Spaceplane stalls out

[DepletedUraniumPenetrator]

Hi.

So the other day I was building a SSTO spaceplane using B9 parts and FAR. Though my knowledge is limited, I know the basic principles of building air-or spaceplanes, but one thing I can't seem to get right is the high altitude stability. Starting at ~12000m, the thing starts stalling very easily when I make even slight movements (and is also unrecoverable once its out of control). I really can't find an explanation for this, maybe it has something to do with the very large wings who are not suited for supersonic speeds? I have attached a few pictures and also FAR stats (which greatly exceed my understanding ).

Javascript is disabled. View full album

Edited June 19, 2014 by DepletedUraniumPenetrator

[i like turtles]

It maybe due to not enough thrust. It would be helpful if u could tell me the weight of the craft and the total thrust:)

[i like turtles]

Also, could you send me the link of the craft? It would be greatly helpful if i test the craft myself

[the_bT]

Try to do the plot in the 4th picture with a higher mach number, this tells you when your plane stalls at mach 0.2 which is not the problem. Maybe it's a problem with shifting COG, your fuel does not seem to be close to the COG...

[DepletedUraniumPenetrator]

Well, it has one Sabre-M Engine during the atmospheric ascent, ideally producing 860kn (?) of thrust while having a weight of 105,66 tons (0.83 TWR).

Here is the craft file: https://www.dropbox.com/s/7jei84jn4nc7yc6/Aenima%20III.craft

[LethalDose]

Try to do the plot in the 4th picture with a higher mach number, this tells you when your plane stalls at mach 0.2 which is not the problem. Maybe it's a problem with shifting COG, your fuel does not seem to be close to the COG...

This would be my guess. As you're burning fuel, your CoM is going to shift aft. Take it for a test flight, and see how much fuel is in your tanks (and which tanks it's in) when it stalls, then to back to the SPH, tweak the fuel to represent your stall conditions and I bet you'll see the CoM shifts behind the CoL. You may need to move the wings further back, or lock the fuel in a forward tank.

[DepletedUraniumPenetrator]

The placement of the jet fuel tank indeed was a problem, but now that I fixed it it still easily stalls.

Javascript is disabled. View full album

[LethalDose]

H... have ya tried building a smaller SSTO? Workin' the bugs out on a little one first?

Anyway, if your still stalling, try moving the CoM still farther forward. More control surfaces far from the CoM help, too. Think of the control surfaces kinda like RCS: the further from the CoM they're placed, the more effective they are.

Edited June 19, 2014 by LethalDose

[Skorpychan]

It maybe due to not enough thrust. It would be helpful if u could tell me the weight of the craft and the total thrust:)

Yes. More boosters are obviously needed.

[the_bT]

Plane has enough thrust. I just tried it and made it to orbit despite being and idiot and exploding all 4 nervas... I suspect this to be related to lack of yaw control authority / control setup in general. I'm going to do a few tests and will try to post an improved design so the OP can compare with the original.

[Hodo]

Your problem with stalls is because of pitch authority. You lack decent control over it as the weight moves around in the craft.

You can get TAC fuel balancer or PWB fuel pumps which will fix this problem of the CoM shifting. Or you can manually do it during flight which is a pain.

I use FAR, DRE and B9, and I have made some pretty massive space plane SSTOs that can haul a good chunk into space, 108tons to be exact. I also can't suggest enough that if you want to make big SSTO space planes you get two more plugins... RCS Balancer ( so it can show your dry and loaded CoM) and Procedural Wings.

[the_bT]

Your problem with stalls is because of pitch authority. You lack decent control over it as the weight moves around in the craft.

You can get TAC fuel balancer or PWB fuel pumps which will fix this problem of the CoM shifting. Or you can manually do it during flight which is a pain.

I use FAR, DRE and B9, and I have made some pretty massive space plane SSTOs that can haul a good chunk into space, 108tons to be exact. I also can't suggest enough that if you want to make big SSTO space planes you get two more plugins... RCS Balancer ( so it can show your dry and loaded CoM) and Procedural Wings.

Have you tried the craft? For me it tends to yaw to one side suddenly and then all hell breaks loose, so far I think I might have cured that by rearranging control surfaces but in the process made it a bit unstable pitch-wise once you're over 30k meters, its fine if you switch on RCS but if you do not you flip. Hm...

Will post refined craft later, need to go for a while now.

[Hodo]

Have you tried the craft? For me it tends to yaw to one side suddenly and then all hell breaks loose, so far I think I might have cured that by rearranging control surfaces but in the process made it a bit unstable pitch-wise once you're over 30k meters, its fine if you switch on RCS but if you do not you flip. Hm...

Will post refined craft later, need to go for a while now.

No unfortunately it will not work for me, I am running Realism Overhaul, which means space starts at 180km altitude not 69.1km. So unless that craft is setup to run Realfuels, and has over 9km/s d/v I dont think I can test it. But I think I know what your yaw problem is. Do you have 2 SABRE engines in the back that are in open cycle, check the thrust on the two of them as soon as the craft starts to yaw. I am pretty sure one of them is dropping power pretty rapidly. I have had this happen more than once and my only guess is it is a fuel flow problem.

