What-If 0058: "Orbital Speed"

What if there was a forum for discussing these?

Moderators: Moderators General, Prelates, Magistrates

scaredyfish
Posts: 5
Joined: Fri Oct 03, 2008 8:35 am UTC

What-If 0058: "Orbital Speed"

Postby scaredyfish » Tue Aug 13, 2013 8:49 am UTC

Orbital Speed

"What if a spacecraft slowed down on re-entry to just a few miles per hour using rocket boosters like the Mars-sky-crane? Would it negate the need for a heat shield?"

"The reason it's hard to get to orbit isn't that space is high up.
It's hard to get to orbit because you have to go so fast."

Image

This is a topic which has bugged me for some time, particularly with all the hype surrounding commercial/private spaceflight these last few years.
Ok, cool, you can experience extended free-fall for a few minutes - but that doesn't really count as going into space does it? (Well, yes, technically it does, but come on...)

Having said that, I do applaud the private companies investing in this area, but mainly in the hope that it will lead to some other more practical technological advances in the future.

The SABRE rocket engine looks particularly interesting.

I don't think we'll be holidaying in space during my lifetime though, and I certainly will not go to space today :P
Last edited by scaredyfish on Tue Aug 13, 2013 8:56 am UTC, edited 1 time in total.

User avatar
rhomboidal
Posts: 754
Joined: Wed Jun 15, 2011 5:25 pm UTC
Contact:

Re: What-If 0058: "Orbital Speed"

Postby rhomboidal » Tue Aug 13, 2013 8:53 am UTC

Speed-walking at 8 km/s to a song? OK, I've heard that listening to music while exercising can help motivate you, but that's ridiculous.

User avatar
Znirk
Posts: 168
Joined: Mon Jul 01, 2013 9:47 am UTC
Location: ZZ9 plural Z α

Re: What-If 0058: "Orbital Speed"

Postby Znirk » Tue Aug 13, 2013 8:54 am UTC

Footnote 9 looks a bit incomplete from here.
Using song beats to help measure the passage of time is a technique also used in CPR training, where the song "Stayin' Alive" is used to .

It's used to give the person applying CPR an idea of what the frequency of heart compressions should be. Apparently even the less-musical among us are fairly good at reproducing the tempo of a song which they've always heard played at the same speed.

For those encountering trouble with the syncopation in Stayin' Alive, I've heard suggestions to use the similarly-paced Queen song, Another one bites the dust.

CharlieP
Posts: 397
Joined: Mon Dec 17, 2012 10:22 am UTC
Location: Nottingham, UK

Re: What-If 0058: "Orbital Speed"

Postby CharlieP » Tue Aug 13, 2013 10:14 am UTC

Znirk wrote:Footnote 9 looks a bit incomplete from here.
Using song beats to help measure the passage of time is a technique also used in CPR training, where the song "Stayin' Alive" is used to .

It's used to give the person applying CPR an idea of what the frequency of heart compressions should be. Apparently even the less-musical among us are fairly good at reproducing the tempo of a song which they've always heard played at the same speed.


http://www.youtube.com/watch?v=ILxjxfB4zNk

(which has the added benefit of illustrating the metric identity crisis we have in the UK, as mentioned in last week's discussion)
This is my signature. There are many like it, but this one is mine.

User avatar
najodleglejszy
Posts: 31
Joined: Sun Jan 02, 2011 9:45 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby najodleglejszy » Tue Aug 13, 2013 10:35 am UTC

CharlieP wrote:
Znirk wrote:Footnote 9 looks a bit incomplete from here.
Using song beats to help measure the passage of time is a technique also used in CPR training, where the song "Stayin' Alive" is used to .

It's used to give the person applying CPR an idea of what the frequency of heart compressions should be. Apparently even the less-musical among us are fairly good at reproducing the tempo of a song which they've always heard played at the same speed.


http://www.youtube.com/watch?v=ILxjxfB4zNk

(which has the added benefit of illustrating the metric identity crisis we have in the UK, as mentioned in last week's discussion)

my favourite thing in the video is that he's actually pushing faster than the song's beat.

segundus
Posts: 2
Joined: Tue Aug 13, 2013 1:24 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby segundus » Tue Aug 13, 2013 1:27 pm UTC

I don't understand this. Geostationary satellites orbit at the same speed the earth revolves. So if you launch one of them, why would you need to gain any speed in addition to the speed you already have from being on the surface of the earth?

User avatar
cellocgw
Posts: 1733
Joined: Sat Jun 21, 2008 7:40 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby cellocgw » Tue Aug 13, 2013 1:30 pm UTC

Speaking of fuel load, here's an absolutely delightful problem, from Engineering Optics With Matlab, Ting-Chung Poon & Taegeun Kim, World Scientific Publishing Co. Pte. Ltd., 2006 .

