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Sock Munkey
2005-Jul-01, 07:52 AM
I'm told that most of the heat of re-entry is caused by air comression rather than friction. This being the case might it be practical to have a sort of "collapseable-air-plow" on an exendable boom on the nose of the spacecraft to split the air enough so that it has room to expand and cool once it's past the boom and in contact wih the hull of the craft?

NEOWatcher
2005-Jul-01, 11:39 AM
I'm told that most of the heat of re-entry is caused by air comression rather than friction. This being the case might it be practical to have a sort of "collapseable-air-plow" on an exendable boom on the nose of the spacecraft to split the air enough so that it has room to expand and cool once it's past the boom and in contact wih the hull of the craft?
My thought would be that the deceleration wouldn't be as great, and it wouldn't slow down fast enough. Besides, the boom would have to be quite strong, which might render it impracticle.

wackywizjr
2005-Jul-01, 11:50 AM
This design was considered, but it was determined that the strength needed to keep the air ram in place would make it so heavy as to be impractical.

Then there is also the safety factor if the plow breaks loose then the craft would be hit by it as it buckled back.

The problem with the current shuttle is the speed of re-entry. What is needed is a way to make the craft slow itself down to near zero relative speed before it enters the atmosphere then there would be very little re-entry heat. A spacecraft entering the atmosphere at about 300 miles per hour would not generate near the compression wave of one traveling 18000 miles per hour.

NEOWatcher
2005-Jul-01, 12:00 PM
The problem with the current shuttle is the speed of re-entry. What is needed is a way to make the craft slow itself down to near zero relative speed before it enters the atmosphere then there would be very little re-entry heat. A spacecraft entering the atmosphere at about 300 miles per hour would not generate near the compression wave of one traveling 18000 miles per hour.
Unfortunately, slow it down, and you dive into the atmosphere anyway.

wackywizjr
2005-Jul-01, 12:16 PM
The problem with the current shuttle is the speed of re-entry. What is needed is a way to make the craft slow itself down to near zero relative speed before it enters the atmosphere then there would be very little re-entry heat. A spacecraft entering the atmosphere at about 300 miles per hour would not generate near the compression wave of one traveling 18000 miles per hour.
Unfortunately, slow it down, and you dive into the atmosphere anyway.

True, but if that is where you want to go then there isn't a problem with it. A fall strait down through the atmosphere from a zero starting speed is not unsurvivable (as was evidenced by spaceship one). The problem is the velocity required to maintain orbit. If the velocity could just be cut in half before re-entry it would make for a much safer ride.

Argos
2005-Jul-01, 12:29 PM
Delta v required for cutting shuttle´s speed by a half (~ 4.5) would take a lot of fuel. It would be enough to boost a rocket from the surface to low orbit, in Mars.

NEOWatcher
2005-Jul-01, 12:32 PM
The problem with the current shuttle snip
Unfortunately, slow it down, and you dive into the atmosphere anyway.

True, but if that is where you want to go then there isn't a problem with it. A fall snip
I'm thinking that you would probably hit the atmosphere before you have any considerable slowdown. You probably could throw a little downward thrust in the mix. (is that the right way to say I want thrust to move me up?) That would keep the dive a little bit easier to handle, or at least keep you out of the thicker atmosphere a little longer to allow more time for slow down.

What about fuel usage? It sounds very inefficient.

gopher65
2005-Jul-01, 12:45 PM
They don't just dive the shuttle straight down AFAIK. I think that they skim the shuttle off the upper atmosphere to slow it down, and then the friction sort of 'pulls' the shuttle downwards. The shuttle couldn't survive a dive straight down at mock 20 (or whatever the exact speed is).



----------_______
-----___
---_
-_
-
-

The shuttle tragectory should look something like that.

EDIT: I'll get this little pick right eventually (looks at the editted 500 times text)

wackywizjr
2005-Jul-01, 12:47 PM
Fuel usage is the part that would be hardest to get past.

The large amount of fuel needed for the slow down would need additional fuel to get it into space as well as the fuel to get the craft into space... It is a loose, loose situation.

Another possible way of doing this though could be an extremely large canopy trailed behind the shuttle. something with gas filled ribs to make it spread out and maybe a mile in diameter. Even in the very thin upper atmosphere this could cause a lot of speed loss very quickly and then could be released when the craft gets to the required speed.

