My disasterous mission to Mir

[pandadude]

It all started out well enough. I docked with Mir after lift off from the KSC in my DG-IV. I spent about 3 days orbiting the Earth whilst attached to Mir. Then I decided it would be time for re-entry on my next pass over the Cape.

Since my last re-entry with AeroBrake which was very accurate, I had become a bit of a fan of that particular MFD, so I used it again.

I used BaseSync MFD to bring my orbit over the Cape, then 180deg away I started the de-orbit burn. My re-entry angle was just over 0.6deg. Soon thereafter, I started re-entering the atmosphere with an AoA of 40deg.

About six minutes to touchdown I noticed a problem. I was on course for the KSC according to AeroBrake, but looking at how quickly I was approaching (using Map MFD), it was pretty clear that I was going to overshoot the landing site. Then another problem. I wasn't losing enough speed. At about 65km altitude, I was still going at least 6km/s. Fearing the worst, I checked the temp of the DG-IV. The nose had just gone over 2000deg in temp and was still rising. Uh oh.

Now this is where I think I did the right thing, but wasn't too sure. I immediately raised my AoA up to about 60deg, hoping it would take some stress off the nose and slow me down more quickly. Obviously I would land off target, but I had plenty of fuel so as long as I survived, I could fly to the SLF. The temp dropped below 2000deg, but then started to rise again. It was more or less at this point I realised the crew were .

All this took place in about 30 seconds, so I just did what came to mind first, but maybe I did something wrong. Well, not maybe, I definitely did something wrong, I'm just not sure what.

Perhaps most annoyingly, the wreckage of the DG-IV landed almost perfectly in line with Runway 33 at the KSC (albeit a few kms to the East). So at least one part of my planning worked.

 

[davidsinn]

Hypersonic flight is very counterintuitive. To climb you need to drop your nose. To descend and increase drag you need to raise it. It's the slowing down quickly that killed you. That kinetic energy needs to go somewhere. You turned it into large amounts of heat. Next time drop your nose and skip up a bit and then hold a steady descent. Overshoot and come back. Or just skip up and out and come around again 90 minutes later.

 

[Quick_Nick]

Be sure to bank a lot while reentering. It can be very helpful!

 

[Linguofreak]

Hypersonic flight is very counterintuitive. To climb you need to drop your nose. To descend and increase drag you need to raise it. It's the slowing down quickly that killed you. That kinetic energy needs to go somewhere. You turned it into large amounts of heat. Next time drop your nose and skip up a bit and then hold a steady descent. Overshoot and come back. Or just skip up and out and come around again 90 minutes later.

Well, it's not exactly hypersonic flight that does that. It's flight with angles of attack past the stall angle of your wings, whatever speed you're flying at. Since you're trying to slow down during re-entry, and since it's convenient to put your heat shielding on the underside of your wings, and since most hypersonic flight at this point involves spacecraft re-entering, high angles of attack and the consequent counterintuitivness are common in hypersonic flight, but they aren't a necessary part of it.

 

[pandadude]

So I should have lowered my AoA...I see. I guess my logic was that raising it would cause the nose to take less of the impact, and the underbelly of the DG-IV to take more. Didn't really work though.

 

[pandadude]

I just gave it another shot and took your advice about reducing AoA to produce lift. It worked really well. I felt it seemed a little almost unreal gaining height whilst re-entering the atmosphere, but as long as it works. I guess it must be pretty hard to plan a re-entry where you can just set it up and know where you're going to land and then not have to make any changes yourself. The first time I did it it must have just been lucky.
Makes me realise how difficult it must be to get it all right in real life. It was easy for me to just reload a quicksave and use your advice second time round, but I don't think even NASA has figured out a way to do that yet.

 

[agentgonzo]

but I had plenty of fuel so as long as I survived

This could be part of your problem.
If the vessel is too heavy (too much fuel), then it won't slow down as quickly as you enter the atmosphere, which will lead to higher speeds and more heat on the heat shield. Dump the majority of your fuel after your reentry burn and before reentry interface.

 

[pandadude]

This could be part of your problem.
If the vessel is too heavy (too much fuel), then it won't slow down as quickly as you enter the atmosphere, which will lead to higher speeds and more heat on the heat shield. Dump the majority of your fuel after your reentry burn and before reentry interface.

