Spacecraft Re-Entry Speed: Why So Great?

OK, so I understand the physics behind orbits, and geosynchronous orbit velocities. But why, during re-entry must spacecrafts still be traveling at some 17,000 MPH?

I’m sure the answer will make perfect sense once I hear it, but why can’t they just slow the heck down to, oh I don’t know a mere 2,000 MPH and then dive into the atmosphere? Why does deceleration have to happen in the atmosphere, as opposed to space?

To decelerate in space, they would have to burn fuel, which means they would have to carry more fuel up into orbit in the first place, which would make the whole business more expensive and inefficient, compared to using the atmosphere to slow the spacecraft down in reentry.

Hmm… I don’t buy it. If they can take enough fuel into orbit to reach the moon, then the amount of fuel to slow a spacecraft shouldn’t be that restrictive?

Most of the fuel for the moon shot was just getting the danged thing off the Earth, and the stages of the rocket that carried the fuel were jettisoned as they were depleted. Once you’re pointed at the moon at the velocity you want in a vacuum, you don’t need much fuel beyond what’s necessary to put you into lunar orbit.

Once the capsule was back (the only piece leftover after the whole mission), it just plummeted through the atmosphere, relying on the Pacific Ocean to cool it off.

Weight, and its associated fuel cost (including the fuel cost of the weight of the fuel) is THE major concern in spaceflight. You want to deal with as little of it as possible.

OTOH, when we landed on the moon, we transitioned from orbit to landing without any atmospheric braking. Less gravity, but still…

Sure. Makes sense. I’m aware of gravity and vacuums. So, to me, that means that it wouldn’t take much fuel to decelerate a spacecraft in a vacuum, right? It doesn’t take much to get you moving, so it shouldn’t take much to reverse the vector, no?

I understand why the atmosphere is used for braking, but what seems counterintuitive is that if they don’t burn up on the way UP; why would they burn up on the way back DOWN?

I guess it’s because they do much of their accelleration on the way up outside of the atmosphere, right?



Atmosphere ain’t in there nowhere.

That’s my guess anyway.

I think that much of the distance to the moon is covered by virtue of momentum. I don’t think it takes much more fuel to escape earth than it does to escape earth and then travel to the moon.

Now ask yourself how much fuel it takes to slow a two kiloton object from Mach 20 to Mach 2 or whatever.

Doesn’t it also burn on the way up?

We’ve been through this in a previous thread that I can’t find at the moment.

It would take nearly as much fuel to slow the spacecraft down as it did to speed it up. So the rockets would need to be twice as big. But that doesn’t tell the whole story. That fuel that’s going to be used to slow it down also has to be lifted into orbit.

The takeoff weight of the shuttle is given in Wiki as 4,474,574 lb and the payload as 53,700 lb. Since the fuel to slow it down is the same as that required to speed it up the payload would be increased to something like 4,400,000 lb. The amount of fuel that would be required to lift that weight into orbit would be enormous and thoroughly impractical.

See, again this doesn’t make sense. (I’m daft; I know.)

Obviously you need lots of fuel to break the gravitational pull of Earth. Not only that, you’re breaking the gravitational force of Earth through a viscous fluid: the atmosphere.

But once in a vacuum, the amount of fuel used to propel, steer, accelerate, decelerate must be fractional. I can’t believe that fuel restrictions are the reason behind having to enter the atmosphere at insanely excessive velocities.

Of course I may be wrong!

The amount of force needed to accelerate an object has a lot more to do with the mass (think weight divided by a constant) than with air resistance. The launch is vertical and the path stays nearly vertical until the thing is out of the really thick atmosphere. There is some extra force required and that’s why I said it takes nearly as much fuel to slow it down as to speed it up.

Anyway, the acceleration you can get with a force F is equal to M/F where M is the mass being accelerated. The force is the net force and on the way up some of the rocket force is used to overcome drag and isn’t available for acceleration. However, this amount is relatively small compared to the force required to accelerate the mass.

So no, being in a vacuum doesn’t reduce the amount of rocket fuel required to slow the things down by much.

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:

Leaffan, the requirement for fuel is large because you have to make a big change/acceleration to the mass of your spacecraft. Lack of atmosphere really has nothing to do with it.

