Space Plane payload fraction mystery

[Merandix]

Dear fellow Kerbonauts, I'm on my way back to where I was when I built spaceplanes back in 0.90 (ok-ish at building and flying them). But not quite. But what is happening now, is puzzling me to no end.

Take one spaceplane. Load it up with 36 tonnes of ballast fuel tank to simulate heavy cargo. Won't quite make it. No sweat, I won't often be launching loads that heavy anyway. Let's be generous, I had to use slightly less than one third of the orange tank's fuel supply to circularise at 75km. So, if I take an 18 tonne tank, it should get up there no problem right?

Well, erm... no... Despite me dropping 18 tonnes of cargo... this thing flies almost the same?! I don't get this. Sure, the orange tank flight was one of my better flights, and I'm still learning on how to exactly put spaceplanes in orbit on the piloting bit, but still, that margin should be caught easily with EIGHTEEN tonnes of cargo weight dropped. I would have thought that with the cargo reduction, I'd be putting it up there, and then merrily flying back. I've been tweaking this thing for hours, and while there definitely are differences in behaviour, the differences maybe amount to a 200-400 m/s difference in dV. That's not what you would expect of a 15% weight reduction <.<

Clearly I'm missing something, any Spaceplane guru's here with brilliant ideas on how to make this thing haul it's cargo up there? I would like it to be able to take at least 18 tonnes up, and get back down while in powered flight. It's 'almost there' but that almost bit is a brain-cracker.

((note, CoL is a little bit behind the CoM, almost touching it)

Edited April 30, 2015 by Merandix

[Blu_C]

Hmmm... it looks like you should have plenty of thrust. What does Kerbal Engineer say your t/w ration is and your deltaV? I'd be curious to know though I doubt either of those is the problem. I'd especially like to know when you swap over to rocket mode.

Also can you describe exactly what you mean by not being "quite able to make it"? Are you getting suborbital? Able to start circularizing but not completely accomplish it?

[Merandix]

It says in the screenshot. It's not Kerbal Engineer, but RCS-build-aid works fine for me too.

no load = TWR of 1,92

18 tonne load = TWR of 1,64

36 tonne load = TWR of 1,43

Just did a testflight without cargo... Holy crap that thing gets off planet quick... faster than most rockets <.< I believe I had circularised in under 4 minutes. Has plenty of manoeuvring fuel left. Not a lot, but enough to get from 75 to 150 km circular orbit, and back, plus a powered landing. That's the sort of difference you expect from an 18 tonne weight difference. Though admittedly, going from 75 km to 150 and back was about 300 dV... so I may be underestimating the difference 300 dV makes. So that answers part of my question...

@Blu_C

I usually have to grab the fuel from the cargo-tank to finish circularisation. Though admittedly with 36 tonnes it was true suborbital before I grabbed the fuel from the orange can to get there, and ended up with 2/3rds of said fuel left... Which obviously does not take into account that I would otherwise also need to get back to kerbin I honestly think I've been underestimating the difference a rather small dV number makes.

Now, for part two of the question, can this thing ferry the intended 18 tonnes up? Or do I need a full redesign?

[Blu_C]

I'd have to fly it myself to be sure, but I came up with several ideas:

1) You are over-thrusting while under a lighter load. I sort of doubt this is the problem, though, as under the new aerodynamic model it is much less significant than it used to be.

2) With those heavier loads you have a TWR of around 1.5, which is what I tend to build my rockets with. In that case the effective payload of your space plane is probably going to be similar to that of a rocket (you know, 90% of the mass needs to be lost on the way up). I'm going to guess that the plane as a whole weighs something on the order of 80 tonnes with no payload (judging by the screenshot). How much of that is fuel? Any weight you can cut from the craft's total weight will probably help you a ton in the long run. I might even go so far as to try and take smaller wings (you will be MUCH lighter when landing so it may be ok). If necessary any weight savings can maybe even be put into fuel.

3) It may be a fundamental flaw with the design. I don't SEE anything wrong, but I tend to favor smaller space planes using the MK2 parts, so this is a bit outside my usual scheme. If that happens to be the case I might recommend taking more RAPIERS and fuel with the base craft, maybe even pushing the TWR up as high to a maximum of 2.5 with sufficient deltaV to reach orbit and then some.

