxidizer would be firing to some degree throughout the duration of the flight; at best, we're probably looking at reducing our oxidizer needs by 20-25% at most.

Rather than focusing on trying to use the atmosphere as a chemical reactant (e.g., SABRE), the goal of the above design is to use the atmosphere as a reaction mass. A high-bypass turbofan engine is highly efficient because only a small portion of the air it intakes is actually burned; the rest is pulled through the engine mechanically and mixed with the hot exhaust to increase impulse and thrust.

As I mentioned above, air augmentation designs have typically been focused on increasing the efficiency of existing rockets; they haven't really been designed to function solely in that mode from the ground up. The goal would be to increase the area of the intake so much that the increase in thrust far outweighs the increase in drag. The total intake area would likely be about the same as the total cross-sectional area of the entire craft.

With a rocket engine, mass flow remains the same regardless of airspeed, but with a rocket-heated thermal ramjet, mass flow increases with speed, which is what allows thrust to keep up with drag.

Airframe integration would still be important, but it would focus on reducing the external surface area rather than on setting up a high-drag forebody compression system.

Just for example, let's consider Falcon 9, converted to a high-bypass thermal ramjet configuration. Here are some of the specs from the standard expendable v1.1:

  • Diameter: 3.66 m
  • Cross-section: 10.52 m2
  • Total thrust: 5.89 MN
  • Exhaust velocity: 2766 m/s
  • Wet mass: 506 tonnes
  • Mass flow at peak thrust: 2728 kg/s
  • Specific impulse: 282 s
So let's imagine adding converting the Falcon 9's single cluster of engines to a pair of thermal ramjet ducts instead, mounted on either side with the engines scaled down and staggered inside the ducting:

falcon conversion.png
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The cross-sectional area has been doubled, but so has the internal volume. Meanwhile, we now have air inlets with a cross-sectional area equal to the cross-sectional area of the entire vehicle.

Let's look at Mach 1 performance, using the conservative assumption that no atmospheric air is being used as an oxidizer. Because cross-sectional area has doubled, the drag on the thermal ramjet is twice that of the drag on the Falcon.

At the same time, however, the Falcon's thrust and mass flow remains the same, while the thermal ramjet's mass flow has increased tremendously. Assuming an air density of 70% STP, the air mass flow through the thermal ramjet is simply ρ*A*v, which comes out to 6387 kg/s; its total mass flow is therefore 9115 kg/s.

The 2728 kg of fuel-propellant expended by the Falcon every second is traveling at 2766 m/s and therefore has a kinetic energy of about 1e10 J. But the thermal ramjet can deliver that same kinetic energy to 9115 kg of "propellant" every second, accelerating it to 1513 m/s. This results in a thrust of 13.8 MN, more than twice that of the Falcon. This means that it also has more than 2.3x the specific impulse of the Falcon, since specific impulse is thrust divided by fuel mass flow.

Thus, the internal-bypass thermal ramjet is an engine that actually increases in both thrust and specific impulse as its airspeed increases, rather than decreasing (as with a pure airbreather) or remaining constant (as with a pure rocket). And that is the case even assuming that no atmospheric oxygen is burned.

The thrust and the specific impulse would continue to climb as airspeed increases; the limiting factor would simply be the strength and durability of the airframe. Of course, since thrust is increasing rapidly, you don't have as much of an issue with thermal loading. However, even if we limit ourselves to an average specific impulse at 2.3x that of the Falcon, resulting in an effective exhaust velocity of 6.36 km/s, then our mass fraction is 76% and SSTO is trivial.

Of course, obviously I would need to do a lot more math to figure out the exact mass fraction using optimal launch trajectory, maximum aerodynamic load, maximum allowable acceleration, likely static mass flow, and so forth. But for a ballpark estimate that's not too bad.

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Re: Stationary ramjet

Postby Neil_Boekend » Fri Sep 25, 2015 7:40 pm UTC

I see. This is a solution for a different problem than I thought.
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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 25, 2015 7:41 pm UTC

Neil_Boekend wrote:I see. This is a solution for a different problem than I thought.

What problem did you think? :)

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Re: Stationary ramjet

Postby Wolfkeeper » Fri Sep 25, 2015 8:27 pm UTC

I've designed stuff like that before. Eventually I worked out that it gave no significant performance advantage. It's heavy and difficult to cool. What you gain when you're atmospheric, you lose when you're outside the atmosphere. And you always have to leave the atmosphere to achieve a stable orbit.