1.1 A laser rocket is accelerated in free space by a photon engine
that emits 10 kW of blue light (A = 450 nm).
a) What is the force on the rocket?
b) If the rocket weighs 100 kg, what is its acceleration?
c) How far will it have traveled in one year if it starts from zero
velocity?
[Courtesy of Adrian Korpel, Professor Emeritus, Univ. Iowa]

I do wonder if the honorable Professor meant to include computation of mass loss to create the blue light thrust :P
https://app.box.com/witthoftresume
Former OTTer
Vote cellocgw for President 2020. #ScienceintheWhiteHouse http://cellocgw.wordpress.com
"The Planck length is 3.81779e-33 picas." -- keithl
" Earth weighs almost exactly π milliJupiters" -- what-if #146, note 7

User avatar
Prosthetic_Lips
Posts: 21
Joined: Mon Mar 25, 2013 2:59 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby Prosthetic_Lips » Tue Aug 13, 2013 1:52 pm UTC

The confusing part to me is that the ISS is feeling 90% of the gravity at the Earth's surface. However, we see that the astronauts feel weightless. Is this due to their sideways movement? And, if so, why don't they keep accelerating until they start really falling into the Earth? Are they "losing" speed due to the curving around the Earth?

Or, are they just feeling mostly weightless? If so, I still have confusion about the acceleration.

User avatar
Cdevon2
Posts: 13
Joined: Wed Feb 03, 2010 5:01 am UTC

Re: What-If 0058: "Orbital Speed"

Postby Cdevon2 » Tue Aug 13, 2013 2:07 pm UTC

Prosthetic_Lips wrote:The confusing part to me is that the ISS is feeling 90% of the gravity at the Earth's surface. However, we see that the astronauts feel weightless. Is this due to their sideways movement? And, if so, why don't they keep accelerating until they start really falling into the Earth? Are they "losing" speed due to the curving around the Earth?

Or, are they just feeling mostly weightless? If so, I still have confusion about the acceleration.


The reason they're "weightless" is that they're in perpetual freefall. However, because they're moving at an incredibly fast speed sideways, by the time they would have hit the ground, they've moved in respect to the Earth so that the ground is essentially in a different place because of the curvature of the Earth. Imagine the ISS was traveling straight East from New York, and that you have a ruler (or an imaginary line, if you prefer) on the ground 1,000 miles long, so that it stays completely straight perpendicular to the pavement at Rockefeller Center. After 1,000 miles of ruler, it's not on the ground anymore, it's a good ways up in the air. If it reached the orbital altitude of the ISS, the ISS would still hit that ruler, but by that time, it would be miles above, say, Italy. The acceleration helps with this: by the time they get a quarter of the way around the Earth, down is East (That is, down in New York is the same direction as East in Italy), so that the acceleration down when they're above New York will help the ISS move East when it's above Italy.
The only 3 laws I need:
Moore's Law
Godwin's Law
Hofstadter's Law

taemyr
Posts: 111
Joined: Fri Jan 26, 2007 12:14 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby taemyr » Tue Aug 13, 2013 2:13 pm UTC

Prosthetic_Lips wrote:However, we see that the astronauts feel weightless. Is this due to their sideways movement? And, if so, why don't they keep accelerating until they start really falling into the Earth? Are they "losing" speed due to the curving around the Earth?

Or, are they just feeling mostly weightless? If so, I still have confusion about the acceleration.



Weightlessness is defined as the absence of all forces except gravity. So the astronauts are indeed weightless.

The reason they "feel" weightless and the reason that weighlessness is defined as above is that movement from a uniform gravity field is indistinguishable from inertial movement.

This means that they do not have any force pulling them towards any wall of the station. - Or rather they do have such a force, but that force is also pulling the station. So the acceleration of the astronaut and the acceleration of the station is identical. Thus relative to the station the astronauts experience no net force in any direction. And since the station is the natural reference for the astronauts they are floating.


The following is a bit ninjaed by Cdevon but;
The sideways movement is indeed the reason that the astronauts, and the station they are in, does not fall down to the earth. Essentially they are always falling against they earth, but they constantly miss, and they miss by little enough that they are left in the same position. It's perhaps easier once you realize that when we talk about acelleration we are taling about a change of velocity. Not necesarrily an increase of velocity. In the case of a circular orbit the strength of gravity is exactly strong enough that to effect the change of direction needed to maintain a constant altitude.

User avatar
moody7277
Posts: 515
Joined: Thu Aug 14, 2008 7:06 pm UTC
Location: Extreme south Texas

Re: What-If 0058: "Orbital Speed"

Postby moody7277 » Tue Aug 13, 2013 2:24 pm UTC

scaredyfish wrote:This is a topic which has bugged me for some time, particularly with all the hype surrounding commercial/private spaceflight these last few years. Ok, cool, you can experience extended free-fall for a few minutes - but that doesn't really count as going into space does it? (Well, yes, technically it does, but come on...)


If it's good enough for Alan Shepard, it's good enough for me.
The story of my life in xkcdmafia:

Tigerlion wrote:Well, I imagine as the game progresses, various people will be getting moody.

User avatar
davidstarlingm
Posts: 1255
Joined: Mon Jun 01, 2009 4:33 am UTC

Re: What-If 0058: "Orbital Speed"

Postby davidstarlingm » Tue Aug 13, 2013 2:34 pm UTC

Awesome that he tackled this right when we've been talking so much about orbital launches, heh.

Prosthetic_Lips wrote:The confusing part to me is that the ISS is feeling 90% of the gravity at the Earth's surface. However, we see that the astronauts feel weightless. Is this due to their sideways movement? And, if so, why don't they keep accelerating until they start really falling into the Earth? Are they "losing" speed due to the curving around the Earth?

Or, are they just feeling mostly weightless? If so, I still have confusion about the acceleration.

Others have answered this already, but why not join them?

