SSTO Re-Entry?

[March Unto Torment]

So, I've got some small problems with SSTO re-entry.

With the help of some lovely users on this forum, the X-50 Valkyrie - a beautiful 390 tonne SSTO - managed to get into orbit, after some minor design tweaks.

However, getting is back is another story entirely. Every time I'm re-entering, when I reach about 50,000m, I enter a violent tailspin.

Unusually, I have actually already succeeded in landing from orbit - I managed to arrest the tailspin less than a kilometre above the surface, which is dramatised in this story story.

However, I'd rather manage to return from orbit without the intense and dramatic tailspin.

So if anyone has any suggestions, I'd really appreciate the help!

[Tex_NL]

Check your remaining fuel. Pump it as far forward as possible to make sure your CoG is still in front of your CoL.

Put some airbrakes near the tail of your plane to both slow down and keep the CoD behind your CoG.

[The_Rocketeer]

The key issue is the effect of drag. Essentially, this is a force that pushes against the nose of the plane. It also pushes against wings and tail fins. If the force is pushing too hard against the nose compared with the tail fins (or possibly delta wing), the craft will tumble off course. High tail drag, e.g. using AIRBRAKES, is the most straightforward way to combat this.

A second factor is the position of the CoM. A CoM nearer the nose will increase the amount of nose-drag needed to pull the craft off course. If you have left-over fuel, it's a good idea to move it to forward tanks for re-entry.

A third issue is that, although a high speed craft can create huge amounts of drag and heat in the high atmosphere, the atmospheric control surfaces won't perform well at hypersonic speeds, so it's hard to keep the plane nose-forward using them. Reaction wheels also don't produce nearly as much mechanical torque as atmospheric drag can. This is also the reason why you're recovering control in the low atmosphere, because your control surfaces become more effective and generate more torque relative to the (now lower) drag force.

So, in short:

Design with a steamlined nose that won't generate so much drag, e.g. nosecones or intakes up front.

Add AIRBRAKES to the tail (and use them)

Use more tail fins, or go for a delta-wing arrangement.

Shift remaining fuel forward until you're slow enough to use your elevons.

[March Unto Torment]

Quick picture of the design I'm using -

As you can see, it's a very much delta-wing, streamlined design. But I'll try adding some airbrakes towards the rear, to focus my centre of drag, and pump the fuel forward (although that may necessitate structural modifications, since most of my fuel tanks are towards the rear).

[The_Rocketeer]

I can pretty much guarantee the fuel mass is the major part of your problem here.

Whereas jets/airbreathers drain all tanks evenly (so the CoM hardly shifts/moves very slowly), rockets drain linearly from the top of a stack towards the engine. This means the CoM moves further and further back until you're draining the last few tanks, when it (usually) starts moving forward again.

You might be able to work some magic with non-crossfeed parts and fuel hoses, so your tanks drain in different directions or all drain towards the CoM.

Edit: your delta wing could be the only reason you managed to regain control - if you had much less control surface (which you wouldn't have without a delta), chances are you'd never have recovered your spin, especially with a wayward CoM.

Edited September 13, 2015 by The_Rocketeer

[tseitsei]

Basically you want lots of weight on the front part of the plane (All remaining fuel should be pumped as far forward as possible. And if that's not enough you might need to get back to drawing board and actually move more tanks/other weight forward). You also want more drag at the rear part of the plane (Airbrakes as far back as possible).

That way your plane will act like a dart (heavy tip on the front and lots of drag on the back) and should fly beautifully nose first and not spin out of control...

[March Unto Torment]

Ah, thank you for the advice. Working on the modifications now.

One question, though - what do I do once I'm in low atmosphere, and want to resume normal flight again?

[tseitsei]

Ah, thank you for the advice. Working on the modifications now.

One question, though - what do I do once I'm in low atmosphere, and want to resume normal flight again?

What do you mean?

Once your speed is slow enough you can just start flying the plane normally.

You can glide down and land if you want or you can start your jet engines again and fly somewhere else if you have fuel left...

Good way to bleed off any extra speed is using airbrakes and if you dont have those just pitching up and down works since you fly with big AoA and that causes a lot of drag. Just don't use too big AoAs when hypersonic. Aerodynamic forces will rip your ship apart...

[March Unto Torment]

What do you mean?

Once your speed is slow enough you can just start flying the plane normally.

You can glide down and land if you want or you can start your jet engines again and fly somewhere else if you have fuel left...

Ah, sorry for the confusion. I was wondering how flying it would work if I put my CoM really far forward; I was having some issues early-on with the Valkyrie with my CoM being way forward of my CoL (moving the fuel tanks back fixed that one right up).

[The_Rocketeer]

You don't want your CoM really far forward, but considering how your fuel tanks are arranged and the overall mass of the craft, I doubt it'll get that far forward. The crucial thing is for it to be somewhat forward of the CoL. The further, the more dart-like, but at low altitude (as u rightly notice) u don't want to have the CoL too far behind the CoM or you have to constantly pitch up to avoid nosediving.

As long as you still have power that shouldn't be too big a problem, but if it's running away from your ability to control, you've probably shifted more fuel than you needed to, or picked tanks that are further forward than they really need to be.

Just don't use too big AoAs when hypersonic. Aerodynamic forces will rip your ship apart...

Usually true, but unlikely in this case considering the tumble described in the link at the top.