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Re: Stationary ramjet

Postby sevenperforce » Fri Sep 25, 2015 8:54 pm UTC

Cooling won't be an issue, not with this kind of air flow. Any rotation of the airflow caused by the peripheral-vane turbine (whether spun up or not) will tend to push the airflow against the walls of the chamber, mixing very efficiently. I can't see that this would necessarily be any more difficult to cool than a conventional rocket.

If you have a strong airframe (we're going for reusability and low mass fraction here, after all) and you use high thrust to counteract high drag while maintaining constant mass flow, then you'd be looking at hitting orbital speed somewhere around 25 km. At that point, it's not going to take much to get up to an appropriate altitude.

In the above example, your vacuum T/W ratio is the same as the Falcon's.

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Re: Stationary ramjet

Postby Wolfkeeper » Fri Sep 25, 2015 9:04 pm UTC

sevenperforce wrote:Cooling won't be an issue, not with this kind of air flow. Any rotation of the airflow caused by the peripheral-vane turbine (whether spun up or not) will tend to push the airflow against the walls of the chamber, mixing very efficiently. I can't see that this would necessarily be any more difficult to cool than a conventional rocket.

Rockets have very low surface area in contact with the hot exhaust, and they deflect the airflow without stopping it. You should do a back of the envelope calculation of how much cooling you're going to need. It's shockingly high, and if you manage to get lower propellant consumption, that means you've got less propellant to play with...

It's not so much that it doesn't work, it's that you end up with a much, much more complex system, and with hardly anything to show for it.

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Re: Stationary ramjet

Postby Neil_Boekend » Sat Sep 26, 2015 6:49 am UTC

sevenperforce wrote:
Neil_Boekend wrote:I see. This is a solution for a different problem than I thought.

What problem did you think? :)

Dragging the weight of the oxidizer up and accelerating it.
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Re: Stationary ramjet

Postby sevenperforce » Sat Sep 26, 2015 12:42 pm UTC

Neil_Boekend wrote:
sevenperforce wrote:
Neil_Boekend wrote:I see. This is a solution for a different problem than I thought.

What problem did you think? :)

Dragging the weight of the oxidizer up and accelerating it.

Ah, yes. There are more gains to be had by using atmosphere as working mass than by trying to use it in replacement of your oxidizer.

Now, what would be extremely cool would be if you could engineer a way to cool and compress atmospheric air and store it in your propellant tanks as they emptied. Your mass would end up remaining constant, which screws terribly with the rocket equation, but you'd end up being able to inject the compressed air back into your exhaust stream at altitude, augmenting your thrust with additional working mass. Some versions of the LACE proposal (Liquid Air Cycle Engine) attempt to do this, but their goal is to compress, liquify, and fractionalize the atmospheric air to use as oxidizer. Even though there has been considerable study in how this might be achievable, it's pretty much universally agreed to be too technically challenging to pull off. Merely cooling and compressing atmospheric air for use as working mass would be a lot easier, though still probably too difficult to be realizable.

Wolfkeeper wrote:
sevenperforce wrote:Cooling won't be an issue, not with this kind of air flow. Any rotation of the airflow caused by the peripheral-vane turbine (whether spun up or not) will tend to push the airflow against the walls of the chamber, mixing very efficiently. I can't see that this would necessarily be any more difficult to cool than a conventional rocket.

Rockets have very low surface area in contact with the hot exhaust, and they deflect the airflow without stopping it. You should do a back of the envelope calculation of how much cooling you're going to need. It's shockingly high, and if you manage to get lower propellant consumption, that means you've got less propellant to play with...

I guess I'm not sure where you're thinking I'd need active cooling at all. The above design does very little ram compression of the duct airflow...the angled jets are designed to accelerate the air, not slow it down. And there would still be very little surface area in contact with hot exhaust, since the angled jets will meet at the center of the duct, surrounded by the accelerating atmospheric airflow that both insulates the duct walls and reduces the heat of the exhaust.

