Efficient Flight Plan For Space Planes?

[BurningSky93]

I've made it my current objective to build an orbital space plane. Now, I've seen in some threads people build itsy-bitsy teeny-tiny spaceplanes that can achieve orbit, and frankly, I'm envious because I just can't do it.

Even copying (or to make myself feel better, reverse-engineering) these designs, I've found I'm still not capable of getting a spaceplane into an orbital trajectory. This means that there's something wrong with the way I'm flying the planes (assuming that these people who achieve orbit aren't exploiting the fuel bug).

I'm currently using mixed jet and rocket engine aircraft.

So, what I'm asking is, what's the most efficient flight plan for space planes? I'm asking for quite a bit of detail here, so something like:

"Fly at theta degrees to the horizontal until you achieve an altitude of Y metres, then ignite your rocket engine... etc etc."

Obviously there will be variations depending on the actual aircraft, but a good general outline would be very helpful.

[silent_prtoagonist]

I've found that the most efficient trajectories are almost the same as the ones I use for rockets. For instance, try pitching up to 60 degrees as soon as you've cleared the runway, then slowly pitch down as you ascend, aiming to be at 45 degrees at 15km altitude. Turn on your rockets at ~12 km, turn off your jets at about 15 km. After that, just keep pitching down, aiming for 30 degrees at 30 km, and level at 45 km, then keep accelerating until your apoapsis as where you want it, coast and circularize.

Also, don't try those super tiny designs at first. I find a reasonable one-kerbal design to reach orbit without too much difficulty usually has about 5 standard tanks of fuel (2000L), to give you an idea of what ballpark you should be in. Once you've figured out how to get that into orbit, start optimizing your ascent, and shaving off fuel until you've worked it down to super-tiny.

[vexx32]

Well, atmospheric engines are typically good up to around 20,000m. From there on, ideally you'd want to be flying East (090) on the ascent, most probably with about a 30-45 degree inclination as you reach the 20km mark. As you pass 20km, I'd advise you switch off all your atmospheric engines and enable rocket engines to provide the requisite thrust. Depending on how you want to do it, you can then use thrust-vectoring rocket engines to pitch up again to about 60 degrees inclination and then burn to an orbit, or try to go vertical and gain a fair amount of altitude before stabilising your orbit.

[bsalis]

It depends on the spaceplane you have. I assume you are talking about an SSTO with jets that can do TWR > 1?

In that cast like others have said. Go vertical after takeoff, then kick off the rocket engines at about 10Km and start your gravity turn. Then disable the jet engines once they drop to below 20% efficiency (which is easy to do if MechJebing, but no harm leaving them running until coasting-to-Ap if doing it manually)

[Suzaku]

Some of those spaceplanes are exploiting the .16 Fuel bug. It is hard to pick out which ones actually work from the ones that are cheats. Wait until .17 before looking at those spaceplanes.

[Serratus]

I have an SSTO, and I just go 45deg inclination from the get-go, right up until my Ap gets to target alt. I personally found it that - at least in SSTOs - adding any atmospheric engines is a waste of time and fuel. They only work fraction of the way up, and after that You have to haul them all the remaining way up (and down if You want to glide!). In non-SSTO designs, they are useful if You drop them at ~15-20Km.

Edit: My wisdom has been shared! And now I have non left

[BurningSky93]

Do you guys fair better with purely rocket space planes or with mixed engine types?

[Serratus]

Tell us if You want to make an SSTO or normal space plane? I'd say if SSTO then go with only rockets (I prefer aerospikes). If not SSTO, then by all means, use air-breathing engines.

[BurningSky93]

Would some of you mind sharing some of our more basic, easy to fly designs, both regular and SSTOs? I've not really seen any good examples of SSTOs.

I also keep having this problem of my planes seemingly having, well, zero lift. Even though I have plenty of wing surface, they often end up going off the end of the runway before taking off, which has resulted in exploding streaks many times.

Is my memory correct in recalling that lift mechanics are being overhauled for .17?

Edited August 30, 2012 by BurningSky93

[EndlessWaves]

I personally found it that - at least in SSTOs - adding any atmospheric engines is a waste of time and fuel.

