[OzAnt]

Hi folks,

I've never really understood this, which to me suggests I'm missing a piece of critical information. My dilemma is this:

Why do spacecraft need to re-enter the atmosphere at speed that causes so much (underbelly) friction?

I've seen shows and read books that talk about having to get the angle of re-entry correct or the spacecraft would 'skip' off the atmosphere. I get the logic of this statement. So, according to the above, if the angle was wrong, spacecraft would 'skip' (I'm picturing a pebble, if thrown right, skipping across the water here) on top of the earth's atmosphere.

Well, with the pebble, once its velocity slows down (a few 'skips' later) it eventually breaks through and sinks into the water - even though it's approach angle was incorrect to enter the water on the first encounter with the water. Back to the spacecraft... If it skipped along the atmosphere, it would most likely self destruct - if not, it would at least probably kill its occupants from the severe jarring. Nevertheless, would it not eventually stop 'skipping' and also enter the atmosphere? And if it did, wouldn't it now do so with a lot less friction and therefore heat generation?

Where I'm going with this is that if the spacecraft decelerated without using the friction through skipping, couldn't it enter the earth's atmosphere with a lot less fanfare? From a practical viewpoint, this would mean less technology in the special heat tiles on the bottom of a space shuttle for instance.

If it didn't mean that, then would it be possible to put up satellites that 'rested' on the earth's atmosphere without ever penetrating it?

You can see I'm confused. I just don't know what I'm missing

[Tormod]

Usually (or rather, in just about all cases I know of), the re-entry is performed with no fuel burning, meaning that the spacecraft has no countermeasures to help it slow down. Therefore the angle of attack is of extreme importance.

In the case of the space shuttle, this means a series of maneuvers where they position the craft so that the bottom side hits the atmosphere first and it remains in that position for a while to absorb the heat, before it can start the glide down to he airstrip (the shuttle lands like a glider planer, meaning there are no engines involved in the landing).

Here is a nice page about re-entry:
Aerospaceweb.org | Ask Us - Atmosphere & Spacecraft Re-entry

BTW moving this thread to the space forum.

[chendoh]

Right on T
There can be more than one S-Turn.....A figure 8....A J turn.
Even a little more altitude to repete the sequence.

Skipping the atmosphere seems more like an Aro-braking mannuver.

Until science devises a way to bring a ship home Quietly,(Propulsion)

We are stuck with what we have.

[Janus]

Originally Posted by OzAnt

Hi folks,



Where I'm going with this is that if the spacecraft decelerated without using the friction through skipping, couldn't it enter the earth's atmosphere with a lot less fanfare? From a practical viewpoint, this would mean less technology in the special heat tiles on the bottom of a space shuttle for instance.

(

The problem is that it would take quite a bit of fuel to do this, meaning the fuel would have to lifted to orbit in the first place (which takes even more fuel). This would either increase the amount of fuel needed vastly, or force one to reduce the amount of useful payload drastically.

For example, if it takes 12 kg of fuel per kg payload to get the payload to orbit and using air braking through friction for the return, it would take more than 80 kg of fuel per kg payload to use the engines to slow down for return, over six times more fuel.

[OzAnt]

Originally Posted by Tormod

Usually (or rather, in just about all cases I know of), the re-entry is performed with no fuel burning, meaning that the spacecraft has no countermeasures to help it slow down. Therefore the angle of attack is of extreme importance.

In the case of the space shuttle, this means a series of maneuvers where they position the craft so that the bottom side hits the atmosphere first and it remains in that position for a while to absorb the heat, before it can start the glide down to he airstrip (the shuttle lands like a glider planer, meaning there are no engines involved in the landing).

Here is a nice page about re-entry:
Aerospaceweb.org | Ask Us - Atmosphere & Spacecraft Re-entry

BTW moving this thread to the space forum.

Thanks Tormod. That's a cool link and a fascinating read. Also, sorry about creating this thread in the wrong forum.