Being in orbit means you're moving fast enough that gravity never really has a chance to pull you to the ground -- it can only pull you around and around in a circle. As Buzz Lightyear would say, it's "falling with style". And since the Space Station is falling around the earth at exactly the same speed as the astronauts, they don't notice.

The cost of getting to Low Earth Orbit comprises three components: the gravitational potential (around 88 MJ for each astronaut; this is the energy they'd hit the ground with if you dropped them from orbit with no initial velocity and there was no atmosphere to absorb part of it), the orbital kinetic energy (around 2,737 MJ for each astronaut), and atmospheric drag during ascent (the booster rockets burn more energy pushing through the lower atmosphere than anything else; around 3,650 MJ for each astronaut). For reference, 88 MJ is equivalent to burning about a gallon of gasoline; 2,737 MJ is equivalent to burning about twenty gallons of gaslone, and 3,650 MJ is equivalent to burning about thirty gallons of gasoline.

marsilies
Posts: 14
Joined: Thu May 10, 2012 3:56 am UTC

Re: What-If 0058: "Orbital Speed"

Postby marsilies » Tue Aug 13, 2013 2:47 pm UTC

Here's a question: If it takes such a high speed (and so much energy to reach that speed) to stay in low-earth orbit, why don't we simply go into a higher orbit?

User avatar
davidstarlingm
Posts: 1255
Joined: Mon Jun 01, 2009 4:33 am UTC

Re: What-If 0058: "Orbital Speed"

Postby davidstarlingm » Tue Aug 13, 2013 2:56 pm UTC

marsilies wrote:Here's a question: If it takes such a high speed (and so much energy to reach that speed) to stay in low-earth orbit, why don't we simply go into a higher orbit?

Gravity drag, mostly. Gravity is constantly pulling you down every second you're moving at less than orbital speed. To counteract that, you have to fire your engines constantly with no benefit other than basically hovering in place. Thus, it's imperative to get into orbit as quickly as possible, then make adjustments; aiming for a higher orbit would waste more energy to gravity drag than you could save.

Fire Brns
Posts: 1114
Joined: Thu Oct 20, 2011 2:25 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby Fire Brns » Tue Aug 13, 2013 3:01 pm UTC

Znirk wrote:Footnote 9 looks a bit incomplete from here.
Using song beats to help measure the passage of time is a technique also used in CPR training, where the song "Stayin' Alive" is used to .

The punctuation is odd but it isn't incomplete. It trails off to avoid redundancy in a comical manner wherein if the sentence were complete the words "stay alive" would be appended to the sentence.
Pfhorrest wrote:As someone who is not easily offended, I don't really mind anything in this conversation.
Mighty Jalapeno wrote:It was the Renaissance. Everyone was Italian.

User avatar
cellocgw
Posts: 1733
Joined: Sat Jun 21, 2008 7:40 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby cellocgw » Tue Aug 13, 2013 3:10 pm UTC

davidstarlingm wrote:
marsilies wrote:Here's a question: If it takes such a high speed (and so much energy to reach that speed) to stay in low-earth orbit, why don't we simply go into a higher orbit?

Gravity drag, mostly. Gravity is constantly pulling you down every second you're moving at less than orbital speed. To counteract that, you have to fire your engines constantly with no benefit other than basically hovering in place. Thus, it's imperative to get into orbit as quickly as possible, then make adjustments; aiming for a higher orbit would waste more energy to gravity drag than you could save.


Well, there's more to it than just orbital mechanics. We put satellites into orbit to do stuff, and the kind of stuff puts limits on desired positioning and/or altitude. The obvious example is Communications Satellites, where geosynchronous orbit is the only way (barring complicated cellular networks of fleets of satellites) to get continuous data transmission to/from specific ground locations. On the other end, surveillance satellites would like to be as low as possible to get the best imaging resolution they can.

ETA: Even from geosync orbits, the Earth rapidly becomes more riverish than seaish.
https://app.box.com/witthoftresume
Former OTTer
Vote cellocgw for President 2020. #ScienceintheWhiteHouse http://cellocgw.wordpress.com
"The Planck length is 3.81779e-33 picas." -- keithl
" Earth weighs almost exactly π milliJupiters" -- what-if #146, note 7

rmsgrey
Posts: 3031
Joined: Wed Nov 16, 2011 6:35 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby rmsgrey » Tue Aug 13, 2013 3:15 pm UTC

I think it's a Heinlein novel where a character observes that the idea of reaching escape velocity in order to get into deep space only applies if you're making essentially a ballistic launch (and then using your engines to compensate for atmospheric resistance) - if you can sustain the thrust, you could get into deep space at walking speed - it would just take longer and far more energy.

The advantage of a space elevator (or similar launch structures) is that the force to keep you from falling back to Earth comes from the structure, not from your engines - it's the difference between standing on top of a ladder and hovering with the aid of a jetpack.

User avatar
Cdevon2
Posts: 13
Joined: Wed Feb 03, 2010 5:01 am UTC

Re: What-If 0058: "Orbital Speed"

Postby Cdevon2 » Tue Aug 13, 2013 3:17 pm UTC

davidstarlingm wrote:
marsilies wrote:Here's a question: If it takes such a high speed (and so much energy to reach that speed) to stay in low-earth orbit, why don't we simply go into a higher orbit?

