Are there any rocket scientists here?

[Elind]

I have a question for someone who understands stuff about spacecraft reentry problems.

We all know how this is done with heat shields and high speeds and decelerations etc.

We also know about techniques called aerobraking where some satellites use the very thin uppermost atmosphere acting on the spacecraft to gently slow it enough to change orbit, without heating or high G forces.

My question is this: Why can one not design a spacecraft with either very large wings, or even something like a parasail that will allow it to "fly" in the very uppermost atmosphere at very high speed (orbital initially) and convert the speed lost by atmosphere resistance into lift, thereby countering the tendency to fall into the denser atmosphere too soon at high speed, by flying UP even while losing lateral speed?

All the while losing speed in very rarified atmosphere, and staying cool, until the speed drops to normal aircraft levels and one simply flies down the rest of the way at subsonic or low supersonic rates, kind of like Spaceship One does in suborbital flight?

 

[Iamme]

Ahhh..like skipping stones on a pond, where the stone doesn't suddenly nose dive in the water (and burn up).

I too have wondered why they can't stop the meteor-like decent into the atmosphere, and why they can't like head the ship in the other direction, by turning around the craft with rocket engines going, and then just free fall or glide back through the atmosphere.

Maybe they could, but don't because perhaps this would require too much fuel and then this perhaps could cause the entire craft to have to be redesigned to carry more fuel.

With your plan, I am afraid you are wanting to use the forces of the atmosphere to your benefit at the same time these forces become the detriment. This is why I see it can't work.

Re-entry into the very outer atmosphere is at speeds of mega thousands of miles an hour. But the ship at that point can't use any aeronautical design to slow it down because the atmosphere is too thin. So it proceeds ripping into the atmosphere, getting lower, into more dense atmosphere. So, right about at the time the craft should start to glow cherry red, you want to put on the brakes. I think that by the time it starts to slow down that it is already too late and will already start burning up.

Your theory sounds a little like someone trying to design a car so that the wind it creates could turn a fan that would power the car. I think it is something like that (folly).

But we'l see what others have to say here.

 

[Jeff Corey]

It would waste fuel to do a retro rocket descent. So they skip on the atmosphere as you said, like a flat rock on a pond. They bleed off the acceleration by doing that very skillfully.
Notice that on the Moon, they couldn't do that due to no atmosphere.
And wings would not be a good idea. The forces involved would probably rip them off, given our present level of material technology.

 

[tracer]

It would waste fuel to do a retro rocket descent. So they skip on the atmosphere as you said, like a flat rock on a pond. They bleed off the acceleration by doing that very skillfully.

Elind's point seems to be that no, they do not skip gently off the atmosphere very skillfully. They slice into the atmosphere like a meteor and rely on a heat shield to bleed off the energy of their rapid deceleration. If they entered at a shallow enough angle to bounce off, and did this trick repeatedly over several progressively-lower-energy orbits, they might be able to spiral into the atmosphere and gently aerobrake down to subsonic speeds without needing a heat shield.

Is that an accurate summary of your post, Elind?

 

[Jeff Corey]

As the acrobat said, "I sure wouldn't try that trick without a net."

 

[Elind]

Elind's point seems to be that no, they do not skip gently off the atmosphere very skillfully. They slice into the atmosphere like a meteor and rely on a heat shield to bleed off the energy of their rapid deceleration. If they entered at a shallow enough angle to bounce off, and did this trick repeatedly over several progressively-lower-energy orbits, they might be able to spiral into the atmosphere and gently aerobrake down to subsonic speeds without needing a heat shield.

Is that an accurate summary of your post, Elind?

That's the idea.

Although skipping all the way out again is just wasteful of time and loss of control. Once in contact with atmosphere, can one not have enough control to stay up where heat friction is low, long enough to lose enough speed to get out of the burn up speed levels?

Wings as such probably wouldn't work in the normal way in such rareified air, but something like a controllable parasail might.

I suspect that if it hasn't been done it might be because the margins for error are very small and control requirements very high, rather than the pure physics.

 

[Elind]

I too have wondered why they can't stop the meteor-like decent into the atmosphere, and why they can't like head the ship in the other direction, by turning around the craft with rocket engines going, and then just free fall or glide back through the atmosphere.

That would work, but require at least the amount of fuel it took to get from, say, a few hundred thousand feet on up to orbit.