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Re: Stationary ramjet

Postby Wolfkeeper » Sat Sep 26, 2015 1:53 pm UTC

sevenperforce wrote:Now, what would be extremely cool would be if you could engineer a way to cool and compress atmospheric air and store it in your propellant tanks as they emptied. Your mass would end up remaining constant, which screws terribly with the rocket equation, but you'd end up being able to inject the compressed air back into your exhaust stream at altitude, augmenting your thrust with additional working mass. Some versions of the LACE proposal (Liquid Air Cycle Engine) attempt to do this, but their goal is to compress, liquify, and fractionalize the atmospheric air to use as oxidizer. Even though there has been considerable study in how this might be achievable, it's pretty much universally agreed to be too technically challenging to pull off.

Not challenging, it just takes vast amounts of liquid hydrogen.
Merely cooling and compressing atmospheric air for use as working mass would be a lot easier, though still probably too difficult to be realizable.

Nope, that's exactly what Skylon/SABRE will do, and the technical reviews have come back 👍 looks good.
Wolfkeeper wrote:
sevenperforce wrote:Cooling won't be an issue, not with this kind of air flow. Any rotation of the airflow caused by the peripheral-vane turbine (whether spun up or not) will tend to push the airflow against the walls of the chamber, mixing very efficiently. I can't see that this would necessarily be any more difficult to cool than a conventional rocket.

Rockets have very low surface area in contact with the hot exhaust, and they deflect the airflow without stopping it. You should do a back of the envelope calculation of how much cooling you're going to need. It's shockingly high, and if you manage to get lower propellant consumption, that means you've got less propellant to play with...

I guess I'm not sure where you're thinking I'd need active cooling at all. The above design does very little ram compression of the duct airflow...the angled jets are designed to accelerate the air, not slow it down. And there would still be very little surface area in contact with hot exhaust, since the angled jets will meet at the center of the duct, surrounded by the accelerating atmospheric airflow that both insulates the duct walls and reduces the heat of the exhaust.


You said your design wasn't a scramjet. In which case there's a normal shock at the inlet which you haven't drawn on your diagrams, and the temperature and pressure goes up incredibly behind that. All of the hardware from the inlet onwards is in contact with that very hot gas.

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Re: Stationary ramjet

Postby sevenperforce » Tue Sep 29, 2015 2:37 pm UTC

Wolfkeeper wrote:
sevenperforce wrote:Now, what would be extremely cool would be if you could engineer a way to cool and compress atmospheric air and store it in your propellant tanks as they emptied. Your mass would end up remaining constant, which screws terribly with the rocket equation, but you'd end up being able to inject the compressed air back into your exhaust stream at altitude, augmenting your thrust with additional working mass. Some versions of the LACE proposal (Liquid Air Cycle Engine) attempt to do this, but their goal is to compress, liquify, and fractionalize the atmospheric air to use as oxidizer. Even though there has been considerable study in how this might be achievable, it's pretty much universally agreed to be too technically challenging to pull off.

Not challenging, it just takes vast amounts of liquid hydrogen.
Merely cooling and compressing atmospheric air for use as working mass would be a lot easier, though still probably too difficult to be realizable.

Nope, that's exactly what Skylon/SABRE will do, and the technical reviews have come back 👍 looks good.

Well, Skylon/SABRE isn't cooling/compressing atmospheric air to use as working mass; it's used as oxidizer. And it's not stored, either.

With LACE, air from the intake is liquified and fractionalized into LOX and other byproducts. Waste hydrogen and the other liquid products are dumped; the LOX is injected into the rocket engine. Some LACE designs are further intended to collect and store extra LOX along the way, so that the oxidizer needed for the final boost to orbit is collected en route rather than being carried from the ground. In this way, the airbreather's burden can be overcome since most of the mass needed at the start of the rocket phase is collected during the airbreathing phase:

LACE storage.png
LACE storage.png (1.96 KiB) Viewed 5276 times

As long as the inlet is large enough that you can collect more oxygen than you need to burn, this design works. You're basically trading lightweight LH2 for heavier LOX as you go up.

But this is the design which I said was too technically challenging to pull off. The weight of the liquifier and fractionalizer systems, along with the extreme bulk of the excessive amounts of LH2 you have to carry...it's just too much. The size of the hydrogen tanks would be so huge that reusability goes out the window.

In contrast, SABRE/Skylon uses a flow-through design. There's no storage, and all the LOX you need for the rocket stage is carried with you from the very beginning.

