Flight Question inverted re-entry?

[Grover]

ive seen it a few times, and seen it mentioned, but what is the difference between a re-entry that is ground-up and a re-entry that is space-up? and what are the practical uses?

 

[Urwumpe]

ive seen it a few times, and seen it mentioned, but what is the difference between a re-entry that is ground-up and a re-entry that is space-up? and what are the practical uses?

It all depends just on which direction the lift vector of your spacecraft has.

Practically, capsules are generally designed to be head-down, so astronauts can easier see the horizon as reference.

 

[Wishbone]

If you're coming back to Earth from the Moon or other planets, your speed will be hyperbolic (in excess of 11 km/s). You have to shed off quite a lot of velocity in the atmosphere, but the "natural" time spent in the upper layers will be too short (you don't dare to go lower due to deadly deceleration loads and heating). To compensate for your speed which will force you out of the atmosphere past perigee, you have to apply negative lift from your aerodynamic surfaces. Which means negative angle of attack, but this also means you're exposing the tender upside of your craft to the flow. Rather than risk damage, you roll 180 deg and fly inverted, with the underside taking the brunt - as designed (if the craft is designed for such feats, that is). am re-reading CAIB report, it's sad as hell...

 

[OrbiterSpore]

did you meaned something like that?

 

[EnDSchultz]

Another way of describing it is that you turn your aircraft upside-down so that when you start reaching the atmosphere, you can use the aerodynamic lift (assuming it's a space-plane) to, instead of pull it away from the ground, help pull you -towards- the ground. This allows you to stay in the upper atmosphere and shed off speed basically indefinitely. If you had done a standard re-entry profile, you'd have just skimmed the atmosphere, shed a little bit of speed, then gone back into space.

You can use an inverted re-entry to allow you to aerobrake from very high velocities without either going back into space or going so low in the atmosphere you burn up. You can "hug" the upper atmosphere and maintain a constant altitude until you're slow enough that you can flip upright again and continue as normal.

 

[Grover]

ahh, so its a way to get your speed back to reasonable levels (as if you'd come all the way down from earth's SOI) so you can re-enter without burning up as you slip through the thick atmosphere below 50k

makes sense now, thanks guys, maybe ill try it one day

 

[Urwumpe]

ahh, so its a way to get your speed back to reasonable levels (as if you'd come all the way down from earth's SOI) so you can re-enter without burning up as you slip through the thick atmosphere below 50k

makes sense now, thanks guys, maybe ill try it one day

No, you will still burn up if you attempt this at too high speed. Flying with negative lift (as it is called, lift force vector pointing down to Earth) permits you to slow down at higher altitude, where the air density is lower and where you would skip out of the atmosphere without lift (since your centrifugal force will overcome gravity)

 

[Grover]

so it allows you to hold your altitude (you still have to go high enough to avoid burning) to change a parabolic orbit that passes through the upper atmosphere into a re-entry that is slow enough to carry out normally without burning up due to extreme high speeds?

 

[Urwumpe]

so it allows you to hold your altitude (you still have to go high enough to avoid burning) to change a parabolic orbit that passes through the upper atmosphere into a re-entry that is slow enough to carry out normally without burning up due to extreme high speeds?

Yes - while you are fast, you fly at negative lift for staying inside the atmosphere, after you slowed down to suborbital velocity (<7.8 km/s), you use positive and negative lift for keeping your altitude high enough.

 

[Grover]

so we wont be seeing any real life craft doing this for a while then... not till we get things on direct approach from mars

 

[Urwumpe]

so we wont be seeing any real life craft doing this for a while then... not till we get things on direct approach from mars

The lunar Apollo missions did that all the time, The soviet Zond spacecraft did even do an skip reentry, in which you leave the atmosphere of Earth at a suborbital trajectory again, and then do a second controlled reentry after some distance. Was needed for the soviets, because of the high latitudes of their land landing sites.

Also, the Space shuttle does sometimes reentry with negative lift - not completely inverted, but with bank angles beyond ±90°.

 

[Grover]

hmm. im wrong again it seems... this is becoming a regular occurrence...

 

[Urwumpe]

hmm. im wrong again it seems... this is becoming a regular occurrence...

Don't worry, the path of a scientist consists always of learning to be wrong in a much better way. You'll never cease making errors, but if your errors become better, you are on the right track.

 

[diogom]

...you use positive and negative lift for keeping your altitude high enough.

What altitude is high enough? 60k?

 

[Urwumpe]

What altitude is high enough? 60k?

The one at which you loose speed as fast as your "payload" can tolerate, but still don't burn up.

 

[Tex]

I've tried coming into Mars atmosphere in the DG4 with hyperbolic speed and I burned up every time. I kept trying higher and higher altitudes, but alas ended up so high that I skip out avoiding excessive heat. Maybe someone who knows it better can confirm, but it doesn't seem designed for that extreme maneuver.

For that reason I use the stock Delta Glider for practicing aerobraking for now.

 

[mjessick]

When you expect to fly through a dispersed atmosphere, one strategy is to reenter with the lift horizontal and at the flight path angle that will bring you to the desired landing site, possibly with some roll reversals. This gives you the maximum amount of margin because if the atmosphere density is less than expected, you can roll the lift more downward. If the density is heavier than expected, you can roll the lift upward to compensate. If you detect a failure, you can put the craft into a fast roll and reenter ballistically (very low lift... trajectory uncontrolled.)

 

[Brycesv1]

ballistic re-entries = fun
ballistic landings = not so much

 

[Urwumpe]

DG4 should be tough for Mars, AFAIR, the autopilot does not permit a full range of bank angles.

I would expect something like reentering with full negative lift at hyperbolic speeds and rotate heads up once the deceleration is good.

With the XRs it should be easier to do that.

 

[Salun]

So I finally decided to not cheat and try doing an entry myself inverted in an XR2.

I found that 63 to 65 K is the optimal PeA to aim for when heading in. The only way to do it is KillROT autopilot. Full up trim and Manual control of the center of gravity.

Once PeA is reached using the Surface MFD adjust pitch to maintain that 2km corridor until sub orbital velocities are reached. Then begins the easy part. Flip it over and engage the normal attitude control AP