Gravity drag, mostly. Gravity is constantly pulling you down every second you're moving at less than orbital speed. To counteract that, you have to fire your engines constantly with no benefit other than basically hovering in place. Thus, it's imperative to get into orbit as quickly as possible, then make adjustments; aiming for a higher orbit would waste more energy to gravity drag than you could save.

Not quite. It's more because the energy we put into it isn't worth the decrease in orbiting speed.

Gonna be bringing some math into this. We'll assume that the orbit of the ISS is nearly circular and that the mass of it is negligible to the Earth's mass (which it is). The orbital speed is calculated as sqrt((G*M)/r), where G is the gravitational constant and M is the mass of the Earth. Both of these are constant, so our orbital speed is proportional to sqrt(r), our orbiting radius (calculated from the center of the Earth). Low Earth Orbit is around 7000-8000km, around where most of our satellites are. Let's quadruple that to about 30,000km above the surface of the Earth. Our orbital speed is only reduced by a factor of 1/2, but we have to get the orbiting object up 21000-24000km, fighting the Earth's gravity the entire way. It's more efficient to put the orbit as low as possible, because lateral movement is much easier than vertical movement, and most of our satellites are in LEO, especially for one so heavy as the ISS.
The only 3 laws I need:
Moore's Law
Godwin's Law
Hofstadter's Law

User avatar
moody7277
Posts: 515
Joined: Thu Aug 14, 2008 7:06 pm UTC
Location: Extreme south Texas

Re: What-If 0058: "Orbital Speed"

Postby moody7277 » Tue Aug 13, 2013 3:33 pm UTC

rmsgrey wrote:I think it's a Heinlein novel where a character observes that the idea of reaching escape velocity in order to get into deep space only applies if you're making essentially a ballistic launch (and then using your engines to compensate for atmospheric resistance) - if you can sustain the thrust, you could get into deep space at walking speed - it would just take longer and far more energy.


This is essentially the "rationale" behind the space flight in The Mouse on the Moon.
The story of my life in xkcdmafia:

Tigerlion wrote:Well, I imagine as the game progresses, various people will be getting moody.

User avatar
Wnderer
Posts: 640
Joined: Wed Feb 03, 2010 9:10 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby Wnderer » Tue Aug 13, 2013 3:41 pm UTC

Why does the fuel source need to be on the spaceship? You have your laser on the Earth plugged into a great big nuclear power station. It fires at the spaceship pushing it higher and higher until it gets to geosynchronous orbit. I think you need to account for the fact the whole setup is spinning at high speed. As the ship goes higher and higher it has to go faster and faster. So even if the laser provides the lift, you still need another force increasing the speed. The same with coming down from geosynchronous orbit. Gravity can pull the spaceship down but to hover over the same place, something is required to slow your rotational speed. But why not build the ship so it reflects the power from the laser at an angle?

I don't know. I think it might be possible to build a sort of cable-less space elevator. It's just not practical to do with rockets.

User avatar
Whizbang
The Best Reporter
Posts: 2238
Joined: Fri Apr 06, 2012 7:50 pm UTC
Location: New Hampshire, USA

Re: What-If 0058: "Orbital Speed"

Postby Whizbang » Tue Aug 13, 2013 4:24 pm UTC

Wnderer wrote:Why does the fuel source need to be on the spaceship? You have your laser on the Earth plugged into a great big nuclear power station. It fires at the spaceship pushing it higher and higher until it gets to geosynchronous orbit. I think you need to account for the fact the whole setup is spinning at high speed. As the ship goes higher and higher it has to go faster and faster. So even if the laser provides the lift, you still need another force increasing the speed. The same with coming down from geosynchronous orbit. Gravity can pull the spaceship down but to hover over the same place, something is required to slow your rotational speed. But why not build the ship so it reflects the power from the laser at an angle?

I don't know. I think it might be possible to build a sort of cable-less space elevator. It's just not practical to do with rockets.


I would imagine the amount of energy needed to lift a ship into orbit using lasers would be more than enough to just fry the ship completely. Randall discusses the Laser Elevator in his Blag, and to lift a squirrel takes a stupid amount of energy. Using a strong enough laser to propell a ship into low orbit would be enough to cook everyone inside. Unless we use a heatshield like we do for re-entry. So, two heatshields, one for take off and one for landing... and a rediculously powerful laser.

gnutrino
Posts: 100
Joined: Sat Sep 06, 2008 9:02 am UTC
Location: Over the edge...

Re: What-If 0058: "Orbital Speed"

Postby gnutrino » Tue Aug 13, 2013 4:26 pm UTC

Reaching orbital speed is hard enough; reaching to orbital speed while carrying enough fuel to slow back down would be completely impractical


But it works in Kerbal Space Program :D.

rmsgrey
Posts: 3031
Joined: Wed Nov 16, 2011 6:35 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby rmsgrey » Tue Aug 13, 2013 4:31 pm UTC

Whizbang wrote:
Wnderer wrote:Why does the fuel source need to be on the spaceship? You have your laser on the Earth plugged into a great big nuclear power station. It fires at the spaceship pushing it higher and higher until it gets to geosynchronous orbit. I think you need to account for the fact the whole setup is spinning at high speed. As the ship goes higher and higher it has to go faster and faster. So even if the laser provides the lift, you still need another force increasing the speed. The same with coming down from geosynchronous orbit. Gravity can pull the spaceship down but to hover over the same place, something is required to slow your rotational speed. But why not build the ship so it reflects the power from the laser at an angle?