I was saying that, rather than liquifying and storing LOX like LACE or cooling and passing-through air like SABRE, it would be cool if you could compress/cool and store air as your propellant tanks emptied in order to use not as oxidizer, but as additional working mass. The more working mass you have at the start of the rocket stage, the better. The challenge is when to collect it, because anything you take with you to the start of the rocket stage has to be accelerated too, eating into your mass fraction.

I guess I'm not sure where you're thinking I'd need active cooling at all. The above design does very little ram compression of the duct airflow...the angled jets are designed to accelerate the air, not slow it down. And there would still be very little surface area in contact with hot exhaust, since the angled jets will meet at the center of the duct, surrounded by the accelerating atmospheric airflow that both insulates the duct walls and reduces the heat of the exhaust.

You said your design wasn't a scramjet. In which case there's a normal shock at the inlet which you haven't drawn on your diagrams, and the temperature and pressure goes up incredibly behind that. All of the hardware from the inlet onwards is in contact with that very hot gas.

Yeah, any foresurface is going to have a shock layer behind which temperature and pressure skyrocket. But that's true regardless of whether you're talking about an inlet or the external surface. Granted, this design has a greater body surface area for the same frontal surface area, but that's not insurmountable.

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Re: Stationary ramjet

Postby Wolfkeeper » Tue Sep 29, 2015 3:06 pm UTC

sevenperforce wrote:
Wolfkeeper wrote:
sevenperforce wrote:Now, what would be extremely cool would be if you could engineer a way to cool and compress atmospheric air and store it in your propellant tanks as they emptied. Your mass would end up remaining constant, which screws terribly with the rocket equation, but you'd end up being able to inject the compressed air back into your exhaust stream at altitude, augmenting your thrust with additional working mass. Some versions of the LACE proposal (Liquid Air Cycle Engine) attempt to do this, but their goal is to compress, liquify, and fractionalize the atmospheric air to use as oxidizer. Even though there has been considerable study in how this might be achievable, it's pretty much universally agreed to be too technically challenging to pull off.

Not challenging, it just takes vast amounts of liquid hydrogen.
Merely cooling and compressing atmospheric air for use as working mass would be a lot easier, though still probably too difficult to be realizable.

Nope, that's exactly what Skylon/SABRE will do, and the technical reviews have come back 👍 looks good.

Well, Skylon/SABRE isn't cooling/compressing atmospheric air to use as working mass; it's used as oxidizer. And it's not stored, either.

It's using cooled compressed air as a working mass; that's precisely what it's doing.

Incidentally, there's this myth that jet engines need oxygen from the air. I mean, they certainly do use it; it's free, why wouldn't you. But that's not the main thrust. 80% of the thrust of jet engines comes from lobbing nitrogen out the back at high speeds. If the air contained only nitrogen, jet engines, including SABRE would work fine, you'd just need a tank of LOX as well as your fuel. Performance would drop quite a bit, but it would still be much better than just burning the fuel and LOX in rocket mode. I mean, turbofans don't burn most of their air at all for example.

With LACE, air from the intake is liquified and fractionalized into LOX and other byproducts. Waste hydrogen and the other liquid products are dumped; the LOX is injected into the rocket engine. Some LACE designs are further intended to collect and store extra LOX along the way, so that the oxidizer needed for the final boost to orbit is collected en route rather than being carried from the ground.

You need to run the numbers, like Alan Bond did. The amount of liquid hydrogen to liquefy the LOX is enormous. And LH2 has the highest heat capacity of any propellant. It works out around 700 seconds Isp or something, really bad. It works a whole heap better if you DON'T liquefy the air; and if you follow that to its logical conclusions you end up with SABRE.

I was saying that, rather than liquifying and storing LOX like LACE or cooling and passing-through air like SABRE, it would be cool if you could compress/cool and store air as your propellant tanks emptied in order to use not as oxidizer, but as additional working mass.

Doesn't work, unless you liquefy it, the density is horrible and your tankage ratio is ghastly, like 1:1 or worse.

I guess I'm not sure where you're thinking I'd need active cooling at all. The above design does very little ram compression of the duct airflow...the angled jets are designed to accelerate the air, not slow it down. And there would still be very little surface area in contact with hot exhaust, since the angled jets will meet at the center of the duct, surrounded by the accelerating atmospheric airflow that both insulates the duct walls and reduces the heat of the exhaust.

You said your design wasn't a scramjet. In which case there's a normal shock at the inlet which you haven't drawn on your diagrams, and the temperature and pressure goes up incredibly behind that. All of the hardware from the inlet onwards is in contact with that very hot gas.