[LethalDose]

Do you have 2 SABRE engines in the back that are in open cycle, check the thrust on the two of them as soon as the craft starts to yaw. I am pretty sure one of them is dropping power pretty rapidly. I have had this happen more than once and my only guess is it is a fuel flow problem.

OMG, I have so had this happen and it sux. It's an air starving problem, which is sort off a fuel flow problem. While the game is now smart enough to throttle back air breathing engines to match the air available, it doesn't always do it symmetrically.

Kashua has posted on this before. Apparently game's logic preferentially delivers air to "the first engine placed on the vehicle". My solution/workaround for this is to put "toggle mode" on action group 1, and make sure I'm paying enough to switch manually. Over 25 km altitude, I switch on my RCS. When I notice it firing in yaw, I know I need to kick it over to closed-cycle.

[the_bT]

No unfortunately it will not work for me, I am running Realism Overhaul, which means space starts at 180km altitude not 69.1km. So unless that craft is setup to run Realfuels, and has over 9km/s d/v I dont think I can test it. But I think I know what your yaw problem is. Do you have 2 SABRE engines in the back that are in open cycle, check the thrust on the two of them as soon as the craft starts to yaw. I am pretty sure one of them is dropping power pretty rapidly. I have had this happen more than once and my only guess is it is a fuel flow problem.

Its not asymmetric thrust as the craft has only one sabre engine in the back. I'm pretty sure it was lack of yaw control that was the problem before, the COM is very far back and there was only one control surface trying to keep yawing in check, while it was doing that it induced a roll as it is necessarily above the COM. The same tail fin had also roll activated so it tried to counteract the roll and as a result didn't do anything as the plane turned into a slip and then bad things happened.

What I did was this:

1. I slightly angled the wings upwards to improve general stability.

2. I made the control surfaces on the wing only act on roll input, the aft horizontal stabilizers work only on pitch input. I would suggest never to mix these two things on large aircraft. Same with yaw and roll on one control surface, as described above.

3. As I believe lack of yaw authority to be the main problem here I doubled the tail fin and also angled the canards 45° and doubled them too. Canards now act on pitch and yaw inputs.

4. Added flaps to the wing and changed the landing gear so take off is not so dangerous. Also did some beautifying on the wing, couldn't help it

5. Unrelated to the stability problems I played around with the action groups. 1 decreases, 2 increases flaps, 3 switches between air and space mode, 4 opens/closes cargo bay.

Craft now looks like this:

With that I could fly to orbit without much trouble at all. Pitched up hard until 15k then held about 10° pitch until 30k, switched to rocket mode and held 10° until apo was 100k or so. Had tons of fuel left, so should be able to circularize. All that without cargo though.

Remaining issues:

1. Once in the high atmosphere where the control surfaces stop working well the drag from the twin tail fins trys to pitch the craft up which can flip you if you do not counteract it with RCS. I added a bit more monopropellant for that on the inside of the cargo bay walls. Also moved the RCS nozzles a bit to the front.

2. I don't know what kind of cargo you plan to carry but as the cargo hold is in front of COG you can't carry heavy stuff. Tried 36 tons, can barely take off and fly but not climb above 10k. I don't think there is a way to fix that without serious redesign though.

The wings can carry at least double that in cargo. Although then you need more engines...

Here's the craft: https://dl.dropboxusercontent.com/u/16335634/Aenima%20III_2.craft

Hope I was of assistance

Edited June 19, 2014 by the_bT
picture

[DepletedUraniumPenetrator]

Wow, amazing! I totally forgot that you can change the behaviour of control surfaces with FAR and this certainly was the reason for the problem. I also did some changes in the meanwhile, I added larger control surfaces on the tail and put one of the large LOF tanks in front of the cargo bay so that the CoM is farther ahead, that increased overall stability alot and hopefully enables larger payloads. Next step will be to merge your brilliant ideas into the craft, so maybe it turns out as a capable SSTO after all I'll upload the craft file when its done.

Thanks guys for all the help!

[the_bT]

Man, I'm enjoying messing up with your design way to much. I decided to try and re-balance the craft a bit and then kinda went beserk. Got rid of all fuel in the fuselage so the COG was more or less dead center in the cargo hold. That allowed to get rid of the ridiculous canard arrangement and way over the top twin tail I had stuck onto the thing to solve the stability issues.

Problem: Craft was no longer long enough to place the wings far enough forward so the COL would be at COG. Also craft no longer had fuel to fly

Solution: Angle wings in the most extreme fashion so COL comes forward, clip wings like a mad man to hide the ugly seams and clip embed fuel in the wing-root.

I present, Lifting Cargo in style*:

*Mind, I have not jet attempted to lift any cargo at all, I fear with higher take off weight a second SABRE engine will be needed...

Edited June 20, 2014 by the_bT

[i like turtles]

It might be easier to add more rockets so your TWR is more than 1

[i like turtles]

How did you get your SABRE engines to work? It doesnt work for me

[i like turtles]

oh thank u

- - - Updated - - -

ur welcome