Deep-space speed challenge

[tavert]

Here's a theoretical solution that came out of a MINLP solver after a few days:

The now-even-more-overpowered 48-7S results in the part count constraint being active, potentially giving another 1500 m/s or so beyond what I came up with in 0.21. It looks like I could have used up to 10 seconds longer burn time since MechJeb actually rounds down the numbers it displays, which I wasn't taking into account in my optimization.

This takes Leonon's "linear staging" concept to a ridiculous extreme. I've actually made use of this glitch unintentionally before to mount landing gear below engines, but never tried mounting whole stages below running engines. If there's a physics-less strut right below the engine, it works fine with no blocked thrust or part damage. Definitely the lowest-mass, lowest-part-count staging method I've seen, though it results in crazy tall rockets (hence this one was put on its side and pointing backwards in the SPH).

Sadly, it tears itself apart in flight. I might have to go back to a more conventional horizontal asparagus setup with prescribed symmetry in each stage and leave more room in the part count for struts...

[numerobis]

Huh; I hadn't heard of that trick! It'll save you decoupler mass on the last couple stages anyway, even if you can't abuse it for the entire flight.

My concept for getting a smaller Eve trip than your best was to stage in a straight line, but with engines outboard, saving you decoupler mass. But you lose in the end (at least in 0.21) because of that last solitary engine: you'd need to save an enormous number of decouplers before you win by having the extra engine in your last stage, or a non-firing engine in the penultimate stage. I must try again

I've gotten the entire ascent automated now: I hit space, and leave; the autopilot spins up the basic jets, then the turbojets, then releases the clamps, autostages until the flight is done, and finally it opens the flight log at the 10-minute mark. Which is good because I get less than a frame per second when the engines come on -- it takes 10s spinning up then 30s until I ditch any substantial parts. I'd better be doing something else in that time!

[tavert]

This morning I was playing around with Eve ascent configurations taking advantage of the inexplicably uprated 48-7S, and with this linear staging trick it should be doable in 3 stages, each with 1 ton of fuel and 1 48-7S. You can add an ant-and-oscar final stage, but it's only good for another 400 m/s or so total. Single digits for the round trip is absolutely doable now, just pick how few ion engines you have the patience to get away with.

[numerobis]

3 engines seems like too little, according to my calculations. But check the least-mass thread for details. This trick definitely opens up some possibilities; I got 16 km/s with 550 parts and surprisingly little mass.

My big unresolved question now is how to build a jet stage with fewer struts. The designs I can think of all end up imparting ridiculous amounts of roll. I'll formulate the problem better overnight I think.

[Nao]

@numerobis

If you don't spam ram intakes, you could try adding some closed radial intakes to increase performance with minimal partcount impact. There are some shenanigans going on with them, i was able to get up to 55% more performance out of the engine by adding closed radials. In one test the engine not only worked at 100% thrust at 1/4 of normal flameout air, but also had more thrust than it should at that fuel flow/speed. (Didn't manage to use this bug with large amounts of intakes so no new Maching bird record but it definitely can be useful). I think they provide "ghost" air even when closed and mess up some calculations.

Edited October 26, 2013 by Nao

[numerobis]

I haven't noticed any strangeness with radial intakes. When you say "1/4 of normal flameout air" what do you mean exactly?

Nevertheless, I am planning on using a few (meaning about 50) radial intakes. At low speed they have much less drag than 30x ram intakes -- as you increase the area of an intake, the drag grows, so that a 30x ram intake has drag 2 by about 100 m/s. And as per my question in the howto section about how many parts is the maximum, I might need to increase the number of intakes per part and use fewer parts. That would be aesthetically unfortunate, but life is pain.

tavert, for your spacecraft tearing itself apart, I'm curious -- do you think it might be that the engines gimbal the wrong way above the center of mass?

In any case, if you can get a 16 km/s stage up, send me the .craft file and I'll fly it in the 10-minute challenge with my ridiculous jet stage.

[Nao]

Don't remember the exact numbers but In one of the test's on the bug I used 1 circular intake and 50 closed ram intakes to go ~2230m/s and above 30km altitude on 100% throttle. The circular intake provided less 0,8 (or was it 0,65?) units of air when the flameout occurred (normally it takes 3,36). The engine fuel flow was smaller than normal (it looks like automatic flameout control, when below minimal air flow, the engine starts to drop fuel flow and thrust by itself), but the thrust was somewhat higher than expected from the speed and fuel flow values too (also thrust go down if you throttle down even thou it looks like engine is limiting thrust by avilable air so theoretically it shouldn't ). Idk what is actually going on there.

Having 1-2 closed radial intakes per ram intake gives lesser but still noticeable boost in performance

If you are using lots of radial intakes, try adding one ram and close all the radials, it should have much better performance.

@ tavert's craft, at first I thought about gimbals too, but then I remembered that its KSPhysics, and it probably can wobble itself to death with out player even touching it at that length. Maybe using MJ to throttle down, putting the flight on rails for one frame then throttle up one every other second it could finish it's burn .

[tavert]

I think I tried flying it in several configurations, with MJ trying to hold a direction, with just SAS, and with nothing, and it would always go snaking around like crazy. The gimbal direction bug certainly doesn't help though. And the fact that the last stage was just drop tanks (that's what the optimizer told me) meant it fell off immediately...

[numerobis]

I built one with just 6 inline stages with three struts between each stage. Even that wobbled like mad on the launch pad until it was under thrust.