Please help me - Why won't my Shuttle work?

[Johno]

Hi all.

So I have caught the Space Shuttle bug. I have wasted hours trying to create a stock NASA shuttle work-alike.

Now, there are a few problems I already understand. She's currently woefully under-fuelled, for one thing. I also haven't had the chance to test the OMS or the RCS, both of which I'll need to get the ship above the atmosphere to do.

But that's just it - I'm doing something systematically wrong, but what?

The design process was as follows:

1) Create an orbiter with viceless flight characteristics, with jet engines. Check.

2) Make the jet engines jettisonable, and test its glide characteristics. Check (although I am yet to test it with the OMS pods and RCS system added; that's something I should do!).

3) Angle the SSMEs so that the stack can launch in an upright direction. Check (with extreme difficulty).

Here's where I've run into strife. The launch now goes generally reasonably well (albeit drifting a little) until the SRBs are jettisoned. It's from there that things start to go rough. The orbiter/ET combination start to wildly tumble.

Now, I know that the Shuttle must do something to prevent this kind of tumbling (which obviously comes from unbalanced engines), but what?

The craft file is downloadable here.

Thanks in advance - I am fully expecting that I made every error known to Shuttlebuilderkind.

[celem]

Well yes, its the unbalanced engines. Im at work so cant test-fly it. But from the picture at that link I can see you've angled the shuttle engines to balance against the srb's. Once you drop those the thrust is way off the centre of mass for the shuttle/ET combo.

In reality the shuttles engines do a lot of vectoring, true. Once the srbs are gone though your engines would need to actually be tilted the other way to compensate for the off-centre thrust, gimballing only goes so far. Strip the srb's inside the vab, then check CoM and CoT to see its characteristics in that stage.

My advice would be to lose the angling of the shuttle engines and instead balance the initial stage of the launch by moving the srb's out of over-all symmetry to offset the thrust/weight of the shuttle (so they sit a little toward the same side as the shuttle, try and bring them closer to your starting CoM).

Even with these alterations you are going to be off-centre once the srb's drop, though the neutral shuttle engine angle will allow them to vector modestly to allow for the ET weight. The remaining control needed to maintain vertical flight is probably gonna come down to SAS and RCS along with whatever lift the shuttle control surfaces can still provide (thin air).

Dont sweat it. Shuttles are hard. Mainly because as soon as you start adopting realworld designs, you have to deal with the fact that they are, by nature, all unbalanced as sin. That and NASA's own tech doesnt perfectly lineup with ours, nor does ksp work quite like the realworld.

Edited December 16, 2013 by celem

[Pursuedtank]

It is almost certainly the weight of the SRB's that keep it balanced, as soon as you drop them the center of mass shifts, this results in the center of thrust not going through the center of mass. The shuttle would need to gimbal its engines to get the center of thrust through the center of mass. However this is where the problems lie. In the stock game the engines don't have a very good gimbal range. You could try to add a large amount of reaction wheels to counter it. Or you could download this mod which would make the whole thing much easier.

[Kasuha]

My ideas:

- make sure the center of mass of the shuttle + fuel tank combo is the same direction from engines both with full and empty fuel tank

- make sure the fuel tank is drained evenly, i.e. its center of mass stays in place regardless of amount of fuel in it

- make sure engines are aiming at this center of mass

- mount your SRBs around this center of mass (i.e. higher than where they are)

- next time you are going to make a screenshot to post somewhere, time warp to daytime first

Edited December 16, 2013 by Kasuha

[el_coyoto]

The people above me all give good suggestions.

You can add fuel lines to help drain the tanks more evenly, check your engines, etc.

I just wanted to add my two cents with a nice trick I discovered a while ago on someone's shuttle and that I described in a similar thread to cope with the thrust and the shifting center of mass. I'll post a direct link to my answer, but the rest of the tread might interest you...

Good luck!