Stationary ramjet

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Wolfkeeper
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Re: Stationary ramjet

Postby Wolfkeeper » Thu Oct 01, 2015 5:16 pm UTC

sevenperforce wrote: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.

With scramjets everything is still behind oblique shockwaves. Oblique shocks characteristically have a sudden big jumps in temperature and pressure across them.

So your claim that there's no forebody compression and no heating is really dubious; indeed if there's no compression from the inlet, then what's the point?

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sevenperforce
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Re: Stationary ramjet

Postby sevenperforce » Thu Oct 01, 2015 7:05 pm UTC

The point is to have an airflow available to use as added working mass without significantly slowing said airflow.

There will certainly be an oblique shock at the inlet, resulting in a fair bit of heat, but it won't be as severe as with a normal shock. More importantly, however, forebody compression can be kept to a minimum because it is not needed. For combusting ramjets and scramjets, forebody compression is required in order to "prepare" the airflow for combustion and expansion. But in a supersonic-central-flow ducted rocket, atmospheric oxygen is not the primary oxidizer and so compression is unnecessary; the airflow can instead undergo nearly monotonic heating, expansion, and acceleration.

The advantage of using the atmosphere as the primary working mass is offset by the fact that your final thrust depends on the change in the speed of the flow, not the absolute flow speed. Your exhaust flow has to be moving faster relative to your engine than your engine is moving relative to the incoming airspeed, or you have negative thrust.

With a combusting ramjet at low supersonic speeds, it's fine to choke your inlet flow to subsonic velocity because you can depend on boosting it up to much higher speeds in your exhaust stream:

ramjet mach numbers.png
ramjet mach numbers.png (4.21 KiB) Viewed 1652 times

With a combusting scramjet, on the other hand, things start to get dicey because even without choked inlet flow, the forebody compression you need for combustion reduces your inlet flow velocity quite a bit and you need a huge jump in exhaust speed in order to make up the loss:

scramjet mach numbers.png
scramjet mach numbers.png (4.97 KiB) Viewed 1652 times

However, if you don't need combustion, then you don't need forebody compression. You'll still have an oblique shock, which will reduce airspeed slightly, but not to the extreme that it does in the other cases, and so your flow can be accelerated from front to back:

scramrocket mach numbers.png
scramrocket mach numbers.png (4.89 KiB) Viewed 1652 times

We tend to think in a reference frame where the airflow has speed and kinetic energy, but it doesn't; it's our launch vehicle which has all the speed. The air is just sitting there. Thus, the high-speed exhaust flow of the circumferentially-mounted ejector rockets should be able to heat and accelerate it backward in much the same way regardless of the forward airspeed of the craft. Of course the air will be accelerated to at least Mach 1 relative to the surrounding medium by the oblique shock, and we have to account for the relative velocity of the ejector rocket exhaust as well, but those aren't dominating factors.

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drachefly
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Re: Stationary ramjet

Postby drachefly » Thu Oct 01, 2015 7:32 pm UTC

sevenperforce wrote:
Wolfkeeper wrote:
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...


TWEEET tweet yellow flag.

Compressing air quickly achieves the opposite of cooling. What did you mean here?

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sevenperforce
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Re: Stationary ramjet

Postby sevenperforce » Thu Oct 01, 2015 7:51 pm UTC

drachefly wrote:
sevenperforce wrote:
Wolfkeeper wrote:
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...


TWEEET tweet yellow flag.

Compressing air quickly achieves the opposite of cooling. What did you mean here?

Quite; SABRE does both. The SABRE engine uses an advanced, compact closed-cycle helium precooler (using liquid hydrogen fuel as the terminal coolant) to supercool the air between inlet compression and turbine compression. The vaporized and heated hydrogen fuel is burned with bypass air in a series of small spill duct ramjets so that it's not totally wasted. Once the inlet compression becomes so high that the amount of liquid hydrogen needed to cool the airstream becomes prohibitive, the front inlet is closed and the engine proceeds under pure rocket propulsion.

It's a promising design. It's just that any liquid hydrogen rocket requires extremely high-volume tanks, which ends up dictating all your design factors. That makes both re-entry and overall reusability much more challenging. Hence the desire to design an SSTO vehicle using a higher-density, non-cryo fuel using an air bypass to vastly exceed the specific impulse AND the dry thrust of a pure hydrogen/LOX rocket.

Wolfkeeper wrote: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.

As a further emphasis of this line of reasoning, it should be noted that Billig ("Propulsion Systems from Takeoff to High-Speed Flight", High-Speed Flight Propulsion Systems 1990) found that a pure ducted rocket has a better mass fraction from takeoff to Mach 3 than both turbojets and afterburning turbojets. The slightly better specific impulse of turbojets, while making them better for cruising flight, is outweighed by the enhanced thrust capability of a ducted rocket and thus loses in terms of acceleration-stage mass fraction.

The same article found, incidentally, that a reusable ducted-rocket SSTO spaceplane would have truly fantastic cruise capabilities if the proper trajectory is chosen, making it quite suitable for serving either as a hypersonic transport or a hypersonic strike vehicle with a larger payload. Of course, he didn't consider the added benefits of a central-bypass ducted rocket.

Wolfkeeper
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Re: Stationary ramjet

Postby Wolfkeeper » Fri Oct 02, 2015 3:15 am UTC

sevenperforce wrote:It's a promising design. It's just that any liquid hydrogen rocket requires extremely high-volume tanks, which ends up dictating all your design factors. That makes both re-entry and overall reusability much more challenging.

Actually, it doesn't. The low ballistic coefficient during reentry makes it easier. Skylon's skin only gets a bit over 1000K, and there's materials that can take that without any problems.

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sevenperforce
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Re: Stationary ramjet

Postby sevenperforce » Fri Oct 02, 2015 2:36 pm UTC

Yeah, Skylon was able to find a way to get good tankage volume as well as good re-entry performance, but it's still not ideal to be forced to design a launch vehicle around a single dominant factor like that.

It should be noted that with a central-bypass ducted rocket, you could get some pretty interesting and unique vehicle geometries. For example, the main propulsive body of your launch vehicle could have a half-ellipse shape around the central bypass, with the intent that the entire vehicle would change attitude slowly in order to have optimal forebody compression and flow-through at all airspeeds.

central with dual.png
central with dual.png (5.99 KiB) Viewed 1526 times

With a large enough bypass cross-section and ejector-entrained thrust augmentation, you could have non-tailsitting vertical takeoff, which is not only more convenient (in terms of payload loading, stability, and so forth) but is also hella cool.

EDIT: I came across this neat NASA study into the feasibility of an "exoskeletal" or "drum-mounted vane" engine, similar to what I was describing earlier. Making it work as a replacement for conventional turbines would require bearing materials more exciting than what we have, but it is a good study as far as a no-moving-parts version would be concerned.


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