Does usable delta v change with engine placement?

[DustInTheWind]

Hello all!

First time, long time, all that.

Anywho, I started a new career mode game once 1.0 hit and am working on a reusable planetary transfer vehicle. I built the thing in sandbox, got it to orbit and worked out a a design flaw that caused a big kaboom. In career mode, I have several contracts I was hoping to knock out in one go: explore Duna, explore Ike, place station in orbit of Duna, fly near the sun, etc... soI figured I'd try MechJeb for the delta V Display in the VAB to make sure I'd have enough delta V to pull them all off in one go, so far so good.

However, once I decided to move my mothership's radial engines to the CoM, the displayed delta V of that stage went from 11,000 to 223. I am using FTX-2 fuel ducts to move fuel to the center of three tanks so as to "auto-balance" fuel loads and keep the CoM as stable as possible for maneuvering purposes to save on mono-propellant and help overall stability. Is MechJeb treating any fuel behind my engines as dead weight, or does a "puller" versus "pusher" configuration really decrease delta V that much?

[icantmakemodels]

nope. it's possible that MJ is having a derp and thinks it'll have less fuel because its connected to a diffrent part. pullers tend to be more stable, so id recommend it.

[cybersol]

Does a "puller" versus "pusher" configuration really decrease delta V that much?

No, puller versus pusher should not matter for dV. You will however have to be careful that the exhaust doesn't fry anything. This issue is likely fuel flow related, so you could save and then fiddle with your fuel lines to check what happens. Failing that, another possibility is to check the MechJeb displays with something like Kerbal Engineer Redux.

[DustInTheWind]

I checked fuel flow several times, and just moving the engines down one tank really increased the delta-v. I guess I just going to have to do a save game and launch it to really find out

[DustInTheWind]

Well, it ended up being a fuel flow problem that wasn't apparent in the VAB. Sadly all hands were lost.

[Red Iron Crown]

Sounds like you found the problem, but I'll add this for posterity: Placement generally doesn't matter, but angling your engines certainly can. If you angle your engines such that they don't thrust directly in the direction of acceleration your vessel will suffer "cosine losses". The stock Heavy Lander craft is a good example of this.

[Warzouz]

Did you change the root part ? I noticed once or twice that changing the root part can mess the staging.

[DustInTheWind]

My planetary transfer vehicle was based on a core of three long Mk3 Liquid fuel tanks with four radially mounted slanted 1.25M to 2.5M adapter tanks. Fuel feed lines went from the fore and aft tanks to the center and then from the center tank to the radial slanted adapters. Tri-couplers went on the adapter tanks for mounting the LV-Ns, and then I built out the rest of the craft. My unmanned version tested in sandbox got into orbit with nearly a full load of liquid fuel at just over 30,000 units (150 tonnes) in just one launch, but when I recreated in in career, I decided to adapt the design to a mid-body puller and add a Mk3 crew cabin, cockpit, and a lots of other extras so it could be an in-orbit resource to quickly knock out contracts.

Unfortunately, the fuel feed lines from the center to radial tanks weren't working correctly. I still had ~6,000 units of fuel and ~7300 of L-Ox in my ascent stage when I made orbit, so I used that on my next burn, when I fired off my injection stage, I didn't use all of the fuel in those tanks, and manged to snag a really low periphrasis for Munar insertion. As attitude changes for VERY large craft are want to do, my periphrasis lowered without me noticing and I ran out of fuel before I figured out it wasn't transferring. A period of frantic fuel transferring (gotta get all four radial tanks with some fuel else asymmetric thrust) ended when the very end of the Pegasus clipped a mountain top, causing the most spectacular explosion I've seen since that one time I turned on infinite fuel, hacked gravity, and reentered atmosphere at 48,000 m/s...