STS Replica losing control during reentry

[Jollyranger]

I built an STS replica mostly based on some tutorials I saw, and if I wrestle with the controls during ascent, It works. The only problem Is reentry. When I reenter, if I pitch up more than 5 degrees it flips over and sways around, going in backwards.

The only way to land it is to deploy the drogues and flip it over. The com is in front of the col even with the fuel drained and it flies well if launched from the runway. (I didn't get to take a screenshot of reentry because I was fighting it.)

the cargo bay is open in the pic where the fuel is drained

              does anyone have a solution?

[Flavio hc16]

the main problem is that your COm is behind the middle of the craft, so it wants to reenter engine first, i have done the sts replica 3 days ago, and solved it using 2 of the monopropellant mk3 tanks, une behind and one in front of the cargo bay, then you have 2 option:

or you use monopropellant engine, or if you have fuel switch mod ( like i do) switch the monoprop tanks for the liquid fuel/oxidizer tank 

https://imgur.com/a/sEruS

Edited March 9, 2018 by Flavio hc16

[Jollyranger]

1 hour ago, Flavio hc16 said:

the main problem is that your COm is behind the middle of the craft, so it wants to reenter engine first, i have done the sts replica 3 days ago, and solved it using 2 of the monopropellant mk3 tanks, une behind and one in front of the cargo bay, then you have 2 option:

or you use monopropellant engine, or if you have fuel switch mod ( like i do) switch the monoprop tanks for the liquid fuel/oxidizer tank 

https://imgur.com/a/sEruS

That might be the best solution, because it needs more monopropellant and it would balance the ship. I will try it when I get home.

[michal.don]

Well, that's the problem with most shuttle replicas in KSP - if it looks like the shuttle, it flies really bad. The three "vector" engines weigh 12 tons, while the rest is pretty light, so the CoM is really close to the back of your shuttle, which is hard to counter. If you want your craft to look like the shuttle, it might help to spam some reaction wheels  in the cargo bay, or even some "ballast" to the front part of the craft to move the CoM forward. If you do not insist on creating a faithful replica, adding a fuel tank in front of the cargo bay should help a lot - it worked for me, at least.

Not saying it flies like a charm, because it certainly doesn't. But it's pretty stable during reentry, while still looking quite similar to the original thing.

I hope this helps,

Michal.don

[Starman4308]

One other issue is that the CoL indicator is a bit misleading. Sure, your lift producing parts are in the back, but there's a lot of drag produced forwards of that that the CoL indicator misses. I suspect the easiest solution is to put a reaction wheel or two towards the front, both as ballast and for control authority.

EDIT: What really needs to be looked at is Center of Pressure, which is the sum of both lift and drag, but KSP doesn't show that.

Edited March 9, 2018 by Starman4308

[Jollyranger]

13 minutes ago, Starman4308 said:

One other issue is that the CoL indicator is a bit misleading. Sure, your lift producing parts are in the back, but there's a lot of drag produced forwards of that that the CoL indicator misses. I suspect the easiest solution is to put a reaction wheel or two towards the front, both as ballast and for control authority.

EDIT: What really needs to be looked at is Center of Pressure, which is the sum of both lift and drag, but KSP doesn't show that.

Normally I would think about stuff like that, but KSP has me thinking more about orbits than pressure and drag. Thanks for pointing it out though! 

[Not Sure]

A big point in your aircraft learning curve is learning the effects of the placement of the COM. Just because the COL is behind the COL, does not mean it will be stable. As you pitch away from prograde, your COL will shift forward and will naturally fly in front of the COM making it unstable.

to be clear, the problem with all KSP shuttles is that the COM is too far back.

of course, with a lot of things, there’s a mod for that called Better COL. It will show you your REAL COL as well as the stability at different speeds, altitudes, and AOA.

You may be able to fix this by clipping some wings into the front and adding weight to the front.

Edited March 9, 2018 by Not Sure