Yeah, any foresurface is going to have a shock layer behind which temperature and pressure skyrocket. But that's true regardless of whether you're talking about an inlet or the external surface. Granted, this design has a greater body surface area for the same frontal surface area, but that's not insurmountable.

It's not insurmountable, it just means your vehicle melts unless you have a very high coolant flow. Rockets have a hard enough time not slagging themselves, and although the temperature is not as high, you have an enormously greater surface area to worry about. Scramjets do better in this area, because they don't have a normal shock, but even they have a really tough time not melting. Indeed, that's basically the whole point of a scramjet, to not reach such ridiculous temperatures.

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Re: Stationary ramjet

Postby sevenperforce » Tue Sep 29, 2015 4:50 pm UTC

Wolfkeeper wrote:
sevenperforce wrote:Well, Skylon/SABRE isn't cooling/compressing atmospheric air to use as working mass; it's used as oxidizer. And it's not stored, either.

It's using cooled compressed air as a working mass; that's precisely what it's doing.

The compressed air is added to the chamber like an oxidizer, not to a secondary expansion bell as it would be in a ducted rocket. Sure, it increases the working mass, but it's also the only source of oxidizer that the engine is getting. Correct me if I'm wrong, but I don't think SABRE burns any of its LOX until it enters pure rocket mode.

Incidentally, there's this myth that jet engines need oxygen from the air. I mean, they certainly do use it; it's free, why wouldn't you. But that's not the main thrust. 80% of the thrust of jet engines comes from lobbing nitrogen out the back at high speeds. If the air contained only nitrogen, jet engines, including SABRE would work fine, you'd just need a tank of LOX as well as your fuel. Performance would drop quite a bit, but it would still be much better than just burning the fuel and LOX in rocket mode. I mean, turbofans don't burn most of their air at all for example.

Exactly. Even a pure turbojet without any turbofan bypass is really functioning as a thermal rocket; it just happens to be using part of the airstream to oxidize its fuel. Isp is thrust divided by fuel flow rate, so when your fuel flow is only a small component of your propellant flow, Isp skyrockets.

As you point out, a vehicle which can use air as propellant will always have much higher specific impulse, even if it's carrying along its oxidizer. Which is why it's very possible that a launch vehicle carrying its own fuel and oxidizer but designed to use air as the primary propellant would have significant advantages.

With LACE, air from the intake is liquified and fractionalized into LOX and other byproducts. Waste hydrogen and the other liquid products are dumped; the LOX is injected into the rocket engine. Some LACE designs are further intended to collect and store extra LOX along the way, so that the oxidizer needed for the final boost to orbit is collected en route rather than being carried from the ground.

You need to run the numbers, like Alan Bond did. The amount of liquid hydrogen to liquefy the LOX is enormous. And LH2 has the highest heat capacity of any propellant. It works out around 700 seconds Isp or something, really bad. It works a whole heap better if you DON'T liquefy the air; and if you follow that to its logical conclusions you end up with SABRE.

Exactly, which is why LACE simply wasn't a good route.

You said your design wasn't a scramjet.

It's not insurmountable, it just means your vehicle melts unless you have a very high coolant flow. Rockets have a hard enough time not slagging themselves, and although the temperature is not as high, you have an enormously greater surface area to worry about. Scramjets do better in this area, because they don't have a normal shock, but even they have a really tough time not melting. Indeed, that's basically the whole point of a scramjet, to not reach such ridiculous temperatures.

It's not a scramjet because it doesn't require supersonic combustion of the airstream, but there's no flow choking. The airstream should remain supersonic throughout the bypass; in fact, the whole concept is that the airstream is continually accelerated rather than being slowed and then accelerated.

A conventional supersonic combustion ramjet must slow down the airstream enough to enable combustion, and it must also use liquid hydrogen as the fuel since nothing else will be able to combust with the airstream under those conditions. This is more like a supersonic bypass ramjet; the airstream is neither slowed nor significantly combusted.

