Need tips for SSTO spaceplane return

[FancyMouse]

Eventually I made my first non-test SSTO spaceplane to deliver >9k LF to my LKO station. It's a ~170t mk3-sized plane with 10 rapiers - flies ok on ascent. The trip to the station doesn't bring me any big problems. Fuels are delivered successfully.

Now return - I got totally lost. I didn't have an idea how much fuel I needed, how low my periapsis should be for the initial deorbit burn, how should I fly on its way back. What I did is, my deorbit burn still kept periapsis above ground. Using 4 airbrakes on board and a high AoA upon entry, my plane slowed to subsonic at 20km, then spinned (no idea why this happened, btw) and stalled until at several hundreds altitude I regained control and used my backup chutes to splash down on the ocean with the craft mostly in one piece.

What's your tips for a spaceplane return? Even better how do I target KSC with the limited fuel budget upon entry?

[Starhawk]

While the exact technique necessarily varies from craft to craft, here's a framework I use for spaceplane reentry.  Set periapsis to 36-40 km directly over KSC.  Hold prograde +10 degrees or so above the horizon.  Pitching very sharply up or down will create a fair bit of resistance to the atmo once you are getting to an altitude of 40 km or so, and this can be used to adjust the trajectory as you begin to get closer to KSC.

There is a fair bit of trial and error involved, but a bit of constant pitch-up should be enough to avoid any dangerous levels of reentry heating.

Happy landings!

[Snark]

If you can get down to subsonic at 20 km, then what you have isn't a reentry problem (you're already through the hard part-- congratulations!), but a flight stability problem.

If it flew fine on the way up but tumbles on the way down, then I'd guess that what happened is that draining all the fuel out of it moved its CoM back, so that it's now behind the CoL and your craft has become unstable.

Would need to see a screenshot to be more specific, but some general advice:

Spaceplanes have a tendency to be tail-heavy when drained of fuel, since the heaviest components (the engines) are often placed back at the rear end.  When a lot of mass is fuel tankage that's way in front of the engines, they can be vulnerable to becoming unstable when that fuel mass is gone.

How to evaluate:  in the SPH, turn on the CoM and CoL markers, and look what happens to the CoM when the plane is full versus empty.  If what you see is that emptying the tanks makes the CoM fall back behind the CoL, that's likely the culprit.

As for how to solve it:  you'd need to redesign your plane in some fashion so that the CoM is farther forward even when drained of fuel.  In other words:  start with the big fuel tank, then build all your spaceplane components around it so that the CoM stays centered on the middle of the tank (or at least, isn't too far away from it).  That way, the CoM won't move much when you lose the fuel mass.

A quick and easy way to test a spaceplane design for this problem: Launch it empty, turn on the infinite fuel cheat so you can run the engines, then try flying it and see how it does. Then revert to SPH, make any needed design tweaks, and repeat as necessary.

In other words, it's a good idea to test your spaceplane design both when it's full and when it's empty. 

Edited January 7, 2016 by Snark

[luizopiloto]

Try lower orbit to 80~75Km before deorbit... Then reduce orbital speed to 2050m/s... maintain an AoA of 30°...

[Temstar]

@Snark is getting to the core of the issue. The issue is not reentry but flight stability. Cargo spaceplanes that are probe controlled with no cockpit is particularly prone to this.

The actual name for this problem in real life is "CGCP mismatch". CG stands for center of gravity which is the same as CoM in our SPH. CP is center of pressure which is approximately the same as CoL in SPH, it's the point where all the drag forces on different parts of the aircraft sum up to.

CG needs to be in front of CP for the aircraft to be dynamically stable. Problem is in a spaceplane with engines at the back and very little weight at the front (ie fuel tanks empty, no cockpit) the CG is very far back.

CP and CoL is complicated because they are constantly changing as the airflow around your aircraft changes. The CoL indicator in the SPH is calculated assuming airflow coming straight ahead, which is not a bad assuming for most airplanes. However as soon as you introduce some AoA the drag on the fuselage generate will change and there's no rule of thumb how it will change since it depends on the shape of the fuselage. Just like how rockets don't handle sudden large AoA change very well and tend to flip if you do that, if you have a very long, light weight fuselage in front of the CoM and your plane goes slightly sideways against the wind that fuselage suddenly introduce a huge amount of drag that's no longer aligned with the CoM and the plane will want to flip.