[Merandix]

I simply want too much I guess. Performs ok with only 10t cargo. Will use it for a 1.0 K-prize entry. Still, if anyone knows the key to heavy lifting with space planes in orbits a bit higher than 75-80 ish km, please let yourself be known

[capi3101]

About where is your takeoff speed, and about how high up are you/what's your speed when your RAPIERs switch over to closed cycle mode?

I have my own set of design standards that I used to use for pre-1.0 FAR (and pre-1.0 stock for that matter); I don't know if the old FAR standards apply yet to 1.0 or not but given that the drag model is now similar, I imagine they could be applicable. You've got an eighteen tonne payload in your screenie - from that I gather the unloaded weight of the plane is around eighty tonnes, right? In old FAR, I'd have planned for a payload fraction of 15% (about the same you'd see from the SR-71 or Concorde), so for eighteen tonne payload, I'd have planned to build a 18/0.15 = 120 tonne plane. Then with RAPIERs for engines, I'd have planned for 9.5 tonnes per RAPIER and round up - 13 RAPIERs. Lastly I'd have planned for 40 units of Oxidizer and an equal amount of Liquid Fuel per tonne - 120 tonnes * 40 = 4800 units. I'd have then built a main fuselage designed to handle the payload, distributed the fuel in a "fuel fuselage" along the sides, and then used a bunch of equations to try and guess a "best fit" wing geometry that would give me low wing loading and a moderate aspect ratio - the details of that aren't terribly important.

For the orange tank - double all those numbers...

With this design, since your plane is eighty tonnes in the SPH, I'd expect it to lift not much more than twelve tonnes (80 * 0.15 = 12) of payload. I'm guessing you've got an excess of thrust leading to excess drag at altitude, and not enough fuel to lift the kinds of payloads you want to lift with the plane.

Caveat: In all my FAR testing, I never got a plane into orbit with a payload of more than six tonnes, though I was in the initial stages of planning to lift an orange tank to orbit immediately prior to the 1.0 release. So take my advice with a grain of salt.

I did, however, get an orange tank to orbit intact with the old stock aero...

Edited April 30, 2015 by capi3101

[Merandix]

Yeah, this was the bit where have to get used to the new aero... However, the old aero kind of brutally said 'no' if you went too heavy on the load, taking off too heavy = fireball at the end of runway. So it sort of surprises me that it still takes off with 36 tonnes in there no problem

Last variant like this in 0.90 put 27 tonnes into orbit with less fuel, but would hardly get off the ground with 36 tonnes in it. This one is, as you say, is more fit to carry 12 tonnes max into orbit... but gets 36 tonnes of payload off the ground (and in a suborbital flight path) no problem (just not in an orbit).That's where the confusion was coming from. Because if it NEARLY makes it to the intended altitude, circularising shouldn't be that big of an issue, right? Well, that was where my assumption went haywire.

Anyway, to answer your questions: Takeoff speed is in the 90-120 m/s range, about halfway down the runway. But I'm generally quite gentle on pulling up, she'll likely fly earlier if I push her. RAPIERS switch over to closed cycle at about 950-1120 m/s and between 23-25 km in altitude. My piloting is a major factor in how well it performs.

To be brutally honest, I usually just sort of eyeball it and then start tweaking it till it works

[capi3101]

To be brutally honest, I usually just sort of eyeball it and then start tweaking it till it works

Perfectly viable way to play the game. And I've been told I rely too heavily on the math before...

Anyway, to answer your questions: Takeoff speed is in the 90-120 m/s range, about halfway down the runway ... RAPIERS switch over to closed cycle at about 950-1120 m/s and between 23-25 km in altitude. My piloting is a major factor in how well it performs.

Okay...so the takeoff speed would suggest you've got about the amount of wing you need. I would think that you'd want to be going higher and faster before switching over to closed cycle, though; don't know if the new stock will let RAPIERs get going faster or not.

Anyways, 950-1120 m/s at switchover gives you a requirement of somewhere around ~1250-1450 m/s of delta-V to make it into space. Your plane is a rocket at that point, so Tsiolkovsky will be able to tell you how much fuel you need once you're up at that point. Long and the short of it - your plane needs more gas, IMHO.