Spacecraft Re-Entry Speed: Why So Great?

Factual Questions
21

[QUOTE=Leaffan]
I can possibly see this for the Shuttle missions, but how does this hold true for the Apollo missions? They didn’t just go up, orbit, and return obviously. Why should a ship with a linear return vector have to enter at 17,00 MPH?

And I’m seriously curious here, I’m not trying to be a prick!
[/QUOTE]

In the case of Apollo (and Gemini and Mercury) the capsules had very big parachutes. They would slow down enough to re-enter the atmosphere, slow down somewhat due to drag (they also had very tough heat shields) then deploy a small parachute, then a big one, then plop into the water. So they used the atmosphere to slow down much like the shuttle does.

22

[QUOTE=Leaffan]
In all due respect, either I’m missing something or you’re not explaining things in clear terms.

I have no idea what you mean by anything quoted above.

:confused: :confused: :confused:
[/QUOTE]
This is going to be harder than I thought.

You said you didn’t understand anything you quoted so we’ll start at the beginning.

Force (denoted by the symobl F) is a push or pull that moves things. When I push my car I’m exerting a force on the car.

Acceleration (denoted by the symbol a) is the rate of change in velocity as time passes. Velocity (denoted by the symbol v) is the rate of change of distance traveled in a particular direction as time passes. That is, if you change either the rate at which you are covering ground or the direction you are going, or both, you have changed your velocity or accelerated.

Mass (denoted by the symbol M) is the resistance of a physical object to acceleration.

It turns out that these three things are connected by a simple equation F = Ma which is Newton’s third law.

By a bit of mathematical legerdermain we can rewrite Newton’s third law as a = F/M. (I think in the post you quoted I wrote M/F and I’m sorry about that.) As I said, the F is the net force that actually results in acceleration. On launch there is a drag force that is low at first because the velocity is low. This drag builds up rapidly with velocity in an atmosphere of constant density. As I said, the launch is vertical and the rocket’s path stays vertical to get out of the densest atmosphere as soon as possible. The drag is low at first because the velocity is low. It builds up to some maximum and then decreases as the atmosphere thins and eventually falls to essentially zero in low earth orbit.

Call the rocket thrust force F[sub]r[/sub] and the drag force f[sub]d[/sub]. The net force on launch is the rocket thrust, F[sub]r[/sub] - F[sub]d[/sub].

That makes the equation for the acceleration on launch a = (F[sub]r[/sub] - F[sub]d[/sub])/M

In a vacuum there is no drag force so the equation for acceleration is merely a = F[sub]r[/sub]/M.

The launch trajectory is configured so as to reduce F[sub]d[/sub] to a very low number as rapidly as possible and then the two equations are essentially equal to each other.

So it takes nearly the same amount of fuel to slow the shuttle down as it now takes to speed it up. And, as I said, if it now takes 4,474,000 lb. of fuel to put it in orbit from a standing start it might easily take 4,400.000 to slow it to 2000 mph. That means that we have to lift 4,400,000 lb. into orbit and we just can’t do that with present technology (I’m pretty sure.)

23

It’s that high velocity that keeps you in orbit in the first place. The moment you fire thrusters to slow you down, you move to a lower orbit, atmosphere or not. If instead you increase your orbital velocity, your orbital altitude goes up. You can’t orbit the Earth at the shuttle’s normal altitude at anything but ~17,500 mph. So by necessity if the shuttle starts to slow down, it’s going to dip into the atmosphere.

24

Edited to add quote:

Two reasons. First, unless you want to spend a long time in the capsule, you’re going to want to come back from the Moon reasonably fast. Second, from an orbit around the Moon to the Earth’s surface is “downhill”. You have to fight Earth gravity to get to the Moon, you are falling into it to return. You either burn fuel to slow down, or you use Earth atmosphere’s friction to do the job.