 

[rwguinn]

That would work, but require at least the amount of fuel it took to get from, say, a few hundred thousand feet on up to orbit.

you have to shed the energy somehow. Orbital velocity at 200km is 7.78 m/sec. (v=(m*G/r)^.5 where G=6.67*10^-11 m^3/(kg*sec^2) and r=radius to satellite from center of earth) see http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/vel_calc.html
for a calculator....

for a 5000kg satellite in a 200km orbit, the Kinetic energy is 151300 J.You also have the Potential energy of position, which is an m*g*h function. g is 9.8 m*sec^2, h is height above the earth (Yes, that is a simplification--we do have to use G, not G, and the gravitation equations--but it gives a rough idea)
We cannot store that energy. We have to shed it. That means heat. Lots of it. The only way around it is to use a lot of energy to resist it--the old Buck Rodgers land on a pillar of fire--a rocket engine--and land on the tail.
Holy "Space Cadets", Mr. Heinlein!

 

[3point14]

I don't claim to understand these things, but unless the 'new' system weighs less than, or the same as the heat shields it would allow one to take off the craft, it's never going to be implimented.

Weight is king when travelling into space, and any new ideas would have to weigh less than the current system. Anything that uses extra fuel isn't going to be any good. Most of the fuel carried in spacecraft is there just to lift the rest of the fuel.

 

[richardm]

So what about the shuttlecock effect used by SpaceShipOne? Will that not work from orbit?

 

[3point14]

Spaceship 0ne never entered orbit, it pretty much went up and straight down - the friction is caused by the craft travelling at many thousands of miles an hour round the earth, spaceship one didn't have this problem.

 

[MRC_Hans]

The core of the problem is that to stay up there, you need orbital speed. The orbital speed is higher, the lower your orbit. So if you edge your way into the upper atmosphere (or skip on it), you are going into a positive feed-back loop:

You get lower
You hit atmosphere and loose speed
You get further down
You hit more atmoshpere and loose more speed
...etc.

In theory, you might build wings that could replace the orbital "lift" with airfoil lift and thus make a controlled transit from orbiting to sustained flight, but that will require you to build a wing system that can operate over a pressure range from a fraction of a millibar to surface pressure (1000mb) and from speeds of about 20km/sec to landing speed. For comparison, a supersonic jet fighter flies within the envelope of perhaps 300mb to 1000mb and 300km/h to 2300km/h.

Hans

 

[Elind]

you have to shed the energy somehow. Orbital velocity at 200km is 7.78 m/sec. (v=(m*G/r)^.5 where G=6.67*10^-11 m^3/(kg*sec^2) and r=radius to satellite from center of earth) see http://liftoff.msfc.nasa.gov/academy/rocket_sci/orbmech/vel_calc.html
for a calculator....

for a 5000kg satellite in a 200km orbit, the Kinetic energy is 151300 J.You also have the Potential energy of position, which is an m*g*h function. g is 9.8 m*sec^2, h is height above the earth (Yes, that is a simplification--we do have to use G, not G, and the gravitation equations--but it gives a rough idea)
We cannot store that energy. We have to shed it. That means heat. Lots of it. The only way around it is to use a lot of energy to resist it--the old Buck Rodgers land on a pillar of fire--a rocket engine--and land on the tail.
Holy "Space Cadets", Mr. Heinlein!

You are right of course, the energy has to go somewhere, but let's separate the different components you mention.

An object that winks into existence at rest above a point on the earth where the atmosphere just starts, (orbiting at the same speed as the earth's spin at that height), would fall straight down at an accelleration of G (a little less at that altitude initially). That potential energy can be dissipated through parachutes or wings as is done every day, and was done by Spaceship One.

The problem is to dissipate the orbital kinetic energy in a manner that limits the heat buildup. That means do it more slowly and avoiding the denser air while above a certain speed.

For the sake of visualization, let's also not consider something the size of the space shuttle, but a smaller scientific satellite, say a couple of hundred pounds, or an emergency capsule with one human in it.

Could that not conceivably be maneuvered at the edge of the atmosphere, dissipating kinetic energy just like the slower speed objects lower down, but at a low enough rate to avoid the burnup problem?

As to how to do it; wings, or body shape will always be optimized for certain speed or density ranges, which is probably impossible for these ranges. Even if one used the "skipping" stone concept, it would probably fail at a lower altitude when the speed is still very high, for any fixed shape; but what of a variable geometry or a combination of "parachute" and wings?

I'm suggesting that it is theoretically possible, but perhaps technically too difficult to control under conditions of little margin for error, but I'm not sure.