The solution is as @Snark says: move your CoM forward. Besides shifting as much heavy stuff to the front of the plane as possible one other solution is use the same strategy that Skylon's design used to counter CGCP mismatch - put the engines at the middle of the plane.

[MagicFireCaster]

Hey! I won't cover all of the explanations that were already done, but maybe i can give you a quick solution to your problem, since probably your problem is issues with your center of mass/lift, i would recommend you to try (if your ssto desing lets you do it) to move all or most your fuel resources near the nose of your plane, since all that weight in the back of your plane is making it inestable probably sending it to the front of your craft will help partially or totally with your stability issue after reentry.

[Snark]

21 minutes ago, MagicFireCaster said:

Hey! I won't cover all of the explanations that were already done, but maybe i can give you a quick solution to your problem, since probably your problem is issues with your center of mass/lift, i would recommend you to try (if your ssto desing lets you do it) to move all or most your fuel resources near the nose of your plane, since all that weight in the back of your plane is making it inestable probably sending it to the front of your craft will help partially or totally with your stability issue after reentry.

Actually, that's the exact opposite of what he needs to do, and the reason he's having a problem is that he's already doing that. 

The issue is that a spaceplane needs to be well-balanced (with a forward CoM) in order to stable in flight.  There are different ways you can accomplish this.  One way is what you suggest, which is "move fuel resources up front".  Yes, that will balance the plane... when there's a full load of fuel.  But if you're relying on the weight of that fuel for your balance, then as soon as the fuel is gone, your stability goes to heck.  That's the problem that the OP is having.

So the way to deal with this is:

• Put your fuel resources in the middle of the plane.  Ideally, you want your "center of wet mass" (i.e. the CoM of all your fuel) to be in the same place as your "center of dry mass" (i.e. the CoM of your plane when it has empty tanks).  That way, the CoM of your overall plane doesn't move when you drain the fuel.
• Then, to achieve balance, move heavy things forwards.  If there's a heavy cockpit, put it as far forwards as possible.  If you have heavy engines on the back of the plane, move them up (e.g. towards the middle of the plane).

 

[MagicFireCaster]

As i get it, he is stuck on orbit trying to land his plane, so if he has fuel left that is the only advice i can give to him, since you and the others already told him the core problem is the desing of his ssto.

[FancyMouse]

I appreciate the explanation, but I should've said I'm aware of some CoM stuff and I doubt it's CoM problem in this particular situation. I've already used RCS build aid to check my empty CoM and it still looks good to me (in case you don't know - red marker is the empty CoM marker). Here's pic:

If that's CoM problem, that doesn't explain why I can regain control and fly reasonably good at low altitude. I only had about 400 units of LF in the hovered tank entering atmosphere, so CoM should be a tiny little bit forward than the empty marker. I highly doubt this is related to my decent profile that I shouldn't have slowed down that much at high altitude - and that's actually my real question.

[Temstar]

When you say spin, do you mean uncontrolled yaw, uncontrolled pitch or a mix of both?

[FancyMouse]

Uncontrolled roll. Plane points retrograde. I had AoA about 60 degree or maybe even higher before that but wasn't able to restore to prograde and then it flips to retrograde near transonic speed.

After I get to low altitude and around 100m/s I did regain control and fly like usual - or at least I can fly horizontally and splash down safely.

Edited January 7, 2016 by FancyMouse

[Temstar]

Just now, FancyMouse said:

I had AoA about 60 degree or maybe even higher before that but wasn't able to restore to prograde and then it flips to retrograde near transonic speed.

Huh? That sounds like uncontrolled pitch or yaw to me. I have seen planes go into uncontrolled roll but I've only seen this happen when SAS is engaged and the SAS was fighting against uncontrolled yaw and/or pitch. So instead of tumbling on two axis, the SAS try to straighten the plane and it ends up tumbling in all three axis. Roll wasn't the cause of loss of control in the first place. You can test this by doing your reentry again, then as soon as you feel you're starting to lose control turn off SAS and don't touch any controls and see in which direction the plane flips.

 

[FancyMouse]

OK now I really find the problem - I locked the fuel inside those big fuselage to ease my life of fuel transfer. However this actually confused RCS build aid - it recognizes locked tanks and included the locked fuel as empty mass. Really my design shouldn't have any reason to push the empty CoM so forward when I slowly look at the picture. After I unlock all the tanks, the empty CoM does fall behind CoL. Now it explains everything.