I don't know. I think it might be possible to build a sort of cable-less space elevator. It's just not practical to do with rockets.


I would imagine the amount of energy needed to lift a ship into orbit using lasers would be more than enough to just fry the ship completely. Randall discusses the Laser Elevator in his Blag, and to lift a squirrel takes a stupid amount of energy. Using a strong enough laser to propell a ship into low orbit would be enough to cook everyone inside. Unless we use a heatshield like we do for re-entry. So, two heatshields, one for take off and one for landing... and a rediculously powerful laser.


A) The reason it takes so much energy is that it's a ridiculously inefficient means of propulsion - if you reflect the laser light away, then only a tiny fraction of the energy actually gets transferred to your ship...

B) Is there some profound reason why you couldn't simply use the same heat shield twice?

C) Compared to Project Orion - the one where you launch a spacecraft the size of a luxury hotel by exploding nuclear bombs underneath it - the heat-shielding of laser-propulsion doesn't sound so scary.

Nicias
Posts: 154
Joined: Tue Aug 13, 2013 4:22 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby Nicias » Tue Aug 13, 2013 4:35 pm UTC

cellocgw wrote:Speaking of fuel load, here's an absolutely delightful problem, from Engineering Optics With Matlab, Ting-Chung Poon & Taegeun Kim, World Scientific Publishing Co. Pte. Ltd., 2006 .

1.1 A laser rocket is accelerated in free space by a photon engine
that emits 10 kW of blue light (A = 450 nm).
a) What is the force on the rocket?
b) If the rocket weighs 100 kg, what is its acceleration?
c) How far will it have traveled in one year if it starts from zero
velocity?
[Courtesy of Adrian Korpel, Professor Emeritus, Univ. Iowa]

I do wonder if the honorable Professor meant to include computation of mass loss to create the blue light thrust :P

a) approximately 30 micronewtons.
b) approximately 300 nm/s^2
c) approximately 160,000 km

The mass loss is insignificant (3 milligrams)

also, the color is irrelvant.

User avatar
Wnderer
Posts: 640
Joined: Wed Feb 03, 2010 9:10 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby Wnderer » Tue Aug 13, 2013 5:00 pm UTC

rmsgrey wrote:
Whizbang wrote:
Wnderer wrote:Why does the fuel source need to be on the spaceship? You have your laser on the Earth plugged into a great big nuclear power station. It fires at the spaceship pushing it higher and higher until it gets to geosynchronous orbit. I think you need to account for the fact the whole setup is spinning at high speed. As the ship goes higher and higher it has to go faster and faster. So even if the laser provides the lift, you still need another force increasing the speed. The same with coming down from geosynchronous orbit. Gravity can pull the spaceship down but to hover over the same place, something is required to slow your rotational speed. But why not build the ship so it reflects the power from the laser at an angle?

I don't know. I think it might be possible to build a sort of cable-less space elevator. It's just not practical to do with rockets.


I would imagine the amount of energy needed to lift a ship into orbit using lasers would be more than enough to just fry the ship completely. Randall discusses the Laser Elevator in his Blag, and to lift a squirrel takes a stupid amount of energy. Using a strong enough laser to propell a ship into low orbit would be enough to cook everyone inside. Unless we use a heatshield like we do for re-entry. So, two heatshields, one for take off and one for landing... and a rediculously powerful laser.


A) The reason it takes so much energy is that it's a ridiculously inefficient means of propulsion - if you reflect the laser light away, then only a tiny fraction of the energy actually gets transferred to your ship...

B) Is there some profound reason why you couldn't simply use the same heat shield twice?

C) Compared to Project Orion - the one where you launch a spacecraft the size of a luxury hotel by exploding nuclear bombs underneath it - the heat-shielding of laser-propulsion doesn't sound so scary.


There other ways to use the laser for propulsion.

http://en.wikipedia.org/wiki/Laser_propulsion

The elevator spends a lot of time in the atmosphere. Why not use pulsed plasma propulsion and explode the air and ride the laser detonated wave into space and switch to ablative laser propulsion once we are out the atmosphere?

User avatar
Whizbang
The Best Reporter
Posts: 2238
Joined: Fri Apr 06, 2012 7:50 pm UTC
Location: New Hampshire, USA

Re: What-If 0058: "Orbital Speed"

Postby Whizbang » Tue Aug 13, 2013 5:02 pm UTC

rmsgrey wrote:
Whizbang wrote:
Spoiler:
Wnderer wrote:Why does the fuel source need to be on the spaceship? You have your laser on the Earth plugged into a great big nuclear power station. It fires at the spaceship pushing it higher and higher until it gets to geosynchronous orbit. I think you need to account for the fact the whole setup is spinning at high speed. As the ship goes higher and higher it has to go faster and faster. So even if the laser provides the lift, you still need another force increasing the speed. The same with coming down from geosynchronous orbit. Gravity can pull the spaceship down but to hover over the same place, something is required to slow your rotational speed. But why not build the ship so it reflects the power from the laser at an angle?

I don't know. I think it might be possible to build a sort of cable-less space elevator. It's just not practical to do with rockets.