25

[QUOTE=Leaffan]
Obviously you need lots of fuel to break the gravitational pull of Earth.
[/QUOTE]

You don’t “break” the gravitational pull of Earth. Orbiting spacecraft stay reasonably close to the earth (the Shuttle orbits at something like 100 miles up) where the Earth’s gravity is a substantial fraction of what it is at sea level. And, of course, they stay in orbit and circle the earth only because of the Earth’s gravity. While in orbit they are in freefall, so there is no sensation of gravity. This fools people into believing that “in outer space, there’s no gravity” - but that’s silly.

Even a trip to the moon doesn’t “break” the gravitational pull of Earth. One hint would be the fact that the moon stays in orbit around the earth. Another would be that the trip back from the moon is again done in freefall, using the Earth’s gravity.

26

[QUOTE=Leaffan]
I can possibly see this for the Shuttle missions, but how does this hold true for the Apollo missions? They didn’t just go up, orbit, and return obviously. Why should a ship with a linear return vector have to enter at 17,00 MPH?

And I’m seriously curious here, I’m not trying to be a prick!
[/QUOTE]
Because all the way back from the moon the spacecraft was basicly falling to Earth, picking up more and more speed as it went. Nothing to slow it down you see. It was actually doing close to 25,000 MPH by the time it hit the Earth’s atmosphere, and it took some delicate maneuvering to keep from burning up, crushing the astronauts inside with unsurvivable G-forces, or skipping off into space again.

As has been said upthread, the amount of fuel necessary to slow down a reentering spacecraft would be so huge that it is less bulky simply to carry a heat shield capable of taking the burn.

I can’t help but get the feeling that what you’re thinking of is something like the rockets in those 1950s space movies- the ones with tail fins and “atomic motors” of such ridiculously high performance that they would simply point their tail down, brake to a halt and then descend to a landing. You could do that if you had a nuclear rocket with an exhaust as powerful as a miniature nuclear explosion. No such available in real life unfortunately.

27

[QUOTE=Xema]
…This fools people into believing that “in outer space, there’s no gravity” - but that’s silly…
[/QUOTE]

:eek: Huh? OK. Gravity exits throughout the universe, and of course there’s the Einsteinian relativity version, but for all intents and purposes there ain’t a lot of gravity in space, no?

28

[QUOTE=Lumpy]
Because all the way back from the moon the spacecraft was basicly falling to Earth, picking up more and more speed as it went. Nothing to slow it down you see. It was actually doing close to 25,000 MPH by the time it hit the Earth’s atmosphere, and it took some delicate maneuvering to keep from burning up, crushing the astronauts inside with unsurvivable G-forces, or skipping off into space again.

As has been said upthread, the amount of fuel necessary to slow down a reentering spacecraft would be so huge that it is less bulky simply to carry a heat shield capable of taking the burn.

I can’t help but get the feeling that what you’re thinking of is something like the rockets in those 1950s space movies- the ones with tail fins and “atomic motors” of such ridiculously high performance that they would simply point their tail down, brake to a halt and then descend to a landing. You could do that if you had a nuclear rocket with an exhaust as powerful as a miniature nuclear explosion. No such available in real life unfortunately.
[/QUOTE]

And there you have it. From most respondents. Ignorance fought.

Thank you.

29

[QUOTE=Leaffan]
:eek: Huh? OK. Gravity exits throughout the universe, and of course there’s the Einsteinian relativity version, but for all intents and purposes there ain’t a lot of gravity in space, no?
[/QUOTE]

There’s increasingly small amounts the further you are away from any mass, but there’s vast amounts (by interstellar standards) when you are talking about the vicinity of the Earth and the Moon. Xema’s point is that people sometimes see astronauts floating around in Earth orbit and think there’s no gravity up there, when of course there’s heaps, it’s just that the astronauts are in free fall.

30

[QUOTE=Leaffan]
:eek: Huh? OK. Gravity exits throughout the universe, and of course there’s the Einsteinian relativity version, but for all intents and purposes there ain’t a lot of gravity in space, no?
[/QUOTE]

Wrong. This is a common misconception.

Do you consider the Space Shuttle in a low Earth orbit to be “in space”?

The force of gravity 600 kilometers up (where the Hubble Space Telescope orbits) is about 84% of the value on the surface of the Earth. Astronauts don’t feel this, however, because they are constantly falling.