It clarifies the problem with my skip principle earth re-entry idea by saying:

If the entry angle is too shallow, the vehicle will generate too little drag and not slow down enough to follow a trajectory down to the surface. The craft will instead skip out of the atmosphere and back into space with insufficient fuel to make another controlled re-entry attempt.

Now, this is acceptable to me - except that it also states:

A more unusual re-entry option that merges features of both ballistic and gliding profiles is the skip entry trajectory. In this case, the vehicle first grazes the outer fringes of the atmosphere and generates drag that slows the craft down by a small amount. However, the vehicle also generates a lift-to-drag ratio between 1 and 4 and uses this lift to pitch up and leave the atmosphere again. This process is repeated several times as the craft skips along the upper reaches of the atmosphere much like a stone skipping across the surface of a lake. This skipping gradually slows the vehicle until it can safely re-enter the atmosphere and fly a more ballistic path to touchdown.

The attraction of the skip trajectory is that a vehicle can travel much farther downrange than either the ballistic or glide options allow. The primary disadvantage, however, is significantly higher aerodynamic heating since the friction heat absorbed during the skips grows at a higher rate and requires heavier shielding to protect the vehicle. As a result, skip entry has never been used for a manned spacecraft. 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.

You'll note that according to the skip principle, the craft 'leaves the atmosphere' during skips. I would have thought that once the craft leaves the atmosphere, heat is no longer being generated, but in fact is being dissipated. So, I'm wondering why the skips couldn't be co-ordinated such that more heat dissipation occurs than friction heat generation.

The only answer I can come up with is that the article is probably inaccurate when it says that the craft leaves the atmosphere during skips. If it truly left the atmosphere, it would become a brick in terms of aerodynamics and have to rely on its space thrusters to bring it back for another friction deceleration maneuver . I'm guessing this process would have to be repeated a lot of times and would require more than the two weeks the shuttle can accommodate its crew for or more fuel than it can store for the space thrusters to continue working.

I guess I'm caught up in a better way to bring a space shuttle back to earth than having to use a method that heats the underbelly of the craft so much. The technology in the ceramic tiles and the glue to keep them attached to the shuttle as well as the carbon-carbon they use (eg: wing edges) where tiles are impractical is apparently staggering.

[max4236]

If your first descent angle is small enough and your spacecraft has wings, you can skip off the upper atmosphere like a pebble a couple of times before you'll sink into the lower atmosphere. The trick is to do it very gradually. Do a quick retroburn with the main thrusters till the descent angle is between -2 and -4 degrees, then flip around, level the ship and wait patiently for about half an orbit. When you're getting close to hitting the atmosphere, around 100km or so, keep the nose of your spacecraft lined up with your velocity with respect to the planet and use the thin upper atmosphere as the lifting medium. At high speeds around 7.5km/sec it acts like it's the thicker lower atmosphere. Just kind of lightly brush the upper atmosphere and do a bunch of mini nose dives. Don't do any sharp turns. Pull up gently when the oncoming air starts to turn orange, just until the velocity is a +2 to +4 degrees ascension, or else the next skip you'll come in too steep. The trajectory makes a giant parabolic arch for each skip. Whatever angle you leave the at is the angle you'll dive back down at the next skip. Treat the lower atmosphere like it's solid ground below 65km on the first skip. Each skip you'll loose a little bit of speed. The altitude limit gets lower on each skip. Takes 4 or 5 skips before you can dive into the lower atmosphere at about 2 to 3 km/sec. Then hit the air brakes and glide on down to your landing site. You have to allow for a really long descent path, sometimes all the way around the planet. You can use just the airfoil control but you'll get a better response if you supplement that with the RCS thrusters because you'll start drifting when the atmosphere thins out again. It might glow a little organge on the nose but it won't get white hot. Takes about 2 to 3 hours. At least on my simulator it works..

[chendoh]

Originally Posted by Janus

"Men are apt to mistake the strength of their feelings for the strength of their argument.
The heated mind resents the chill touch & relentless scruntiny of logic"-W.E. Gladstone.