The thing about jet engines is that they're incredibly efficient and can get you to 10-15km on a fraction of the fuel (and thus weight) of a rocket engine. Think of them as longer burning SRBs.

I asked a very similar question a week or two ago and didn't get much in the way of answers, I think everyone must still be experimenting.

From my experiments I've concluded the following:

1. Wings are essentially wind deflectors and only produce any effect when at an angle to the airstream. You'll never take off with wings parallel to the runway - you either need to rotate the entire machine back or place the wings at an angle so they generate lift when flying horizontally.

2. Wings seem to have some sort of thrust multiplier effect. I've built machines with a sub-1 TWR that can lift just over 100 tons on a single 100 thrust jet engine. Unfortunately of the machines I've built so far the aerodynamics have always interfered with the ability of such a weak engine to rotate the machine into a correct angle for rocket and/or cause unstable upper atmosphere flight after the rockets were lit.

3. The number of wings required for takeoff seems to scale roughly linearly with speed and weight. Doubling the number of wings will halve the speed needed for takeoff, while doubling the weight will double the number of wings required for the same takeoff speed.

4. Around about 25-35° seems to be the optimum wing angle to generate lift and it drops off sharply over 45°.

Edited August 31, 2012 by EndlessWaves

[vexx32]

#4 isn't entirely correct. Look at the Magic Turbines. They generate lift constantly, practically regardless of the angle they're at, so long as they're moving. Last I heard, the list scales entirely linearly, with no "stalling angle" as you'd expect from real-life scenarios.

[BurningSky93]

#4 isn't entirely correct. Look at the Magic Turbines. They generate lift constantly, practically regardless of the angle they're at, so long as they're moving. Last I heard, the list scales entirely linearly, with no "stalling angle" as you'd expect from real-life scenarios.

Whoa, I might be misunderstanding, but are you saying the lift produced increases as the angle of attack increases i.e. it's maximum at 90 degrees?

[silent_prtoagonist]

Would some of you mind sharing some of our more basic, easy to fly designs, both regular and SSTOs? I've not really seen any good examples of SSTOs.

Here's and old design of mine, pretty gentle to fly and plenty of fuel margin.

http://kerbalspaceprogram.com/forum/showthread.php/11214-The-K-Prize-100-reusable-spaceplane-to-orbit-and-back?p=168116&viewfull=1#post168116

[bsalis]

... I personally found it that - at least in SSTOs - adding any atmospheric engines is a waste of time and fuel ...

Yes although that may be true from an efficiency point of view that might make sense but from a fun point of view it might not. Personally I like to fly my SSTO under jet power both before and after orbit. After takeoff I often turn around and do a low level pass over the KSC facilities before heading to orbit. Why? Because I can.

I'll post that SSTO this evening, with the matching re-fueling truck.

EDIT...

Would some of you mind sharing some of our more basic, easy to fly designs, both regular and SSTOs? I've not really seen any good examples of SSTOs.

Here it is...

It is a pretty simple, and easy to fly SSTO due to ample control surfaces. It...

* Is easy to keep straight on the runway when taking off

* Will get airborn before the end of the runway. To do this don't pull back until you are near the end.

* Will fly well when full of rocket fuel. It's a little nose heavy but that will just help to stop it flipping.

* Can climb vertically under jet power to 10km. Sometime during this ascent, disable the jet fuel tanks to preserve some fuel for flying upon return. Engage the rockets past 10km, then disable the jets when the efficiency drops off. Be gentle with the gravity turn once you lose the directional jet thrust.

* Will easily get to 100km circular orbit with fuel to spare for return. Remember to enable the jet fuel tanks and jet engines once the rocket fuel is all spent.

* Can (just) do a Mun free return - I have done this.

* Is not too hard to transition back to flight after re-entry. I'm not sure if any spaceplane makes this pleasant.

* Is easy to land when empty of rocket fuel since there is plenty of lift for the weight. It is also more balanced when empty, so be careful not to flip it when flying fast. Also, since theres ample control surfaces, best to fly now with Caps on for the finer control.

Note that this spaceplane uses the MechJeb and Fuel Transfer plugins. The tanker also uses the Carts mod.

Edited August 31, 2012 by bsalis