To what extent does that make a difference? I know above 19t the DG-IV cannot re-enter or land, but I had about 17t weight altogether the first time I tried and failed here.

All I know is I've managed re-entry with the same weight before, but it had never ocurred to me that more weight might be a problem.

TBH, I think it was just me trying to maintain too high an AoA, but too much fuel probably was a factor too.

 

[Scarecrow]

I just gave it another shot and took your advice about reducing AoA to produce lift. It worked really well. I felt it seemed a little almost unreal gaining height whilst re-entering the atmosphere, but as long as it works. I guess it must be pretty hard to plan a re-entry where you can just set it up and know where you're going to land and then not have to make any changes yourself. The first time I did it it must have just been lucky.

Well I don't suppose I can speak for everyone, but in my experience, reentry is a very dynamic phase of flight, requiring almost constant adjustment of AoA, and bank too in the earlier stages.

 

[ar81]

S-turns
[ame="http://www.orbithangar.com/searchid.php?ID=3424"]S-Turns animations[/ame]

 

[pandadude]

Well I don't suppose I can speak for everyone, but in my experience, reentry is a very dynamic phase of flight, requiring almost constant adjustment of AoA, and bank too in the earlier stages.

Ok maybe I've just taken the movies too seriously where they always have the space craft hurtling through the atmosphere and they make it look like they just keep going downwards the whole time.

 

[Dr Pepper]

To what extent does that make a difference? I know above 19t the DG-IV cannot re-enter or land, but I had about 17t weight altogether the first time I tried and failed here.

The less weight makes a huge difference. For example, I once tried to land the XR5 at the cape with full fuel (started in orbit) tanks. I ended up bombarding Southwest Asia with flaming chunks of spaceship. I tried it again with only 10% in the tanks and I re-entered without incident.

 

[pete.dakota]

It's not at all rare for a re-entering ship/capsule to use AoA and its lifting body design to actually hold altitude during re-entry. It's actually the safest way. Earth's atmospheric density doubles with about every 5/6km you drop. If a body isn't creating lift for itself (ballistic), it will experience a rapid thickening of the atmosphere, without losing enough of its energy to beat the curve, as it were.

In the DGIV, I make the ship as light as possible before re-entry. I've usually used all the main fuel for orbit insertion (dumping about 50% before launch), and then use the RCS for orbit adjustment and de-orbit. The DGIV has a very generously sized RCS fuel tank; you could probably dump down to about 100 units before re-entry. Imagine a sheet of paper compared to a sheet of lead. The lighter you are, the quicker you slow down; so you're going slower at any given time as the atmosphere thickens - compared to if you were heavier.

A light DGIV or Shuttle - with a high AoA - will create a lot of lift in the upper atmosphere. And will probably want to skip back out if the wings are not banked. It's easy to think that the S-turns are there to change direction; but that's really only their secondry purpose. The idea is that banking rotates the angle of lift being produced to a more parralel position relative to the Earth's surface - so the ship can stay at it's current altitude and continue to lose energy in a controlled manner - and also control the rate at which it falls through into the thicker atmosphere.

Use the DGIV's re-entry assit AP to hold your AoA and bank angle. And set yourself up a user-friendly quicksave you can jump straight into (for instance, fuel dump, alignment, and de-orbit already done).

 

[agentgonzo]

I've never really been able to use S-turns effectively in the DGIV or change direction during reentry. I always seem to get down to about 2km/s before I can change direction. What AOA do you use in the DG to change direction early on in re-entry?

 

[pete.dakota]

Anywhere between 30 and 40 early on. Don't expect the kind of turning rates you'd get whilst flying around subsonic - it will be very gradual. Most of the directional work is done with the de-orbit burn. As long as your orbit has you passing roughly over your landing site, you shouldn't really need S-turns to change direction; only to maintain altitude and bleed off speed. Of course, if you do one long bank to the left, you have to counter it by doing a similar bank to the right after. This is where AerobrakeMFD is priceless. You MUST use it.

 

[n122vu]

I noticed your reentry angle was about 0.6 deg. Is this what everyone commonly uses when flying the DGIV? I've always used the suggested 1.1-1.2 deg angle and been pretty successful at not burning up...