I would imagine the amount of energy needed to lift a ship into orbit using lasers would be more than enough to just fry the ship completely. Randall discusses the Laser Elevator in his Blag, and to lift a squirrel takes a stupid amount of energy. Using a strong enough laser to propell a ship into low orbit would be enough to cook everyone inside. Unless we use a heatshield like we do for re-entry. So, two heatshields, one for take off and one for landing... and a rediculously powerful laser.


...

B) Is there some profound reason why you couldn't simply use the same heat shield twice?

...


I don't know. Is there? I thought heatshields were a 1 time use thing. *Looks up heat shields* Huh. I guess not. Are the current shuttle heat shields the right material for laser propultion? Do we just glue some solar sails over the tiles?

As you can see, I am not educated in space flight. I just took the easy answer of saying that lifting a squirrel off the ground using lasers is impractical, so a ship must be too.

User avatar
keithl
Posts: 602
Joined: Mon Aug 01, 2011 3:46 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby keithl » Tue Aug 13, 2013 5:57 pm UTC

Heat shields ablate. The temperature near the heat shield surface is far higher than any material can stand, so they evaporate, carry off some of the energy from the solid shield via heat of evaporation, and the ejected material keeps absorbing heat for a fraction of a millisecond while it is accelerated and blown away by the plasma stream. There might be a better way to do it, but many engineers and scientists spent many billions of dollars learning how to do heat shields, and how not to do heat shields. Gotta deliver those warheads, on time and intact, or it spoils the whole war. :-(

DanD
Posts: 257
Joined: Tue Oct 05, 2010 12:42 am UTC

Re: What-If 0058: "Orbital Speed"

Postby DanD » Tue Aug 13, 2013 6:19 pm UTC

keithl wrote:Heat shields ablate. The temperature near the heat shield surface is far higher than any material can stand, so they evaporate, carry off some of the energy from the solid shield via heat of evaporation, and the ejected material keeps absorbing heat for a fraction of a millisecond while it is accelerated and blown away by the plasma stream. There might be a better way to do it, but many engineers and scientists spent many billions of dollars learning how to do heat shields, and how not to do heat shields. Gotta deliver those warheads, on time and intact, or it spoils the whole war. :-(


Ablative heat shields ablate. Thermal soak heat shields don't.

User avatar
cellocgw
Posts: 1733
Joined: Sat Jun 21, 2008 7:40 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby cellocgw » Tue Aug 13, 2013 6:27 pm UTC

rmsgrey wrote:- if you reflect the laser light away, then only a tiny fraction of the energy actually gets transferred to your ship...
.


No, that's an incorrect assessment. What matters is momentum transfer, and by looking at conservation of P in the system, you see that a reflected photon, having changed its momentum by 2*p (boldface here indicating a vector -- the photon reversed its direction), the maximum momentum transfer to the spacecraft comes with 100% perpendicular reflection.
https://app.box.com/witthoftresume
Former OTTer
Vote cellocgw for President 2020. #ScienceintheWhiteHouse http://cellocgw.wordpress.com
"The Planck length is 3.81779e-33 picas." -- keithl
" Earth weighs almost exactly π milliJupiters" -- what-if #146, note 7

User avatar
keithl
Posts: 602
Joined: Mon Aug 01, 2011 3:46 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby keithl » Tue Aug 13, 2013 7:15 pm UTC

There actually is a "sky-crane-like" manuever that can help with re-entry a little bit.

Orbital mechanics 101: Throw a ball into the air. After a few seconds, it comes down. Throw it twice as fast at the same angle. It takes twice as long to come down (4 times farther away). In both cases, the ball is actually in orbit, though the orbit intersects the earth and is 99.99% beneath the surface.

Orbits in space are like that - higher orbits are slower. Higher orbits have more gravitational "potential energy", but objects orbit more slowly in kilometers per second, or in orbits per day. An object at 200 km altitude has a velocity of 7.784km/s, and at 1200 km altitude as a velocity of 7.253km/s. The higher orbit has a larger circumference, so the object appears to move even slower. That is why a geostationary orbit at 35,786 km altitude takes a whole day to circle the earth, and the moon (at 385,000 km ) takes a month.

If you took two objects of equal mass, one in higher orbit and one in a lower, and connected them together with a superstrong, low mass tether, the tether would pull taut, the higher object would speed up faster than its natural orbital velocity, and the lower object would slow down slower than its natural velocity.

If the tether was 1000 kilometers long, connecting between 1200 km and 200 km objects as before, the higher object orbits at 8.059 km/s, and the lower object orbits at 7.027 km/s . The tether has 1.71 millinewtons per kilogram of object weight of tension on it, that is, the masses feel a microgravity of 174 microgees. For comparison, a tether strong enough to build a space elevator has to be able to hold an object at 1 gee, 6,000 times more force, and be 100 times longer.

Now, disconnect the tether to the lower object. It is now orbiting at 10% less velocity than it needs to maintain orbit, and it reenters, with 20% less energy than it would naturally. You only need 80% as much heat shield (sorta, it's complicated). And the higher object is moving faster than it needs to stay in a circular orbit - it moves into a higher, elliptical orbit.

Ivan Bekey, former Director of Advanced Studies, wrote about this. If the upper object is the International Space Station, the tether is ordinary kevlar, and the lower object is a space shuttle on the way home, then you can lower the shuttle down 200 kilometers of tether, slow it down, and speed up ISS. Normally, ISS slows down due to air drag. ISS is reboosted with rocket engines and fuel hauled up from earth. If you did the tether manuever often enough, you would not need to haul up fuel.