 

[Elind]

The core of the problem is that to stay up there, you need orbital speed. The orbital speed is higher, the lower your orbit. So if you edge your way into the upper atmosphere (or skip on it), you are going into a positive feed-back loop:

You get lower
You hit atmosphere and loose speed
You get further down
You hit more atmoshpere and loose more speed
...etc.

In theory, you might build wings that could replace the orbital "lift" with airfoil lift and thus make a controlled transit from orbiting to sustained flight, but that will require you to build a wing system that can operate over a pressure range from a fraction of a millibar to surface pressure (1000mb) and from speeds of about 20km/sec to landing speed. For comparison, a supersonic jet fighter flies within the envelope of perhaps 300mb to 1000mb and 300km/h to 2300km/h.

Hans

Right. I agree (reading this after I wrote the previous). I'm not expecting a working design here, but I take it you agree it might be possible, however technically difficult?

Let's try to do it with a really small object. That would help the design in the sense that we don't have to imagine building for strength that we may not have materials for, like supporting a space shuttle.

 

[Elind]

I don't claim to understand these things, but unless the 'new' system weighs less than, or the same as the heat shields it would allow one to take off the craft, it's never going to be implimented.

In that sense yes, but unless an object is carried within another entry vehicle, the whole craft has to be designed within the reentry parameters and heat problems etc.

Perhaps a more gentle approach would allow other advantages of use or design?

 

[rwguinn]

Right. I agree (reading this after I wrote the previous). I'm not expecting a working design here, but I take it you agree it might be possible, however technically difficult?

Let's try to do it with a really small object. That would help the design in the sense that we don't have to imagine building for strength that we may not have materials for, like supporting a space shuttle.

First off, my first post was off by a factor of 10^3^2, or 1,000,000. The velocity is km/sec, not m/sec. Sorry about that. Dadgummed metric system( )

Ok--your small object has less mass, and proportionally a smaller surface area to dissipate the heat from. That is why small meteors burn up, while big ones occasionally get through.
How long do you figure that you want to take to get down? Let's use a 500kg object in LEO (Low Earth Orbit) at 20km. That is a mere 1.5130 X 10^10J. that is 14 million BTU. and you still have to get rid of it. That's a lot of heat, and if you want to dissipate it slowly, you need a LONG time.

 

[strathmeyer]

The answer is bascially because the velocity required for a spacecraft to bounce off the atmosphere is so high; once it because too skip it's still going so fast it's pretty much the same as diving right in.

Still don't understand why a parasail wouldn't work, though.

 

[Terry]

The answer is bascially because the velocity required for a spacecraft to bounce off the atmosphere is so high; once it because too skip it's still going so fast it's pretty much the same as diving right in.

Still don't understand why a parasail wouldn't work, though.

Because a parasail is effective in a subsonic flow regime, and re-entry to the tenuous upper atmosphere puts you in a hypersonic regime. Fluids behave very differently depending on the nature of the flow.


--Terry.

 

[Elind]

First off, my first post was off by a factor of 10^3^2, or 1,000,000. The velocity is km/sec, not m/sec. Sorry about that. Dadgummed metric system( )

Ok--your small object has less mass, and proportionally a smaller surface area to dissipate the heat from. That is why small meteors burn up, while big ones occasionally get through.
How long do you figure that you want to take to get down? Let's use a 500kg object in LEO (Low Earth Orbit) at 20km. That is a mere 1.5130 X 10^10J. that is 14 million BTU. and you still have to get rid of it. That's a lot of heat, and if you want to dissipate it slowly, you need a LONG time.

But it's not all heat dissipation is it, anymore than falling under a parachute is heat dissipation. It's also transfer of momentum from one object to another (parachute to the air).

The whole point is to minimize the heat part. As to how long, I don't know how to do those calculations realistically, but what's wrong with one or two orbits? A few hours?

 

[Elind]

Because a parasail is effective in a subsonic flow regime, and re-entry to the tenuous upper atmosphere puts you in a hypersonic regime. Fluids behave very differently depending on the nature of the flow.


--Terry.

Fair enough, but can we not call it a drogue then, with some basic directional bias?

I guess the question is simply whether it is possible at all to gain lift in any usable form in the very tenuous upper parts of the atmosphere? However if you get drag you have a force. If you have a force, why can't you direct it even partially where you want to?

Perhaps I should have asked if there are any aerodynamic engineers here?