Heavy SSTO spaceplanes

[DoToH]

I play KSP since 0.17 or 0.18, mostly sandbox mode, with some occasional career mode tries, because I never found career funny. I can go and return almost anywhere in kerbol system with rockets (didn't bother to return from Eve, but it's a matter of packing more DV...)

With 0.90 I decided to give creer mode a serious try, and now I'm stuck with something didn't bother to do before: make a SSTO spaceplane capable of lifting heavy payloads. I have some desings that get to orbit, but without payload and with little DV to spare. I can lift more than 300 tons with SSTO or multi stage rocket lifters, but that approach is too much expensive for early/mid career.

I read some threads about optimal ascent profiles with spaceplanes and many people say to climb to 35K or more on jets. My spaceplanes only get to 25K on jets, then flameout makes impossible to climb more. At 25k I can't build up enough horizontal velocity to reach orbit easily.

So.... how do you manage to get higher than 30k on jets?. The only way a ever got that high on jets was with a little spaceplane fully covered with intakes, and I want to stay away from intake spamming if posible.

Important note I forgot to mention: stock aero. Never used FAR (only made 3-5 planes before) and I want to see how the aero for 1.0 behaves before installing FAR

Any help is welcome

Edited March 1, 2015 by DoToH

[Wanderfound]

The absurdities of the stock drag model are particularly punishing to large spaceplanes. While it is possible to make alarge spaceplane in stock, it ain't gonna be easy, and it won't be very useful when you're done.

[Alshain]

The first thing you need to understand is the way the game currently allocates air. If you place intakes and engines in symmetry, they will ALWAYS be directed to one engine, any air "overflow" goes to the next engine and the next and the next and so on. This leads to your asymmetrical flame outs. The way to get around this is to turn off symmetry mode and place Engine -> Intake -> Engine -> Intake because the last engine placed is the one the intake is assigned to.

You can also do Engine -> Intake -> Intake -> Engine -> Intake -> Intake if you want more intakes.

The last options is a mod called Intake Build Aid.

(Note that this problem will affect you in stock or with FAR)

[GoSlash27]

The way I do it is by limiting my climb rate. NMT 500 m/sec up to 25 km altitude, then NMT 100 m/sec up to 32 km altitude. Once at 32 km, maintain altitude until periapsis is established, then use just enough throttle to maintain it.

It sounds like you're climbing too rapidly, which will cause you to be too slow at altitude, which runs you out of air.

You need to have low enough wing loading to climb efficiently at terminal velocity (about 1.0 Cl per tonne), enough intake area (about .018 m^2 per engine) and enough engine (about 1 turbojet per 15 tonnes or 1 RAPIER per 10 tonnes.

As an aside... lifting 300 tonnes with a spaceplane sounds like a nightmare. A vertical turbojet lifter would make a lot more sense for that job IMO.

Best,

-Slashy

[DoToH]

Engine -> Intake -> Intake -> Engine -> Intake -> Intake

Worked!!! Reached orbit with 1300 DV to spare with a desing that barely made orbit before.

As an aside... lifting 300 tonnes with a spaceplane sounds like a nightmare. A vertical turbojet lifter would make a lot more sense for that job IMO.

300t rocket lifter was a way to say that I can handle medium complexity rocket designs without problems. Lift the same payload with planes is way off my objective. I just want to do some satellite and rescue missions (and maybe refuel my station up to a orange tank) to LKO without needing the old "go out and push" to deorbit.

Edited March 1, 2015 by DoToH

[Pecan]

If you can land reasonably close to, or at, KSC a SSTO rocket only needs an extra 200m/s dV or so to do a 2-drogue-assisted powered landing for ~98% recovery of costs.

[DoToH]

I can land a heavy SSTO rocket at KSC, but my fuel efficiency at precision landing is terrible. Part recovery would be ok, but fuel costs....

[capi3101]

Design does play a lot into how well a given spaceplane will perform. Since you're in stock aero, I can give you guidelines; I recently made the switchover to FAR and am still learning there.

So I design stock aero planes along these lines -

* 25% payload fraction. A lousy spaceplane can usually still hit this threshold; most plane designs will actually get you at least 27% or higher (as compared to 15% for asparagus). Start with your payload design, multiply it by four, and assume that'll be the final mass of your plane. So if you've got a 300 tonne plane, you better not be trying to lift more than 75 tonnes of payload...