There are problems, of course. Something could hit that long tether. When the shuttle is released, all the stored elastic energy in the tether can make it do crazy things. Crazy things near ISS are Not Acceptable. But it is fun to think about.

Here's my calculations, libreoffice/openoffice spreadsheet: http://server-sky.com/slides/tetherxkcd.ods

User avatar
keithl
Posts: 602
Joined: Mon Aug 01, 2011 3:46 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby keithl » Tue Aug 13, 2013 7:31 pm UTC

keithl wrote:There actually is a "sky-crane-like" manuever that can help with re-entry a little bit

If you rotate the two objects end over end on the tether, you can accentuate the velocity difference, a lot. Hans Moravec calls something like this a "Rotovator". This greatly reduces the velocity of the lower object. If you can manuever a launched object very precisely and rendezvous with the end of the rotating tether, the object can latch on and get pulled up to the rest of orbital velocity.

But you must deal with vibrations and stored elastic energy in the tether. There's no natural damping in space like there is in an atmosphere, and damping with a mass at one end, shock absorbers, etc., is not very effective for something very long with a finite speed of sound. This is a big problem for hypothetical objects like space elevator cables. They are simple in concept, as static objects, but distributed nonuniform mass and tension and gravity and angular momentum makes them fiendish to model and implement and keep under control.

rmsgrey
Posts: 3031
Joined: Wed Nov 16, 2011 6:35 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby rmsgrey » Tue Aug 13, 2013 7:42 pm UTC

keithl wrote:
keithl wrote:There actually is a "sky-crane-like" manuever that can help with re-entry a little bit

If you rotate the two objects end over end on the tether, you can accentuate the velocity difference, a lot. Hans Moravec calls something like this a "Rotovator". This greatly reduces the velocity of the lower object. If you can manuever a launched object very precisely and rendezvous with the end of the rotating tether, the object can latch on and get pulled up to the rest of orbital velocity.

But you must deal with vibrations and stored elastic energy in the tether. There's no natural damping in space like there is in an atmosphere, and damping with a mass at one end, shock absorbers, etc., is not very effective for something very long with a finite speed of sound. This is a big problem for hypothetical objects like space elevator cables. They are simple in concept, as static objects, but distributed nonuniform mass and tension and gravity and angular momentum makes them fiendish to model and implement and keep under control.


Though if you can get the maths right, you can, in principle, use the vibrations to dodge objects in intersecting orbits.

User avatar
Pfhorrest
Posts: 3713
Joined: Fri Oct 30, 2009 6:11 am UTC
Contact:

Re: What-If 0058: "Orbital Speed"

Postby Pfhorrest » Tue Aug 13, 2013 7:50 pm UTC

There is an art, or rather, a knack to orbiting.

The knack lies in learning how to throw yourself at the ground and miss.
Forrest Cameranesi, Geek of All Trades
"I am Sam. Sam I am. I do not like trolls, flames, or spam."
The Codex Quaerendae (my philosophy) - The Chronicles of Quelouva (my fiction)

User avatar
keithl
Posts: 602
Joined: Mon Aug 01, 2011 3:46 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby keithl » Wed Aug 14, 2013 3:00 am UTC

rmsgrey wrote:
keithl wrote:But you must deal with vibrations and stored elastic energy in the tether. There's no natural damping in space like there is in an atmosphere, and damping with a mass at one end, shock absorbers, etc., is not very effective for something very long with a finite speed of sound. This is a big problem for hypothetical objects like space elevator cables. They are simple in concept, as static objects, but distributed nonuniform mass and tension and gravity and angular momentum makes them fiendish to model and implement and keep under control.


Though if you can get the maths right, you can, in principle, use the vibrations to dodge objects in intersecting orbits.


Sadly, there's a lot more to it than getting the maths right. There are many more degrees of freedom in the tether than there are in the attachment, so making it jiggle correctly to deal with the next half dozen collider candidates requires monumental calculations based on an incredible amount of precise measurement, both of the position and velocity of all the bits of tether and of all the potential colliders themselves. Complex series of perturbations must be launched days or weeks in advance. I presented some of this at the Space Elevator conference a couple of years ago. They were making calculations based on an average of two avoided colliders per day. But the rate is random, and the frequency distribution is a Poisson. Over 20 years, there would be six days when you would have to deal with as many as eight potential collisions.

A tapered space elevator cable is especially worrisome, because there is a big lump of fat cable through most of the middle, with relatively thin cable stretching down to the ground side attachment. There are also lump masses of climbers whose future movements and positions will also change predicted wave propagation. Move the ground attachment a kilometer sideways, and the lump in the middle will move - eventually. But it's like pulling on the Queen Mary with kite string; changes are slow.

Seemingly simple structures, like stretched strings, can do amazingly complex things. That's why some pianists and string musicians get big audiences.

MarkW
Posts: 13
Joined: Mon Feb 22, 2010 5:58 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby MarkW » Wed Aug 14, 2013 4:10 am UTC

rhomboidal wrote:Speed-walking at 8 km/s to a song? OK, I've heard that listening to music while exercising can help motivate you, but that's ridiculous.
I was hoping for a mention of the Hyperloop proposal, but then realized that SpaceX' proposed pods only travel at a paltry 0.3 km/s.