Very nice Quote Janus, may I use it in the 'feel of blood' thread?

[TheBigDog]

If the craft is going faster than escape velocity when it exits the skip then you need thrusters to recover. If it is slower than escape velocity then it will by virtue of earth's gravity skip again and again, each time losing energy and eventually reentering. There would seem to be a randomness to this that precludes using it as a predictable reentry technique.

At the highest regions of the atmosphere it is very cold, and the atmosphere is very thin, so by slowing at the top of the atmosphere you have the best opportunity to for gradual and relatively cool slowdown. As you get into the thicker atmosphere the friction generates so much heat that nothing can easily survive. So there is a slim angle between random skipping and a death dive that is targeted for reentry.

Bill

[OzAnt]

I'm guessing NASA have spent some time on this idea too, given how expensive it is to mess around with ceramic tiles, industrial space-strength glue and carbon-carbon for where the tiles just won't do (wing edges etc).

max4263, I'm guessing that your simulator - impressive as it is - is concerned with flight trajectories in relation to speed and gravity. Whilst it sounds like it can even extrapolate heat from friction deceleration on the fringes of the atmosphere, I'm guessing it can't monitor heat transfer into the shuttle.

I believe, after much reading since asking about this, that the problem is heat dissipation taking too long (the parameters being the rather narrow range of temperature humans can stay alive in) in this whole equation.

In my mind, I've likened friction deceleration to a conventional disc brake. It generates heat very quickly whilst doing its job. I've seen telemetry from a camera pointed at a race car's disc brake, it's pretty spectacular how quickly the whole disc can glow orange under heavy braking. That heat is now transferring itself into the shuttle's cabin. During the 'skip' cycle where the shuttle is still travelling within escape velocity it probably can't dissipate enough heat in time to stop the cabin temperature increasing.

After several skips, the temperature has climbed in the cabin to unworkable levels. Bear in mind the whole point of this exercise is to move away from the need for expensive ceramic tiles/glue and even more expensive carbon-carbon.

I know those tiles are amazing on the shuttle. I saw on a Discovery show once a guy point a blowtorch at the tile until it glowed orange. Within seconds of removing the blowtorch, he could touch the tile with his bare hand!

So, to allow for the cold of space to extract that heat, we're back to orchestrating the 'skip' cycles to take longer. The simulator would probably show this as pretty tricky to achieve without thrusters employed - as we're looking for parabolic skips that are large enough to stop heat generation completely so that heat dissipation can in fact occur.

The best analogy I can give is that over truck driver braking procedures going downhill. I know a truck driver is taught not to ride the brakes. It's better for a truckie travelling downhill to:
a) brake more heavily to slow down more than needed
b) remove his/her foot from the brake
c) coast (in a low enough gear that prevents too rapid acceleration)
a) repeat step (a)

than to:
a) ride brake (in the same low gear) so truck doesn't get away from him/her

Another point is that disc brakes are cooled by airflow. With the shuttle, air flow would necessarily mean heat generation from friction. This means the shuttle would rely on radiant heat dissipation only and dissipation would work most effectively when there was no friction braking occurring to return heat back into the equation.

Soooooooo... I'm more clearly now understanding the complexities and constraints of trying to achieve a 'gentle' re-entry that could allow shuttles to be produced more cheaply (ie: tile-less).

Mind you, I've appreciated all of your thoughts on the matter - it's certainly been helpful and thought provoking.

[Tormod]

Originally Posted by OzAnt

I'm guessing NASA have spent some time on this idea too, given how expensive it is to mess around with ceramic tiles, industrial space-strength glue and carbon-carbon for where the tiles just won't do (wing edges etc).

The first clue is that they are retiring the shuttle and going back to the capsule re-entry model from the Apollo era. Much cheaper to use a simple capsule - the Russians have shown over the years that it is both safe and cheap.

In fact, I think the airplane design of the space shuttle is it's main problem. It makes it extremely expensive to maintain.