If a spacecraft was 600 km up and not moving (for whatever reason), it would simply fall straight down until it impacted the Earth. The force of gravity causing this fall would increase as the craft fell from 84% of the value on the surface to 100%.

The reason that orbiting spacecraft don’t hit the Earth is because they are moving so fast “sideways” that as they fall toward the Earth they actually “miss” the planet. This is the definition of a stable orbit, and this is why orbiting objects must have a huge speed, or they will not stay in orbit.

It takes large rocket engines to impart this speed to a spacecraft. Here is the key point: it takes just as much thrust to slow a spacecraft back down as it took to speed it up in the first place during launch.*

This thrust can be imparted to the craft by using the rocket engines again, but you’d have to haul up just about as much fuel as the rocket used on the way up. It’s much easier to slow the spacecraft down using atmospheric braking.

One thing that most people don’t realize is that the point of the large rocket engines during launch is not simply to get a spacecraft off the planet. Have you ever watched a video of a rocket launch? They don’t go straight up. About 20 seconds after launch, they start rolling over so that the spacecraft can get the orbital velocity it needs to stay in orbit.

[sub]*Actually, it takes slightly less because the craft had to overcome atmospheric friction on the way up.[/sub]

31

[QUOTE=robby]
Wrong. This is a common misconception.

Snip-snap

It takes large rocket engines to impart this speed to a spacecraft. Here is the key point: it takes just as much thrust to slow a spacecraft back down as it took to speed it up in the first place during launch.*

Snip-snap

[sub]*Actually, it takes slightly less because the craft had to overcome atmospheric friction on the way up.[/sub]
[/QUOTE]

I would expect that on a single stage to orbit spacecraft… scratch that, I wouldn´t expect that even on a SSO craft; by the time the space shuttle or a capsule like the Soyuz gets up there it has shed a very large percentage of it´s mass in expended stages/boosters and fuel.
Now of course current spacecraft shed weight more or less continuously as they go up (with some hiccups as stages are discarded), so it´s not as if they have to haul all the launch weight up to orbit, so I guess there should be an average of the mass as it´s accelerated up and away; that average it´s certainly going to be much less than the orbital mass, and as such it would require a lot less energy to slow it down.

The analogy I can think of is a truck pushing a shopping trolley, it takes a lot of energy to get the whole thing moving, but after the trolley separates from the truck it will take much less energy to stop it than it would have taken for the whole trolley/truck assembly.

Of course, having said that it´s probably still less expensive to go the way of heatshields, but I´m wondering if the weight saved by, say, making away with the shuttle´s heat shielding couldn´t be used to bring enough fuel up to slow it down to a non-meteoric de-orbiting.

32

What if the capsule is on a treadmill?

33

[QUOTE=Mangetout]
What if the capsule is on a treadmill?
[/QUOTE]

Won´t someone fish-slap this fellow? :smiley:

34

[QUOTE=drewbert]
It’s that high velocity that keeps you in orbit in the first place. The moment you fire thrusters to slow you down, you move to a lower orbit, atmosphere or not. If instead you increase your orbital velocity, your orbital altitude goes up. You can’t orbit the Earth at the shuttle’s normal altitude at anything but ~17,500 mph. So by necessity if the shuttle starts to slow down, it’s going to dip into the atmosphere.
[/QUOTE]
**drewbert ** gets it in one. It is impossible to slow a spacecraft to 2,000 mph before deorbiting.

35

[QUOTE=Fear Itself]
**drewbert ** gets it in one. It is impossible to slow a spacecraft to 2,000 mph before deorbiting.
[/QUOTE]

Actually it’s possible, only very, very impractical. To do this you need to keep constant thrust against gravity pull of Earth, which in turn demands huge amounts of fuel, which in turn makes whole ship absurdly heavy and… well, just check **David Simmons **posts.

And by “possible”, I mean theoretically possible, not that we could do it without developing technology exceeding currently available.

36

[QUOTE=puppygod]
Actually it’s possible, only very, very impractical.
[/QUOTE]
Agreed. I was thinking in terms of Apollo technology.

