Flight Question Re-entry issues. Damaging the DG during banks

[shadowfax]

I'm following the 3rd checklist in the main manual (landing the Delta Glider). I'm keeping speeds where the list tells me to. Everything goes fine as I descend out of 100k for 50k, but then I enable the aero surfaces and shut down RCS as instructed. I start the 90 degree banks, and the ailerons get torn off.

So I tried not following instructions and just keeping the ship straight (and even kept the aero surfaces off in case flutter is modeled - is it?) and all I do is shoot back upward as I bounce off the atmosphere. I can tell that if I kept this up I'd eventually slow down enough to descend, but the speed would still be high at first, and I'd eventually need aero controls if I want to land on an actual runway.

So! What am I doing wrong? I'm using very minimal control inputs to roll and pitch (Saitek X45, not keyboard) but that doesn't seem to help in keeping the ailerons attached. The other two checklists worked perfectly for me, so I figure it has to be something I'm missing, or some piloting skill I don't yet have.

If it matters, my copy of Orbiter is stock except for the sound pack.

Thanks

 

[Mojave]

The easiest thing to do is, turn off the damage/failure simulations. Else, you might be able to keep it on, if you reenter at a more shallow reentry angle.

 

[shadowfax]

Are the damage simulations broken? If not I'd rather learn to do it right. Else I might as well just nosedive straight at the ground and call it good

 

[orbekler]

You know, in past I had the same identical problem. Ailerons broke, or I was bouncing off for thousands km away from target.

IMHO, checklist 3 starts too near to KSC to get a proper landing. Deorbiting should be done about 1/2 orbit before target base.

Another difficulty I had was that I couldn't find a way to vary vessel Center of Gravity during descent and force the proper AoA, and that's mandatory to balance drag, lift and temperature.

So I left stock Deltaglider for XR vessels (and DGIV), being XRs my preferred choice.
With these last I learned reentry, even if they're more complicated and more damage prone.

 

[Tommy]

As mentioned above, the stock DG isn't really suited to the high Angle of Attack style of high drag re-entry. You need to fly it in more like an airplane. Most of your troubles are from going too low before you reduce your airspeed enough.

There are a couple of tricks to a low Angle of Attack re-entry, and one is de-orbiting far enough away from the target. I often de-orbit 3/4 of an orbit before my target - so my periapsis is 1/4 orbit before the target base. This leaves plenty of room for aerobraking. I de-orbit by lowering my PeA to about 60k altitude.

It also helps to have AerobrakeMFD, look for it at Orbit Hanger. This will help you end up on target. I think you need to hit the [PRJ] and [MOD] buttons after you select a target, and you'll get a view that shows a map, and a crosshair.

The main trick to a good Re-entry is Energy Management. You have a LOT of energy to start with. You have Potential Energy (from your altitude) and Kinetic Energy (from your velocity). You want to reduce those energy levels so that you get down to about zero when you reach the target.

We can reduce the Potential energy by slowing down using aerodynamic drag. As we lower the velocity, we can begin to slowly reduce the Potential energy by converting to to Kinetic energy. In other words, as we lose speed, we can begin to lose altitude.

What I do is:

1; De-orbit as outlined above - keep PeA at 60k to 65k altitude.

2; I'll engage the airbrakes, and as I near 75k altitude I'll watch my VS and VACC - I want to level off at around 63k altitude. I use elevator trim to manage my decent rate.

3; I'll also use bank to affect my decent rate (higher bank angles = faster decent rate) if I need to adjust my course to reach my target (cross range).

4; Using elevator trim and bank, I start to SLOWLY lose altitude to about 55k altitude, but want to have less than 6km/s airspeed when I get that low.. I'll be watching AerobrakeMFD - and at first my projected path will extend far beyond my target. As I go on, the projected path will shorten. Eventually it will be too short - and won't reach the base. At this time, I'll set the trim and bank so that the path ends at the target - and just leave them there until I'm close to the base.

5; By the time I'm close to base (100k or so) I should be down to about 20k altitude, and a bit under 1km/s airspeed. From there it's just a matter of aligning with the runway and landing.

Re-Entry (in any glider) is difficult, and takes some practice. If you are damaging the DG, you need to lose more speed before you decend down to under 63k altitude.

 

[shadowfax]

Ahh ok. That's my whole problem, then. The checklist didn't mention any Vmax info, so I figured when it said you'd notice air effects at 100, and the next line said to start banking at 50, it wanted me to go ahead and descend from 100 to 50. So I was ending up at 50 with over 7km/s.

I'll practice what you said as soon as I get home. Thanks!

 

[Tommy]

Keep the decent rate at 20m/s or less from 65k and lower. Once you get to the second half of step 4 and have set trim and bank for target, you can usually ignore the VS. You may "rollercoaster" a bit - but just keep the projected path on target and you should be fine.

 

[orbekler]

As mentioned above, the stock DG isn't really suited to the high Angle of Attack style of high drag re-entry. You need to fly it in more like an airplane....

That's why is not suitable for realistic reentry too, nor to learn it, though it's wonderfully useful to learn orbiting and docking.
Actually you can easily do a descent, but à la Sanger, bouncing several times before dropping.

 

[Tommy]

I'm not even sure it's less realistic, given that the DG isn't exactly realistic to begin with. A low AoA re-entry generates less g-forces and airframe stress, but it creates high temperatures for a longer period of time.

A high AoA re-entry is used in reality because ceramic tiles, like any other insulator, are subject to "heat soak". Eventually the heat gets through. The High AoA and high drag method is used because it loses velocity faster - meaning you are down to reasonable airspeed before the heat soaks through your tiles.

It's not unreasonable to think that by the time we can come up with engines like the DG has (or any of it's variants), we can come up with a better insulator as well. The low AoA method is less stressful - keep in mind that the highest G-forces astronauts face is during re-entry, not ascent. Low AoA re-entries rarely exceed two G's.

That said, bringing an XR-2 in direct from Mars, and landing with hull temps still in the yellow, is much more fun!

 

[shadowfax]

Your advice worked. I was able to get to the surface without dying. Unfortunately I ended up off the coast of Scotland instead of the Cape, but that's just a matter of learning when to do the de-orbit burn. For my sessions last night I was more worried about verifying that I could get down through the atmosphere alive than about hitting a specific target.

 

[Tommy]

Nice work. Now it's just a matter of practice. As your technique gets more consistent from flight to flight, you'll be able to improve the accuracy.

 

[SandroSalgueiro]

I could never give tips better than Tommy's.

Overall, I hate the DGIV aerodynamics. Have you tried the XR2?

 

[orbekler]

Nice work. Now it's just a matter of practice. As your technique gets more consistent from flight to flight, you'll be able to improve the accuracy.

6500 km - Call it accuracy!
It's more like to be stranded in a emergency flight.

When i was reading "TUTORIAL: CONCEPTS FOR ATMOSPHERIC ENTRY" by Sanchez, I was wondering why I couldn't do as explained with stock DG, but it was easy to figure out: it was impossible (as already you said about AoA).

Switching to DG XR-1 will help more than eyeballing target, though at the beginning it's a bit frustrating.