Also, as you slow the spacecraft down, it is no longer in anything close to an orbit and will accelerate towards the Earth, you will need to constantly use fuel to provide thrust to oppose the Earth’s gravity and keep going slow.

And they don’t burn up on the way up because they aren’t going fast enough.

If you wanted the rocket to decend from orbit at the speed of an elevator (so that the astronauts can sit upright sipping their tea), you would need the engines firing, with enough thrust to counter the gravitational pull down while maintaining, say, a steady nonincreasing decent rate of 25 feet a second, for the whole duration, would require a HUGE amount of fuel. Huge. HUGE! Easier to just let the sucker drop into a big enough puddle of water…

Lookit the stats for the lunar lander. Lunar lander - Wikipedia This craft was designed to do just what you want (a powered landing at a safe speed).

Would some rocket scientist please do some math for us?

How long did the descent engines burn?

What is “Engine specific impulse”? (I think the “N” is a Newton, but it’s been a long time since I even heard that unit of measurement.)

Wiki says descent stage mass w/fuel was 22783 lbs, the ascent stage 10300lbs. The ascent engines had 5187lbs of fuel, the descent engines had 18000lbs, or a little more than 3 times as much… (But then again, the descent engines had to slow the whole lander, and the ascent engines had to lift just the upper ascent module portion… hmmm.)

Correct me if I am wrong but I am under the impression the biggest drag on the shuttle lifting off is gravity, not the atmosphere.

I think what Leaffan is saying is that decelerating in a vacuum, mostly free of the earth’s gravity probably consumes as much fuel as accelerating under the same conditions. Which is much less than what’s necessary to escape Earth’s grasp

I think you may be confused about the difference between velocity and acceleration. In a vacuum, with no resistance, things tend to maintain a constant velocity (speed relative to some other object, like the earth.) That’s why once you’re in orbit, it doesn’t take much more fuel to get to the moon. Line up the ship, fire the engines, and off you go and you don’t stop until you reach lunar orbit.

Acceleration is a change in velocity. To achieve any change in velocity, whether speeding, slowing down, or changing direction, you need more energy. In space, your only real choice is rocket fuel. Unless you’ve got a photon sail on board. But we haven’t invented those yet.

Now, to get into earth orbit, you must act against the earth’s gravity. As it turns out, acceleration and gravitation are actually kinda the same thing so the math is pretty simple. To get into earth orbit requires a rocket that can accelerate you from standing still on the launch pad to about 17,000 miles per hour. Then you can shut the engines off.

Now you’re in a vacuum, and your engines are off, which means you continue moving at a constant velocity. You are traveling 17,000 miles per hour relative to the surface of the earth, and you have to slow down back to a standstill to get back there. You need to apply the same amount of force to slow you back down to zero as you did to get you up to 17,000. That’s a change in velocity which means it’s acceleration.

When you blasted off, you had some humongous tanks of rocket fuel to get you up to orbital speed. You also had the advantage of being pointed in the direction you were traveling – like a bullet – so air resistance is pretty low.

To accelerate back down, the shuttle re-enters at an angle, using its wings to create drag, and eventually flattening out to horizontal flight. It then flies around a bit back and forth with the express purpose of slowing itself down against the atmosphere. This creates a lot more resistance than shooting straight through, causing the shuttle to get quite toasty indeed.

From launch to earth orbit takes a shuttle about ten minutes. To glide the shuttle to a landing takes significantly longer.

Maybe it helps to remember that rocket thrust is different than pushing stuff with your hands. If you’re in space and everything is weightless, it’s true you can move a heavy, 1 ton barbell with a single finger. But that’s because you are pushing against the floor or wall of the spacecraft with your feet or some other part of your body.

Also, you impart a small change in speed with a small force from your finger. But you observe that change in speed as signficant since the barbell moves.

The difference between at rest and escape velocity is a gigantic change in speed. Also, it must be accomplished by rocket thrust.

Force = mass times acceleration means when you move the barbell with your finger, you actually move the spacecraft in the opposite direction when you push with another part of your body. But in space, there’s nothing to push against, so the engines get thrust by throwing off mass. Usually, this is hot gas, which has less mass than the spacecraft, but is also moving away at higher speed.

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!