Mini Shuttle flies ass first.

[BechMeister]

Hello! I am currently building a small refueling station in Low Kerbin Orbit.  I made a small emergency shuttle/escape craft. It looks like this:




As you can see its very small. It has ~600 Δv - which is more than enough to correct orbits after a disaster and have some say in were it lands. It has parachutes too.

Now my problem is I have now tested its re-entry characteristics.. and even though it had 2t out of 2.6 ton Monopropellant to counter balance the mass of the engines it would still flip and fly ass first - even at AoA of 10° 

The only time it managed to not flip was when i told it to follow the prograde all the way down.. and lets just say that Bob seized to exist because man.. he hit the ground fast.. I dont think it went sub 1000 m/s - which means had heat been a thing.. i guess he would never have touched the ground. 

The plane becomes controllable again at 400-800 m/s (depending on how well the flip maneuver goes) - but once heat is introduced.. i dont think it will make it to that point if it doesn't stay belly down.

I have a few ideas to "fix" the problem:

1. Put decouplers on the engines to drop them after re-entry burn... that would remove the mass (how ever i would like for them to stay...)
2. Try some how add more mass to the nose..

are there any one who have any other ideas? Or is this craft to small to be possible to get stable... and i should really just get pods for escape crafts?

Edited April 21, 2023 by BechMeister

[18Watt]

The problem seems to be the COM is too far aft, or the COL is too far forward.  

One suggestion for having less mass it the back:  Does that small shuttle really need 2 engines?  I suspect a single engine would be more than adequate.  In fact, just for doing orbital maneuvers and a de-orbit burn, a single small engine would be more than adequate.  That would help reduce the mass in the rear of the plane.

Another suggestion is either move the wings aft, or add control surfaces at the very back of the plane, which would move the COL aft.

Finally, one reason your COL is so far forward is because the MK2 cockpits and fuselages produce some lift themselves.  That isn't the end of the world, and can usually be compensated for by reducing engine mass at the back, and adding wing area at the back.

If you are not using the COM and COL display tools in the VAB, those are very helpful in evaluating how changes you make affect the relationship between those two values.

 

[BechMeister]

Oh wow.. I dont know why i didn't think about reducing it to one engine.  I think its because it was an older design from a time were i just wanted as high a lift to weight ratio as possible. But yeah one engine would help a lot. Funny story. I was testing a new iteration were i put 2 batteries inside the nose to place more mass at the front. And i was looking at other ways to get it into space. I put two stack separators on the two engines and when i staged after getting into space on a sub orbital path it broke one engine off.. I have yet to find a way to stage that wont break 1/2 engine off. But I did get to test the flight characteristics... and with the new changes I was able to some what control it on its ascent down with a AoA on 10% 

I also tested the parachutes as it was landing over water. I had placed the schutes in accordance with the center point of mass when the tank is full, brain farting that the center of mass would shift forward wen its empty. So it hit the water with the nose down (which i dont think is ideal)

But yeah.. No Brainer. Ill just make one engine - It will also remove the problem with stack separation breaking an engine off. 

Can i ask you what COL  and COM is short for? i cant really figure it out... 

[fulgur]

Centre of Lift and Centre of Mass.

Refer to this wonderful tutorial; it taught me 90% of what I know about KSP aircraft. It should still work for KSP2 because the aerodynamics haven't changed much, although the drag model (in KSP1 and 2) is now better.

 

Edited April 22, 2023 by fulgur

[BechMeister]

Changes:

- changed to one engine.
- Reduced Monopropellant from 2t go 500kg (which should be more than enough for orbital maneuvers)
- Re-located the parachutes to be placed the at the empty center of mass.
- Docking port is at full center of mass to make it easier to rotate

First test flight:

- It glides a lot better now. Really easy to maneuver 

Next is a re-entry test

10 minutes ago, fulgur said:

Centre of Lift and Centre of Mass.

Refer to this wonderful tutorial; it taught me 90% of what I know about KSP aircraft. It should still work for KSP2 because the aerodynamics haven't changed much.

 

Ah! yeah those acronyms makes a lot of sense

[18Watt]

9 hours ago, BechMeister said:

Can i ask you what COL  and COM is short for? i cant really figure it out...

If you are on a PC and mouse hover over the underlined terms, a definition should pop up.  They did that with several terms in the forums, if you see a term or acronym underlined in a post there is a good chance it can be mouse-hovered over to produce a definition.

I have not figured out how to make that work on a touch-screen mobile device, such as an iPad.

Glad you are getting better results!

[BechMeister]

Oh its amazing to fly now.. its probably the most controllable craft i have now.. It can rotate on its belly AoA 45° I think i even did 90° at some point. I can control my suborbital path same way the shuttle did by jawing left or right. Really easy now. Thx for the help