User avatar
keithl
Posts: 602
Joined: Mon Aug 01, 2011 3:46 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby keithl » Wed Aug 14, 2013 4:50 am UTC

BTW, there is yet another tether trick you can do, for a space elevator from Mars. The space elevator is a string between mass in orbit and the ground. The Martian gravity well to the surface is 20% as deep, so materials can be 20% as strong.

But we don't need a self-supporting space elevator. Phobos orbits Mars in 8 hours, in an orbit between 6130 and 5850 kilometers above the Martian surface. A tether hanging straight down from Phobos will skim the top of the (moving) atmosphere at only 530 meters per second, fairly easy to rendezvous with.

But wait, there's more! The bottom of the tether can swing like a pendulum. With the right taper ratio, the bottom will swing more than the middle, amplifying motion. If the pendulum is timed so that at downstroke the bottom moves relatively backwards at 530 meters per second, it is stationary relative to the surface. If that is timed to coincide with Phobos periapsis (the lowest part of the orbit), the cable can touch down on the surface, at zero relative velocity. If there is a platform on the end, and you predict accurately where it will touch down, you can simply walk aboard, and soon you will be 280 kilometers up and moving at 1300m/s in the direction of orbit at the forward stroke. 2120 m/s of delta-V will put you in Mars orbit at 3420 m/s. Alternately, you can climb 5850 kilometers up the cable to Phobos.

But wait, there's STILL more! Run a loop of tether on rollers around Phobos, and around a large swinging pulley that touches down on Mars at periapsis, rolling across the landscape at 70 meters per second. Snatch onto the cable at the bottom and hang on as it rotates around the back of the pulley and up towards Phobos. After 23 hours, you will reach the rollers on Phobos; grab another braking cable and slow down. Ride the down cable to return to Mars. The whole contraption is powered by the orbital velocity of Phobos. Coriolus acceleration keeps the up and down cables bowed apart.

For a really wild ride, don't swing the pulley, let it roll continuously on a track around the equator, dragged behind Phobos, speeding up and slowing down as Phobos moves towards apoapsis or periapsis. Lay out a few kilometers of "launch" cable between the attach, and the payload forward of that, with another cable forward of the payload to a detach point on a ground anchor. As the roller comes by and the launch cable attaches to the cable around it, the attach gets yanked around the pulley and upwards, pulling the payload up and forwards. With the right lengths of launch cables (mad handwaving here), you should be moving upwards and forwards at 500+ meters per second in 15 seconds or so, and get to Phobos (and the medics, ready to fix your broken bones) in three hours.

Getting off Earth would be a heck of a lot easier if we had a trillion ton second moon in equatorial orbit at Phobos altitude. Too bad that the orbit would be unstable :-( The earth's gravity well is steeper, so we would need unobtainium tether material, staged pulleys, and a bunch of other annoying complications. Still, it would be a lot easier than a space elevator running to a counterweight well past geosynchronous. And think of the sport, ground-huggers dodging the bottom pulley wherever it touches down!

Mikeski
Posts: 842
Joined: Sun Jan 13, 2008 7:24 am UTC
Location: Minnesota, USA

Re: What-If 0058: "Orbital Speed"

Postby Mikeski » Wed Aug 14, 2013 6:11 am UTC

segundus wrote:I don't understand this. Geostationary satellites orbit at the same speed the earth revolves. So if you launch one of them, why would you need to gain any speed in addition to the speed you already have from being on the surface of the earth?

The same angular velocity, yes. Not the same linear velocity. The outside edge of a record goes a lot faster than the edge of the label.

segundus
Posts: 2
Joined: Tue Aug 13, 2013 1:24 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby segundus » Wed Aug 14, 2013 9:47 am UTC

Thank you Mikeski! I should have looked up how high a geostationary orbit is (about 6 times the radius of the earth, to my surprise) and thought about it. Wikipedia has a nice to-scale picture of satellite heights on the "Medium Earth orbit" page.

User avatar
AvatarIII
Posts: 2098
Joined: Fri Apr 08, 2011 12:28 pm UTC
Location: W.Sussex, UK

Re: What-If 0058: "Orbital Speed"

Postby AvatarIII » Wed Aug 14, 2013 11:25 am UTC

just about exactly 1000 miles.


uhuh, more like just about 1000 miles, or just about exactly 1142.328 miles

WIMP
Posts: 17
Joined: Wed May 25, 2011 6:59 pm UTC

Re: What-If 0058: "Orbital Speed"

Postby WIMP » Wed Aug 14, 2013 12:22 pm UTC

segundus wrote:I don't understand this. Geostationary satellites orbit at the same speed the earth revolves. So if you launch one of them, why would you need to gain any speed in addition to the speed you already have from being on the surface of the earth?


The Earth spins at something like 1000 miles/hr at the equator. If you launch a satellite from the equator, it receives Earth's 1000 mph East velocity. Then you climb way, way up to geostationary orbit. Your orbit is now much larger than the Earth's ~24,000 mile circumference, and at 1000 mph, you cannot complete a full orbit in 24 hr. You need to add velocity, so you're covering your much larger orbit in the same amount of time as the slower-rotating Earth covers its smaller rotation.


Return to “What If?”

Who is online

Users browsing this forum: No registered users and 4 guests