Edited September 13, 2015 by The_Rocketeer

[Tex_NL]

Ah, thank you for the advice. Working on the modifications now.

One question, though - what do I do once I'm in low atmosphere, and want to resume normal flight again?

Get TAC Fuel Balancer. It's one of the easiest ways to quickly transfer fuel from one tank to another without causing unevenly balanced tanks.

[ForScience6686]

Get TAC Fuel Balancer. It's one of the easiest ways to quickly transfer fuel from one tank to another without causing unevenly balanced tanks.

I disagree with this. It is easily doable in stock, you just need to select proper tanks. My advice is to make sure in the sph that your col is properly placed when you have the fuel remaining that you are returning with. Generally I am returning with empty tanks so there is not guessing at where my fuel level will be. Then I'll overshoot the ksc and turn back to land so I don't have to worry about not having enough glide to return.

[Vegetal]

After my craft is subsonic and in stable flight, my method for balancing fuel for landing is this:

Get it level and engage SAS to keep it flying stable.

Observe pitch control indicator.

Pump fuel to the back until such indicator is just a smidge above middle.

There, your aircraft is now rebalanced to post-reentry flight and ready to land.

[The_Rocketeer]

After my craft is subsonic and in stable flight, my method for balancing fuel for landing is this:

Get it level and engage SAS to keep it flying stable.

Observe pitch control indicator.

Pump fuel to the back until such indicator is just a smidge above middle.

There, your aircraft is now rebalanced to post-reentry flight and ready to land.

Yeh, this ^^. I think a lot of people miss this bit of handy info when testing new designs.

[March Unto Torment]

Ah, that's what the pitch indicator is for! I always wondered what the use of that was.

Anyhow, thanks for the advice, everyone! I'll try a test flight this evening. I've added airbrakes and SPH testing indicates that I should be able to shift my CoM significantly forward.

With luck, I'll release my design after further testing - I'm very happy with her performance. When I landed her, even after my dramatic tailspin, I had close to a hundred tonnes of fuel left in the tanks. I'm pretty sure that, once I've worked out exactly how much fuel I need to use up to get to orbit and adjusted accordingly (so that I land empty), I'll get at least 120 tonnes to orbit in a single run

Oh, and regarding the aerodynamic forces, this is a very solidly constructed spaceplane. Plus, I use KJR.

[Vegetal]

The control indicators are very useful. You can see if your ship is reaching the maximum control authority with it, which is very important, especially for spaceplanes.

[p1t1o]

The first thing that popped into my mind, that I dont think anyone has mentioned yet is this:

If you are "flaring" your craft to create drag in the high atmosphere, a-la space shuttle, you may be well above your stall AoA. At high altitudes this doesnt matter so much as aerodynamic forces havn't built up too much. But once you enter the thicker air lower down, your aircraft is now stalling, producing very little lift, and your surfaces won't give you much control, you will be unable to correct any drift and you will go into a spin.

This will happen even if our fuel is balanced perfectly.

If managing your CoL/CoM via fuel transfers doesn't resolve the issue, try de-flaring sooner as you descend into the lower layers of the atmosphere.

[The_Rocketeer]

Ah, that's what the pitch indicator is for! I always wondered what the use of that was.

Strictly speaking that's not what the indicator is for, but it is a convenient use for it. Basically it shows you how much of your maximum control input is currently being used (by you or by the SAS). In other words, if the needle has risen above the line to nearly at the top, you will only be able to pull up a little bit harder, but you'll be able pitch down very hard (which may have drastic unwanted effects on your flight). If you're flying without SAS by using the trim controls, they're also handy for showing you where you've set your centre to (this is pretty similar to what the SAS does).

The same goes for the yaw and roll indicators, tho in game these get much less use than RL because there's no wind.

[March Unto Torment]

Ah, thank you. That's going to be very useful.

[Alshain]

I plan my planes so they come back with no fuel these days. Admittedly it took a lot of practice to land a plane from orbit with no power though.

When I come in, I do it steep. I don't overshoot and come back around unless I really screw up. I can usually come in at a 15 degree glide slope just fine and flare at the runway. That's pretty much how the real deal did it (I think it was 12 degrees but our navball isn't that precise). It's real easy once you get the tricks to unpowered landing down. Start at ~70km and put your PE at about 25km while you are on the other side of the planet from the ocean outside KSC. From there fly in straight just below horizon till you slow down enough to can see where the end of your arc collides with the planet. If you pull up, your arc will typically extend out a little, and if you push down it will collapse in, so just use that trick to keep the end of it near the KSC (remember, the atmosphere is going to keep slowing you down so don't overdo it). When you get in close, just land. Hopefully you've practiced the landing before attempting to go to space.

Edited September 15, 2015 by Alshain

[March Unto Torment]

I plan my planes so they come back with no fuel these days. Admittedly it took a lot of practice to land a plane with no power though.

That's the plan, at least later on; it's pretty fuel-efficient in atmosphere, so I'll probably only pack ~1,000 units of fuel for re-entry, allowing for about 120t to orbit. Unfortunately, it turns out that there's not much which weighs that much which actually fits inside her cargo bay I'm planning on soon lifting a full-sized Jool-capable ship in two SSTO launches, though, and even then, the only reason for breaking it up is because the standard ship is too long for the cargo bay.