TT38K Radial Decouplers add Delta V?

[Torch]

Hey all, I have an interesting one here! I have built an SSTO to ferry kerbals to my space station. When I initially built it, I had TT38K Radial Decouplers between the fuel tanks to create some space between the tanks. This version worked great. In redesign, to get rid of unnecessary weight, I removed the decouplers, and all of a sudden, the craft wouldn't even get off the ground! In doing some research with MechJeb 2, I discovered that the radial decouplers were adding 472 m/s of delta V, and allowing the craft to accelerate from 80 m/s at the end of the runway (wouldn't take off) to 120 m/s (would take off).

Here are pictures with MechJeb windows visible so you can see for yourself:

SSTO without TT38 Radial Decouplers between fuel tanks

Anyone know whats going on here?

Here are links to the craft files, with the MechJeb2 Units remove, ie, these are stock:

http://www./download/g4h6geww2e6m1a1/SSTO_Mk_VIIa.craft

http://www./download/uqf4k5otl249s5b/SSTO_Mk_VII.craft

Thanks for reading, I am interested to hear if anyone else has experienced this, and is there a way to get my spaceplane airborne without the decouplers!

[Kimberly]

Decouplers you don't intend to use mess up the delta-V display, because it assumes you're going to shed weight/attached engines even though you won't actually do so. I don't know why it would make an actual difference in flight, though.

[Torch]

Weird Huh? I get that they could mess up the delta V displayed, but actually make a performance change? That I didn't expect.

[iamthemoose]

I had a similar situation earlier with Engineer where my rocket lost ~3000m/s dV when I changed something insignificant.

[Torch]

@iamthemoose, did you actually lose 3000 dV, or did Engineer just display that you lost 3000 dV?

[Johnno]

Decouplers you don't intend to use mess up the delta-V display, because it assumes you're going to shed weight/attached engines even though you won't actually do so. I don't know why it would make an actual difference in flight, though.

This is what's going on. Engineer (and mechjeb I suspect) work by looking at the staging list to do the maths for you.

If you can't take off after reducing mass by removing decouplers it's likely because the center of mass has been moved, even if slightly. Try readjusting your landing gear, moving them closer to center of mass.

And iamthemoose, changing your staging is in no way insignificant for figuring out your delta-v.

[carazvan]

Think the problem here is that with the decouplers it was 'using' the fuel in the center for the side engines but without it's only using the fuel in the side tanks.

Try adding a fuel line from the center tank to the side tanks to see if that fixes it.

[Torch]

@Carazvan, good thought, I obviously had fuel lines from outer to inner tanks, and I thought adding lines as you suggested from inner to outer might work also, alas, no, same result. I understand how the dV could get buggered up with decouplers that you don't use, but I just don't get why that would cause a performance problem. Adding the additional fuel lines had no effect on the acceleration, still just 80 m/s at the end of the runway, not enough to achieve flight :-(

[Johnno]

Tried your craft to see if I could help, but there's some non-stock part still on it. 'small adapter', think it's on the nose, I tried forcefully changing the craft file but KSP didn't like that.

Edit: Further digging found some mention of lazorsystem docking camera as well.

Edit 2: Ok I got the craft loaded, moved the landing gear (as I initially suggested) and it took off fine. However I noticed straight off you barely have any control surfaces and the plane is really heavy. The fuel lines aren't properly drawn and worst of all, you're using rocket fuel tanks for jet engines. Huge waste and huge mass addition.

You should swap those tailfins for winglets, or add control surfaces to them. Move the rear landing gear closer to the center of mass and reduce the overall weight by swapping out those rocket fuel tanks the jets are using.

Also there's no point to have decouplers attaching the tanks unless you intend to drop them. Just attach the tanks to eachother and strut them.

Edited July 20, 2013 by Johnno

[Torch]

Wow, thanks for all the work on that, greatly appreciated, sorry about the lazor, forgot all about that. Do you know how I can change this to answered?

[Johnno]

Wow, thanks for all the work on that, greatly appreciated, sorry about the lazor, forgot all about that. Do you know how I can change this to answered?

Edit post - Go Advanced - there should be some prefix thingie there I hear.

Edit the first post that is.