* Pick what kind of engines you're using. If you go with RAPIERs, you want one for every full thirteen tonnes; if you go with a combination of Turbojets and rockets, you'll want one Turobjet for every full fifteen tonnes. Round up to the next multiple either way. 300 tonnes = 20 Turbojets. That rocket's starting to look a LOT more attractive...

* You'll want an intake area of .03 per Engine; this is not airhogging. I've done planes with nine Ram intakes per engine before; three per engine is WAY easier to design and generally looks cleaner too. What you can do is build "engine units" starting with an Engine Nacelle along your craft's centerline. Slap an engine on the end (if you're going with Turbojets and rockets, put a trio of 24-77s with each jet - that's probably more than you really need!!) and then move the Nacelle off to the side of the craft. Put a tricoupler on the other end, and then put Ram Intakes on that. That'll get you a good Nacelle design. Of course, what Alshain told you is 100% true - if you just leave it at that without rebalancing the intakes and engines, you'll be more subject to asymmetric thrust. So, use the engine units for design purposes, and then tear off the Intakes and Engines and rebuild them properly, or get Intake Build Aid - I'd recommend the latter personally; that mod is just FAR too useful for spaceplane users.

* Fuel wise, you'll want 40 units of Liquid Fuel per (theoretical) tonne of craft, and 25 units of Oxidizer per (theoretical) tonne of craft. So 300 tonnes - you'll want 12000 units of Liquid Fuel and 7500 units of Oxidizer. My advice here is to use rocket fuel tanks and drain off some of the oxidizer - big thing there is that it'll keep your part count down. Which brings us to...

* 1:1 Total Lift Coefficient per tonne. If you've got a 300 tonne plane, you want the total of the lift coefficients of all your wings and lifting bodies to add up to 300. Delta Wings have a lift coefficient of 2, so you'd need 150 Delta Wings at a minimum.

Now, I won't discourage you from trying to build a monstrosity like this. I will say that I myself have successfully lifted a plane to orbit that was about 130 tonnes before, with the purpose of lifting a full orange tank to orbit -

If you're serious about wanting to lift payloads with planes in stock aero, I can't do much better than to suggest you check out DocMoriarty's KSP Space Plane Construction and Operation Guide. It was designed for stock aero and focuses on use of RAPIERs, and it has an extensive section on the design and flight of transporter spaceplanes. It was written for 0.24.2, but the general principles inside it will be applicable until the current stock aero is replaced in KSP 1.0 (I must be the only one who thinks Squad should go to 0.91 from 0.90 instead of 1.0, but that's a discussion best left for a different thread.)

Anyways, good luck. If you do get the 300 up, let us know - cuz that would be a sight, no doubt.

EDIT: Incidentally, if you need a rocket booster that'll lift a 300 tonne payload, holler; I designed one a few months back that'd do 450 without breaking a sweat. I'd have to find that thread again, but...

Edited March 2, 2015 by capi3101

[Ion Man]

The intake build aid, as mentioned above, is very nice. Just let's you assign everything very nicely. You just need to do every engine and intake piece by piece. I assign two shock cones to each engine and after I either stage to rocket engines, or switch the mode on the rapiers with action groups assignment, I have the intakes assigned to another action group to close them, reducing drag between 32 and 60k km.

If you use FAR, your engines will run out smoothly and you won't need much micro management. Closing the intakes is always a good idea, though.

[LordFjord]

The intake build aid, as mentioned above, is very nice. Just let's you assign everything very nicely.

Cheers and thanks

You just need to do every engine and intake piece by piece. I assign two shock cones to each engine and after I either stage to rocket engines, or switch the mode on the rapiers with action groups assignment, I have the intakes assigned to another action group to close them, reducing drag between 32 and 60k km.

You don't really need to manually assign assign intakes to engines (and its buggy). Just hit the button to automatically distribute the intakes to engines or hit F7.

It doesn't matter if the intakes that are feeding the engine are not in line with them or where on the vessel they are placed. It's all about the intake area and that it is equal for each engine.