In a typical ducted rocket, the chamber must be long enough to allow diffusion between the rocket thrust and the airstream. It also depends on ram compression in order to force the airstream around the rocket and into the secondary expansion chamber:

typical ducted rocket.png
typical ducted rocket.png (1.29 KiB) Viewed 5249 times

In contrast, the "inside-out" version of this, the central bypass rocket I'm proposing, does not rely on ram compression because the airflow is entrained by the circumferential thrusters even from a standstill. It doesn't need to be as long, because the exhaust injection is angled with respect to the airflow and thus produces greater turbulence and immediate mixing. The duct can be more closely incorporated into the rest of the launch vehicle rather than being wrapped around the outside of it. Finally, because of the central bypass, the airstream will not be significantly slowed during hypersonic flow conditions:

central bypass rocket.png
central bypass rocket.png (2.57 KiB) Viewed 5249 times

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Re: Stationary ramjet

Postby Wolfkeeper » Tue Sep 29, 2015 7:47 pm UTC

sevenperforce wrote:In a typical ducted rocket, the chamber must be long enough to allow diffusion between the rocket thrust and the airstream. It also depends on ram compression in order to force the airstream around the rocket and into the secondary expansion chamber:

typical ducted rocket.png


Yup, ramrocket, ducted rocket, ramjet. All different names for the same thing, or different versions of the same thing.
In contrast, the "inside-out" version of this, the central bypass rocket I'm proposing, does not rely on ram compression because the airflow is entrained by the circumferential thrusters even from a standstill. It doesn't need to be as long, because the exhaust injection is angled with respect to the airflow and thus produces greater turbulence and immediate mixing. The duct can be more closely incorporated into the rest of the launch vehicle rather than being wrapped around the outside of it. Finally, because of the central bypass, the airstream will not be significantly slowed during hypersonic flow conditions:

central bypass rocket.png

Congratulations! That's called a "SCRAMJET"!!!

At supersonic speeds, if there's a normal shock across the inlet, it's a ramjet, if there's not, if the flow isn't slowed, it's a scramjet. That's the defining difference between the two. Some devices can work in either mode, but that's a scramjet.

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Re: Stationary ramjet

Postby sevenperforce » Tue Sep 29, 2015 8:13 pm UTC

In the typical usage I've seen, both "scramjets" and "ramjets" are presumed to involve combustion of the airstream, regardless of whether that airstream is choked (ramjet) or remains supersonic (scramjet).

A ramrocket/ducted rocket, on the other hand, does not use combustion of the airstream. In a ducted rocket, the oxidizer for combustion is carried along normally, and the airstream is only used to increase propellant mass, not as part of the combustion process.

I'm not proposing airstream combustion; I'm proposing an inside-out ramrocket that entrains the airstream from a static start and can freely function without choking in scramrocket mode.

Supersonic flow with airstream combustion: scramjet. Supersonic flow without airstream combustion: not a scramjet.

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Re: Stationary ramjet

Postby Wolfkeeper » Tue Sep 29, 2015 9:22 pm UTC

If the airflow doesn't become subsonic relative to the body of the engine, then it's a scramjet.

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Re: Stationary ramjet

Postby Copper Bezel » Wed Sep 30, 2015 10:49 am UTC

That seems a little unfair. You admit yourself that ramjets and ramrockets are "different versions of the same thing." A scramrocket must certainly qualify as a "different version of" a scramjet, and perhaps significantly so. I can't keep up with the aerospace engineering in this thread, and I don't think sevenperforce has made a very convincing case, but I don't think playing definition games helps anyone.
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Re: Stationary ramjet

Postby sevenperforce » Wed Sep 30, 2015 1:17 pm UTC

The definitions game really doesn't matter to me. If you feel a strong compulsion that unchoked-flow ramrockets ought to be considered a subclass of scramjets, that's fine. But a non-combusting/self-oxidizing "scramjet" avoids almost all of the major problems with "traditional" combusting scramjets, like poor compression ratio, low thrust, and excessive heating. A scramrocket, unlike a conventional scramjet, cannot unstart. Call it what you like, but the central-bypass entrainment ramrocket avoids the weight and drag costs of traditional ducted rockets while providing high impulse and high thrust from a standstill right up to orbital speeds. Thus, it can't really be handwaved as suffering from the usual problems of combusting airbreathers.

With the right frontal geometry, I suspect that the heat flux at even high-hypersonic airspeeds could be rendered nearly negligible. Heat flux at those speeds is a function of compression, not friction. With no inlet forebody compression, heating at the wall of the inlet should be minimal. And with mass-flow-multiplied thrust, the higher drag of a blunter external forebody can be counteracted, allowing heat to be carried away in the external normal shock.


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