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Wizard From Oz
2009-Apr-19, 09:52 AM
A bit of a hyperthetical. If we had a space shuttle with unlimitied power supplies. Would it still need to go through basically the same sort of re-entry process we see today or could it achieve the same result in a much more gentle way?

Paul Beardsley
2009-Apr-19, 09:58 AM
By "power supplies", do you mean unlimited thrust? (I don't think the phrase is usually taken to mean that.)

Wizard From Oz
2009-Apr-19, 10:06 AM
By "power supplies", do you mean unlimited thrust? (I don't think the phrase is usually taken to mean that.)

Yes sorry. I am just trying to establish wether disapating the energy involved from being in orbit and comming to a complete stop on the ground, is a matter of brute thrust force or not.

cjameshuff
2009-Apr-19, 04:08 PM
Yes sorry. I am just trying to establish wether disapating the energy involved from being in orbit and comming to a complete stop on the ground, is a matter of brute thrust force or not.

It's not just thrust that's the issue, it's delta-v, or more precisely, orbital energy. Just 1 g at maximum output would allow you to slow to a stop over some point on the ground in orbit and come straight down as slowly as you please. The issue is that doing this will take more fuel than you want to carry.

You must achieve the same change in kinetic and potential energy that you did to get into orbit, so reaching the ground from orbit on rocket power takes a vehicle roughly as big as one designed to reach orbit from the ground (the details of gravity drag and structural requirements are different, since you launch at low vertical speed with full fuel tanks, while you land at low vertical speed with near-empty ones, but to a first approximation the rockets needed are the same size). Getting that vehicle into orbit in the first place then requires a launch rocket capable of launching the payload and a rocket that itself would be capable of launching the payload into orbit.

It's also more difficult to control such a landing, and involves unpleasant failure modes. Soyuz capsules have survived unintended uncontrolled reentries on several occasions...they fall back into a passive, ballistic reentry trajectory. A vehicle relying on engine power for landing that found itself without working engines or with control problems would either enter too fast and burn up, or embed itself in the ground.

astromark
2009-Apr-19, 07:20 PM
"A bit of a hypothetical." You have that right,.To slow from 17500kph to allow the craft to simply drop back to Earth... Two major issues, One you would need to carry fuel to do that and, then fuel to slow that descent for a safe landing... as cjameshuff. has said so well. Launching such a heavy vehicle is beyond us. The method used thus far where Earths atmosphere is used to cushion re-entry and be used to help slow the vehicle. Its cheap efficient and it works, most of the time... The shuttle program was a great innovation and if funding were not an issue would be the method of choice. A re-usable craft that can fly, controlled back through the atmosphere is a great idea., and it works. Bringing the shuttle program to a end is a big step back wards. Now to wander away from the OP...'Could we hand NASA to the United Nations and fund it regardless.?' For the good of humanity.

Wizard From Oz
2009-Apr-19, 11:30 PM
Thanks for the responses guys. What sparked it was watching a science fiction film were the ship came gently down. I was curious if there were other factors involved.

Astrmark - The Shuttles biggest weakness were the compromises. It was like trying to design a brick that could float, sink or fly as required.

Rather than a UN agency, perhaps an international agency. With NASA ESA and whatever the Chinese call their operations, working on combined project

Oh and North Korea, with their desire to peacefully explore space ;)

Delvo
2009-Apr-20, 03:32 AM
What sparked it was watching a science fiction film were the ship came gently down. I was curious if there were other factors involved.Was the ship in orbit before it did so? If not, what was its motion like, relative to the Earth? A lot of what the long flaming re-entry paths we use are for is to slow down a previously orbiting object from orbital speed, but how much slowing you need to do changes radically if you weren't orbiting in the first place. (It's not needed at all if your speed happens to have been close to zero relative to the Earth's nearest surface point to you. It might be needed even more if you were doing something faster than orbiting.)

astromark
2009-Apr-20, 09:28 AM
In science fiction they can do all sorts of magic... just watch ET... a relatively large mass ship just floats down to Earth without a lot of obvious energy usage. Later it leaves by going straight up while a small crowd watches from far too close.
In science fiction they can do what is seemingly impossible.
I must add that Alexander Graham Bell would not have thought his telephone could be contrived as a cellphone... We have lots to learn.