37

[QUOTE=Ale]
The analogy I can think of is a truck pushing a shopping trolley, it takes a lot of energy to get the whole thing moving, but after the trolley separates from the truck it will take much less energy to stop it than it would have taken for the whole trolley/truck assembly.
[/QUOTE]
I don’t think so - a significant part of the reason you needed the whole truck was to get the trolley moving at sufficient speed - sure, it weighs less now it’s just a trolley, but if you want to slow it down, you’re talking about expending a fair bit of fuel, which weighs a lot, which means you need even more fuel to slow down the trolley and the fuel, which weighs even more, which means you need… and so on, until, before you know it, it is actually another truck that you need to slow it down.
Worse, your starting energy budget must then include sufficient provision to lift not only the vehicle, but the massive braking system too. It would be easier to build a big ladder and just climb up, and by ‘easier’, I mean ‘really, really difficult’

38

[QUOTE=robby]

It takes large rocket engines to impart this speed to a spacecraft. Here is the key point: it takes just as much thrust to slow a spacecraft back down as it took to speed it up in the first place during launch.*

This thrust can be imparted to the craft by using the rocket engines again, but you’d have to haul up just about as much fuel as the rocket used on the way up. It’s much easier to slow the spacecraft down using atmospheric braking.
[/QUOTE]

You’re saying two different things here. The first (amount of thrust required for ascent & descent are equal) is true. The second (“just as much fuel”) is not, because fuel and engines have mass, and you have to get all of your descent fuel up to orbit. David Simmons has made the point pretty well, but I’ll try to illustrate it with some real numbers until Stranger on a Train shows up. Tsiolkovsky laid out what most aerospace people call “the rocket equation”, which will go a long way towards explaining this, I hope. In English, it states that

In symbols, that’s

The problem you’re asking is why we can’t slow down the rocket to a more reasonable reentry velocity. Current reentry velocities for the orbiter are on the close order of 7,500 meters per second. Let’s look at what it would take to cut that in half to 3,750 meters per second.

Engine efficiency is limited by the state of the art in engineering and rocket science. The most efficient liquid fuel engines get specific impulse values on the order of 500 seconds (higher is better) for high-thrust applications. The shuttle’s Orbital Maneuvering System – the engines used for de-orbit burn – have an I[sub]sp[/sub] of 400 seconds, so we’ll use that value. Gravity is obviously not something we can change. We’ll assume 84% gravity in accordance with the previous posts. Mass ratio is the trick. The orbiter’s empty mass (from Wikipedia, rounded up) is 70,000 kg. You can do the math yourself if you remember how to do logarithms. If we want a change in velocity of 3,750 m/s, we will need the logarithm of the mass ratio to equal 1.13885.

That means that the mass ratio (full-to-empty) must be 13.77, so

So now you have a space shuttle that weighs 963,900 kg (instead of the sporty, lightweight shuttle of today), which is nine times the mass that the Saturn V could lift to orbit. To get a payload of that size to orbital velocity using the Shuttle’s engines would require almost 48.3 million kilograms of fuel.[sup]1[/sup] That’s about 25 times the current takeoff mass of a fully-loaded Space Shuttle.

It’s far more practical to use the atmosphere to slow you down. You don’t have to bring it with you, because it’s always there. As long as you can design a thermal protection material that is lighter than twelve Space Shuttles, you come out ahead of the game.

  1. Checking my math on this is left as an exercise to the OP. :smiley:
39

[QUOTE=Jurph]
It’s far more practical to use the atmosphere to slow you down. You don’t have to bring it with you, because it’s always there.
[/QUOTE]
Not if my plan succeeds.

40

Incidentally it is possible to orbit the Earth at 2000mph - you just have to be in a rather wider orbit than the Moon’s to do it (the Moon pokes along at a mere 2250mph or thereabouts). But that doesn’t help 'cos you still have to lose an industrial buttload of potential energy, and there’s no way to pee it away in a vacuum, so it obligingly turns into kinetic energy on your way down, meaning you hit the atmosphere at what is technically termed “a fair old lick”.

Well yes, once we build a space elevator we’re laughing. :slight_smile: