A question about de-orbiting

[none12345]

All the vehicles potential and kinetic energy must be dissipated. Not all the energy used to get it into space in the first place, which includes all the energy used to life the fuel and the expendable parts etc.

In the case of the shuttle, thats something that weights what like 250,000 pounds....and is traveling at like 26,000 miles/hours, thats a lot of kinetic energy. And then you have somethign that weights 250,000 pounds, 150 miles up in the sky, thats a lot of potential energy. (those numbers are the best i can remember, without looking up the real data, i could be way off).

To answer the OP, yes one could design a craft that does not experience blazing reentry heat. Others have already said why we dont, but we can if we wanted to.

You likely wont see a space craft designd that way until we can find a high thrust source other then chemical rockets. Or crack some other form of propulsion that doesn't dump huge amounts of mass out of the back end.

 

[vattas]

"Russian Soyuz capsules continue to use ballistic entry paths today and usually touchdown on land in Siberia."

Soyuz only uses ballistic entry mode as a backup. Primary mode orients spacecraft to have some lift, reducing G-loads.

Soyuz reentry modes

A related technique known as aerocapture has been applied to unmanned craft, though the method is typically used to slow a vehicle and enter orbit around a planet rather than as a means of re-entry."

AFAIK aerocapture (when incoming vehicle uses atmosphere to go into orbit without rocket braking) has never been used even for unmanned vehicle. Another technique - aerobraking - is often used for Mars probes to achieve desired orbit. But these two techniques has almost nothing to do with reentry.

 

[CalliArcale]

"Russian Soyuz capsules continue to use ballistic entry paths today and usually touchdown on land in Siberia."

Soyuz only uses ballistic entry mode as a backup. Primary mode orients spacecraft to have some lift, reducing G-loads.

Yep -- the G load goes from a reasonably comfortable 2.5 or so to a gut-wrenching 8 or more. Some who have experienced it found it exhilarating; others kissed the ground after getting out. ;-) Either way, though, the ballistic option is a fantastic backup. It may not be pleasant, but you will get to the ground alive and well, which is the point of the whole exercise. I'm not aware of any other manned vehicle which has ever had a backup reentry profile which can be selected after reentry has begun. All the rest, you're committed from the moment you deorbit.

A related technique known as aerocapture has been applied to unmanned craft, though the method is typically used to slow a vehicle and enter orbit around a planet rather than as a means of re-entry."

AFAIK aerocapture (when incoming vehicle uses atmosphere to go into orbit without rocket braking) has never been used even for unmanned vehicle. Another technique - aerobraking - is often used for Mars probes to achieve desired orbit. But these two techniques has almost nothing to do with reentry.[/quote]

You are correct; aerocapture has been proposed but never used. The risk of failure is too high at this point for anyone to accept it; they'd rather just send along enough prop to do the orbital insertion burn. Aerobraking to circularize the orbit was pioneered by Mars Global Surveyor (at least, I think it was the first to do it successfully) and is now commonplace for Martian spacecraft.

Also, I believe aerobraking was used to deorbit Magellan. (It's not like they needed to deorbit, but it let them study the atmosphere.)

As far as the skip reentry, that was actually the intended reentry profile for the lunar Soyuz spacecraft, and it was successfully demonstrated by an unmanned lunar Soyuz craft, as part of the Soviet lunar program. However, the program was canceled before any humans got to ride in it. The reentry pattern isn't for the faint-hearted; it's apparently easier to get a double-skip reentry wrong, and the ballistic option does not exist at those speeds. (Well, it exists, but it's not survivable.) There were many lost vehicles (though one was only lost due to flight rules which called for self-destruction if there was a chance it might come down over non-Soviet-bloc territory), and the Proton booster was problematic, having a troubling tendency to explode on the pad. (Hard to believe the highly reliable Proton booster of today had such inauspicious beginnings, but it did.)

 

[dangineer]

"The vast majority of spacecraft returning to Earth follow a ballistic entry trajectory. Here, the vehicle generates very little aerodynamic lift resulting is a lift-to-drag (L/D) ratio less than 1. The craft instead plunges into the atmosphere and falls through it under the influence of gravity and drag. This drag force slows the vehicle so that parachutes can be deployed for a soft touchdown. The landing point is predetermined by conditions when the vehicle first enters the atmosphere, and the pilot has no control over the capsule's trajectory or its landing point once he leaves orbit and begins the ballistic plunge. Since the craft essentially falls vertically through the atmosphere, its downrange distance, or ground track, from the point where it first entered the atmosphere to landing is relatively small. Manned space capsules, like those used during Mercury and Gemini flights, all followed ballistic entry trajectories to a splashdown at sea. Russian Soyuz capsules continue to use ballistic entry paths today and usually touchdown on land in Siberia. "

http://www.aerospaceweb.org/question/sp ... 0218.shtml

I think this website is a little incorrect. If a spacecraft is reentering from orbit, it is bound to have a relatively small (~10 degrees) flight path angle (FPA). A ballistic trajectory generally maintains a constant flight path angle throughout the reentry profile (since there's no lift). With a small FPA, you wouldn't fall vertically. If you wanted to have a large FPA, you would need to increase your deorbit deltaV. To fall nearly vertically, you would need a deltaV on the order of a few kilometers per second, which would need a considerable amount of fuel.

