Spaceplane Launch Profiles

[DChurchill]

So, I've tried spaceplanes without too much success. I've gotten to LKO a couple times with barely enough dV to deorbit. This time, I started out with the turbojet/ramjet/aerospike and once this latest career got to the point where I had rapiers, I replaced all that with a pair of rapiers.

My problem is this: my results are very inconsistent. One time, I'll get it to 80K without enough dV to circularize, the next I can't seem to get to out of the atmosphere. I'm sure it's the profile I'm flying. And, honestly, my profile is not really thought out.

I'm tired of not getting to orbit. Can someone help me out?

[AeroGav]

This is a series of videos showing me launch a Ramjet/Nerv spaceplane.  Of course it's likely to fly very different to yours.

General tips - use mechjeb if you're in the experiment phase, makes things more repeatable and less effort.  

Adjust angle of attack to get the best lift/drag ratio you can , this minimises "gravity losses", though also bear in mind thrust/drag ratio (might be worth going supersonic at 2.5:1 lift drag ratio if it enables your engine to get in the power band, eg. having 500kn thrust and 200kn drag vs  50kn thrust with 40kn drag subsonic with 9:1 lift drag ratio)

Overall though, don't fret too much about when to go supersonic.  80% of the fuel is burned after your air breathers shut down.

What matters is 

a) getting as much speed and altitude out of the air breathers as possible, so the rockets have less of a gap to bridge to orbital velocity

b) how you fly the rocket mode ascent

 

for a), know the optimum speed for each engine, where is makes max power. There's a thread on that somewhere .  Ramjet gives max power at mach 3, Rapier mach 3.75.      Make sure you're at a decent alt (17km+) before hitting mach 3+ or you'll melt.   Once you have hit optimum speed, you can either accelerate in level flight, but every knot faster will result in less thrust and more drag, or you can climb while maintaining same speed.   The thrust starts halving every 2km above 22km so generally i accelerate mostly level here, until we're getting near mach 4.5.   After that point, jet engine thrust really falls off a cliff so use any excess power to climb

for b), if you have a high thrust weight ratio, it doesn't make a lot of difference how you fly and it's probably more similar to a rocket ascent. But you might be penalising yourself with excess engine mass.  If thrust weight ratio is lower, then  managing   lift:drag, AoA and temperature is absolutely paramount.  Watch the second video in that series for details.

Edited May 23, 2016 by AeroGav

[bewing]

It really depends a lot on whether you are building an MK1, MK2, or MK3 spaceplane. MK1 parts have much lower drag than the others, and not much fuel -- but I think they are the easiest. MK2 parts have much higher drag, and store the same piddly amount of fuel -- so they are probably the hardest to get to orbit. MK3 stuff is for going totally overboard -- the main problem there is flyability and durability, I think.

After that, I find that the whiplash engines give the best airbreathing properties. If the nuke rocket engines can give you enough thrust to accelerate, then usually they outperform the rest. If that's not enough thrust, then maybe a vector?

And you must have enough wing to lift you efficiently through the first 10km of atmosphere. Then it's just a matter of minimizing drag, part count, and cost.

So it comes down to some design considerations: how many kerbals? Do you want to lift cargo? And if so, how big?

 

[mk1980]

in my experience, you can pretty much brute force your way out of the atmosphere with rapiers or a combination of aerospikes & whiplash. basically, just put a shielded docking port on the nose of the plane so it doesn't burn up at high speeds. my usual ascent profile is to climb up to about 10 km fairly steeply, then level out, breach the sound barrier and resume a shallow climb to gain as much speed as possible before switching over to rockets. 

the high heat resistance of the shielded docking port makes sure your plane can survive the heat, so you can "milk" the jet engines quite a lot longer than with the mk1/mk2 cockpits. it's not an elegant solution, but it gets the job done efficiently. 

the key here is that jet engines deliver lots of thrust at high speeds and as you climb higher, drag gets lower, so you can keep accelerating even though the engines lose thrust at higher levels.

with a shallow climb from ~10 km upwards, rapiers typically top out around 1600 m/s at 20km+; the whiplash is less efficient at high speeds and tops out around 1350 m/s. from there, you basically switch over to rockets and keep accelerating. not really sure if it's worth climbing steeper when you switched over. i guess it depends on the arodynamics of the plane. don't try that with an mk1 or mk2 cockpit at the nose of the plane - they will just explode (no idea if the mk3 cockpit is any better, i don't think i ever used that thing)

the nice thing is- since you're already at 1600m/s before you switch over to closed cycle, you don't need a whole lot of oxidizer to accelerate further until you reach orbital speeds.

i'm sure a more elegant solution with optimzed drag and angle of attack can get the job done with less fuel consumption, but the simple brute force approach works pretty well with anything from small crew shuttles that only transport some kerbals to giant cargo planes that lift substantial payloads to orbit.

also, the docking port obviously comes in handy once you're in orbit and it basically works a heatshield during reentry.

[DChurchill]

This is exactly what I needed. Thanks, guys.