George
2005-Jul-01, 12:52 PM
This design was considered, but it was determined that the strength needed to keep the air ram in place would make it so heavy as to be impractical.

Then there is also the safety factor if the plow breaks loose then the craft would be hit by it as it buckled back.
Wouldn't some sort of drag chute overcome these obsticales?

wackywizjr
2005-Jul-01, 12:58 PM
See above post

NEOWatcher
2005-Jul-01, 01:15 PM
This design was considered, but it was determined that the strength needed snip
Wouldn't some sort of drag chute overcome these obsticales?
The force is distributed among the entire airframe. There would be some considerable force on the chute (much more than the landing chute) therefore, therefore more structure would be needed to distribute that force within the airframe.
Besides, as you slow down and encounter thicker atmosphere, I would think it would be necessary to contiually shrink the size of the canopy. Maybe lots of canopies that periodically drop off. But again, more points of failure.

madamwitty
2005-Jul-01, 01:17 PM
Fuel usage is the part that would be hardest to get past.

I agree - using aerodynamic drag is a very effecient way to slow down the vehicle without the mass penalty of fuel. That's also why aerobraking (http://en.wikipedia.org/wiki/Aerobrake)/aerocapture (http://en.wikipedia.org/wiki/Aerocapture) is of interest in many planetary spacecraft missions. The tradeoff is between carrying a lot of fuel, and carrying a (usually *much* less massive) heat shield.

George
2005-Jul-01, 09:46 PM
This design was considered, but it was determined that the strength needed snip
Wouldn't some sort of drag chute overcome these obsticales?
see above post
Oops. Sorry, I must have been typing when you posted.

I would be suprised something that large would be best. I would think a smaller one would provide improved trajectory, lighter in weight (considering the chords, too), stronger and longer life. Of course, you could incorporate both, or more. Differenet chutes to "fit the occasion" ( or altitude). :)

Wouldn't the chute also allow for a slightly less re-entry angle allowing for more deceleration time, due to the chutes pull, before the chute failed?


The force is distributed among the entire airframe. There would be some considerable force on the chute (much more than the landing chute) therefore, therefore more structure would be needed to distribute that force within the airframe.
Besides, as you slow down and encounter thicker atmosphere, I would think it would be necessary to contiually shrink the size of the canopy. Maybe lots of canopies that periodically drop off. But again, more points of failure.
It might not be too hard to modify the rear structure to accomodate this. A chute would produce pure tension into a fixed point (knuckle, likely) which requires less structure to support.

Jens
2005-Jul-02, 05:20 AM
What is needed is a way to make the craft slow itself down to near zero relative speed before it enters the atmosphere then there would be very little re-entry heat. A spacecraft entering the atmosphere at about 300 miles per hour would not generate near the compression wave of one traveling 18000 miles per hour.

How about just putting wheels on it, and hitting the brakes as you near the reentry point?

I saw them do it somewhere. Probably a Disney or Warner Brothers cartoon. :D

George
2005-Jul-02, 11:31 PM
How about just putting wheels on it, and hitting the brakes as you near the reentry point?
"So what kind of brakes would that be", he asked foolishly? :)

Sock Munkey
2005-Jul-03, 04:44 AM
Umm... Air brakes! :roll:
But seriously, now that I think about it, if the shuttle could just carry a little more fuel, it's main engies are strong eough to hold it up while it uses the thiner air to lose orbital velocity. A mach two unpowered glide needs no troublesome heat tiles.

George
2005-Jul-03, 09:51 PM
But seriously, now that I think about it, if the shuttle could just carry a little more fuel, it's main engies are strong eough to hold it up while it uses the thiner air to lose orbital velocity. A mach two unpowered glide needs no troublesome heat tiles.
I'd bet the engine alignment would have to be altered or gimbaled. Either would be a major headache for too little gain. ["Too much squeeze for the juice", as one friend of mine says.] Rephrased, the imbalance of forces look horrible for this manueuver.

However, to keep it up in the thinner air does make sense. I suspect the risk of loosing trajectory is the problem. However, a drag chute should greatly reduce the risk by stabilizing the re-entry angle. [I am just playing with this and do not pretend to be well versed in any space engr.]

Sock Munkey
2005-Jul-04, 05:51 AM
I was thinking of the shuttle being in a nose-up position to present the most drag at the highest alitude. The shuttle engines do gimbal enough to maintain balance on it's butt.