 

[garyegray]

Reentry for returning Apollo spacecraft from the Moon had to be fast and hot because there was not enough fuel on board to slow the craft down. As such, the Apollo capsule was designed to withstand a high speed return and the capsule had aerodynamic lifting and roll programs to bleed off speed and reduce G forces and heating.

 

[Quasar99]

De-orbiting also has some implications for Mars. the same physics apply but the atmopshere is very different. In fact, as has been circulating around NASA for some time, getting from Mars orbit with a human spacecraft to the surface is a very difficult that has not been solved.

The upcoming (launch 2011) Mars Science Lab spacecraft will be the largest ever vehicle to land (hopefully) on Mars. It will require an enormous 30+ ft wide "aerosheel" to absorb the heat of a ballistic entry and 50+ ft parachute

It turns out, however, that Mars' atmopshere is thick enough to slow down in (and possibly burn up if done wrong) but NOT thick enough to slow down a very large and heavy object fast enough before it hits the ground. The robotic Mars rovers Spirit and opportunity experienced about 40 g's just from the first bounce of the air bags.

The MSL spacecraft will do a gentler touchdown using a large rocket powered "sky crane" to lower the lander (no airbags) and with wheels ready to go on the surface.

The trick for a larger (and heavier) landing craft - one containing humans - is that the deceleration in the atmopshere and touchdown have to be surviveable. We cannot just bring tons of rocket fuel to power our way to the surface....
maybe there will have to be a fuel truck (ship) waiting there for us in orbit ahead of time just for this pourpose.

First you would still have to dip into the Mars atmosphere deeply to get caught into Mars Orbit to start with, because the approach velocity to Mars will be about 12,000 mph.

Then you would have to rendezvous with the tanker and prepare for a landing.

Maybe you need another fuel tanker already on the surface to get back up into space.

As you can see, the physics are quite daunting, but everyone has assumed until now that it is solvable. I'm not so sure... Some of the Mars scenarios seriously being considerd would actually have people stuck in orbit (or do a flyby) and never make it to the surface - how frustrating!!






Check ou this video "Six Minutes of Terror"
http://www.youtube.com/watch?v=tZRXwRyb ... re=related

 

[aphh]

So in principle, it is possible to build a re-entry vehicle without any sort of complicated heat shield: Simply decellerate actively using your engines instead of atmospheric drag. Of course this requires a lot more fuel, but perhaps that's where the orbital fuel depots another poster mentions come in handy.

There's a lot more to it, I'm sure, but it would be an interesting design excercise.

For starters, you can't simply decellerate to 0 in space, or you start falling. In the lower densities of the upper atmosphere, your terminal velocity is going to be so high you will still burn up in the lower, denser atmosphere. So you need to find a velocity profile that allows your wings to generate lift as soon as possible (to stop you from falling) yet at the same time does not melt the vehicle.

Jeroen

Decelerating using engines would cause steeper entry angle, so the re-entry would occur faster. This does not reduce the heat load introduced by athmosphere, only the time to bear the heat load would be less.

Steeper entry angle = less air molecules hitting you with more energy
Shallower entry angle = more air molecules hitting you with less energy

Net amount of heat load is the same.

But If you had the same amount of fuel for decelarating than you had for accelerating, you could ofcourse land slowing the velocity down to zero at the end, so the thermal loads would resemble those of getting to orbit, only occur backwards. Having twice the fuel means having to increase the fuel for lift off and getting to orbit, so you see that this will not happen anytime soon, the problem is too big.

 

[Quasar99]

An actual Inflatable test

Just by sheer coincidence, this morning (Monday August 17) NASA successfully launched, deployed, and tracked an inflatable thermal aeroshell launched to 130 km altitude from a sounding rocket over the Atlantic Ocean.

See this lead story on Spaceflight Now

http://spaceflightnow.com/news/n0908/17wallops/


This is awesome news - first time ever - and the story discusses exactly what I mentioned about the dangers of the thin martian atmopshere. Please read the story - it is short and very well written.

 

[EarthlingX]

Most of discussion so far revolved around deorbiting in atmosphere and using atmosphere for braking. What about powered descent ? Most of celestial bodies in our Solar system, where gravity is low enough to expect humans to be able to survive, don't have atmosphere, so couldn't powered descent be most general solution ?
If you need at least twice the fuel for ascent and descent for powered option AND have a fuel depot at your destination (surface or orbit) AND assume you don't leave parts of your spacecraft behind on descent and ascent, wouldn't that mean you could get better fuel/structure ratios ?
Or in short: Couldn't we have a SSTO ship that could land and take off from most of the moons and smaller planets in our Solar system with a help of orbital and planetary fuel depots ?