Help with Space Shuttles

[SaturnianBlue]

Hello! I haven't been around here for a while since I spend most of my time in the Spacecraft Exchange, but I need help for my Space Shuttle, which I'm using for my recreation of the Galileo and Ulysses spacecraft.

I've almost managed to get the Center Of Mass and the DCoM matched, but about 1 minute in the flight, it starts to fight SAS control. I don't want to place a bunch of SAS really.

Solutions tried -

-Use IR to angle engines (Didn't work, Engines pushed into hinge when thrusting).

-Change where the fuel is to try and make it so the CoM does not slip.

-Counterweights (Fairly ineffective for such a heavy craft)

-Stronger engines to try and improve gimbal.

-Slight angle of wings.

-Countless adjustments in Engine tripod angling.

Notes-

Mods used include procedural parts, tweak scale, KJR, IR in cargo bays.

I still want to keep the craft fairly realistic in apperance and performance, because i'm using it for a recreation of the Galileo mission.

[Vegetal]

RCS Build Aid is showing quite a lot of torque on that configuration, 3350 kNm. I always try to put this value close to zero when referencing the average CoM (ACoM), but sometimes you will in fact need to play with power limits during launch.

Stock engines are also quite bad for shuttle replicas, they have too low a gimbal range.

[SaturnianBlue]

I've managed to get it to more like 500 for either one of them, at the cost of the other being more like 3000.

Edit: Ohhhh! So there is a ACoM! I'll try and work on that.

Edited October 12, 2015 by SaturnianBlue

[SaturnianBlue]

I took a look at the ACoM, and the craft was mildly stable, but the problem still happens at 6Km.

[SaturnianBlue]

A few more tweaks, still mostly the same even with the ACoM at almost 0.

[SaturnianBlue]

Anyone have any more suggestions to fix the issue?

[GoSlash27]

SaturnianBlue,

Have you examined how the main tank drains and where you're feeding the engines from?

You want it set up so that the CoM shifts towards the SSMEs in a straight line along their center of thrust as the main tank drains.

Best,

-Slashy

[Empiro]

I don't use RCS build aid (I have Engineer instead, which shows torque force but not the direction), so I just drain and undrain the tanks manually.

Have you tried to see where your CoM is when the SRBs detach? In my own designs, I calculated how much fuel would be burned by SRB separation, and looked at where the CoM was at that point when the SRBs detached. That became the "full" CoM, so the average CoM was between that and the dry CoM. I then minimized the thrust torque on the average CoM by tilting the main engines.

Also, I've found that moving the tank up results in a smaller overall change in thrust torque (because it causes the CoM shift in a straighter line along the engines' thrust).

I then minimized thrust torque for the first part of the flight by following a similar procedure (except that the "empty" CoM is when the SRBs burn out). I set the fuel levels to the midpoint between launch and SRB separation, and attempted to minimize the thrust torque. I changed the CoT at that point by simply moving the boosters up and down (it also changes the CoM, but since the fuel was all set at the average point, I was still minimizing the average torque).

I still ended up using Vernor engines for extra pitch control simply because the engines don't have enough gimbal (I just had to put 1 on each side of the nose of the craft), but I found that I could get through 90% of the flight without needing to use them.

Edited October 13, 2015 by Empiro

[SaturnianBlue]

SaturnianBlue,

Have you examined how the main tank drains and where you're feeding the engines from?

You want it set up so that the CoM shifts towards the SSMEs in a straight line along their center of thrust as the main tank drains.

Best,

-Slashy

I have a fuel line going to the cockpit. I have a LFO tank with the main fuel ( 14K liquid fuel) at the CoM. I'm not sure it's flowing in a straight line though.

[SaturnianBlue]

By taking a look and trying to do some bits of the suggestions, it's more stable, but simply not enough. While it makes no attempt to violently pitch at 7 Km, it still pitches down, though it does manage to survive for plenty time. Not enough stability for orbit.

[SaturnianBlue]

The Space Shuttle is still suffering from the same problems,I add a pair of vernoers to the SRBS, but it's still not working well.

[Vegetal]

Do you try fiddling with thrust limiters during flight? The lack of good gimbals on stock engines frequently forces you to do that with shuttles. You have to watch the pitch indicator on the lower left corner, if it goes too close to it's limits, it's time to decrease power somewhere.

Does it pitch down before or after SRB separation?

[SaturnianBlue]

Do you try fiddling with thrust limiters during flight? The lack of good gimbals on stock engines frequently forces you to do that with shuttles. You have to watch the pitch indicator on the lower left corner, if it goes too close to it's limits, it's time to decrease power somewhere.

Does it pitch down before or after SRB separation?

Not really with specific engines, but I wildly change throttles in flight. It pitches before separation, at about 6Km, and while it can eventually pull out at 14Km, by that time it has fallen to only 20 degrees, which is usually not a good launch trajectory in my experience.

[Vegetal]

Well, pitching down in this case is relative to the horizon or the craft? Shuttles go upside down....

Anyway, try fiddling with thrust limiters during ascent, it may be the only way to go with this design of yours. I also see you have tweakscale, you could try to make the SSMEs with big "Thuds", as they have a wide gimbal range.

I'm thinking it's very possible to make this design work if you use the limiter approach during flight though.

[SaturnianBlue]

It's mostly a concern of weight, persay if the weight if thuds will be more than skippers in proportion to nozzle size and thrust. It's pitching down relative to ground, I fly my shuttle upside down like IRL. I'll try thrust limiters though.

edit: yep, thuds weight a lot more for same thrust... Low ISP was another concern too.

I have tried to fiddle with fuel amounts in the editor, at 1/3 SRB fuel where I believe it happens most likely, but still suffers rather badly.

Edited October 17, 2015 by SaturnianBlue