How to read Spaceplane dV?

[m4rt14n]

Can anyone explain me how to read these data?

I'm using mechjeb to tell me the dV of the Spaceplane. It's showing total dV 32,441 m/s. Then for each stages:

0: Turbojets -> 30018 m/s

1: Nerva -> 2600 m/s

2: Lander engines -> 177 m/s

I'm mostly concerned about having enough dV to go interplanetary. Does it mean that in LKO I would have roughly 2600 m/s to play with?

Thanks a bunch.

Edited February 17, 2014 by m4rt14n

[Taki117]

The 2600 is assuming you don't use any fuel for your Nerva. Inevitably you are going to use a tiny amount to get into orbit. Your best bet is to dock at a space station and empty your liquid fuel tank and fill up your LF/OX tanks to head interplanetary, then you might actually have more than the 2600m/s to play around with.

[helaeon]

I see you have the different engines in their own stages. If you click and drag them so that engine is at the bottom of the staging stack (2 in this case). MJ will show you the maximum possible delta v that engine would have with the fuel supplied to it.

Your real delta v from there is entirely up to how you pilot it. The way MJ shows delta v is assuming you'll stage exactly like you have it arranged as soon as that stage goes dry (like on a rocket). Usually you don't fly SSTOs that way.

Generally on SSTOs its best to be short on the oxidizer and be tapping the rockets just as your liquid fuel starts to equalize with your oxidizer levels. I tend to use LOX tanks anyway so if I want to top it all off for my space engines at a space station then you don't have a whole lot of liquid fuel sitting around, or empty space, you can't use.

[m4rt14n]

Okay I switched the staging around so Nerva goes to 0 and it's showing dV of 2900. Yes I use turbojets all the way to 40km then punching in Nerva around 50km to go orbital using action groups.

Trying to make sense all this dV stuff. Usually I just over-engineer rockets with massive dV and winging it to go to space. Unfortunately KASA decided to cut funding for space station so no refueling for the meantime.

Thanks.

[Claw]

I usually get more creative with how I flow the oxy and the fuel so MJ give better numbers, but this isn't always ideal since you may want all fuel available to all engines.

If you want accurate dV numbers from MJ for your space plane, your best bet is configure the fuel the way it would be when you use a particular engine. Then put that engine at the bottom of the staging stack. MJ will then output a dV number for you for that engine/staging point in your profile.

So for the example you posted, your NERVA has more than 2600 dV available. It's just showing that because it's trying to push all that extra liquid fuel that your TurboJets would eat up. So adjust your liquid fuel to levels you normally see once you're in orbit (after your TurboJets have gotten you there). You can leave your engine staging the way it is, or move your lander stage up higher to be more accurate. Then the bottom line of MJ will tell you the NERVA dV available for your trip.

[m4rt14n]

I'm trying to separate tanks for Jets and Nerva because of weird shifting issues. Since I'm using the T-joint to merge two tanks together to 1 Nerva, if the jet fuel tanks feed into it, it will use the right tank first shifting the weight to the right. Or something like that. Not really sure what's going on.

Also in theory, the jet fuel would be used halfway first, shifting weight to the front. Then in space the Nerva would finish rocket tanks shifting weight backwards again...

[Claw]

2:1 couplers do not feed fuel correctly when flipped around. The problem is that only one connector on the dual side connects. So half your fuel stack isn't feeding the NERVA. You'll have to find a place to route fuel lines if you use the multi-couplers like that.

BTW, even with separate tanks, MJ assumes you use all the fuel of a stage before activating the next stage. So the numbers are better but still tend to underestimate dV.

[m4rt14n]

Okaybthat makes sense. I guess i can try sticking a non fuel crossfeeding decoupler and feeding fuel lines directly from the other tanks to the emgine.