Help with spaceplane design

[Pahimarus]

Hi all,

I'm a fairly experienced kerbal player but novice when it comes to aircraft. I watched Scott Manley's playthrough and am attempting to build my first spaceplane. So far I haven't been able to get my apoapsis above about 45k. My flight profile is to climb to 10,000m, accelerate until I am about 1100 m/s and my whiplash thrust is >300KN. I then pitch up and climb at about 20 degrees activating my rocket engines when my whiplash thrust drops below 150 KN and then close the intakes and shutdown around flameout. I'm also having trouble with some roll instability all throughout and overall flight instability between about 20 and 35km. Thanks in advance.

Design

[Spricigo]

Bring me memories 

 

[bewing]

Yeah, I'm sticking with what I said on Spricigo's thread.

 

[IronMaiden]

2 hours ago, Pahimarus said:

Hi all,

I'm a fairly experienced kerbal player but novice when it comes to aircraft. I watched Scott Manley's playthrough and am attempting to build my first spaceplane. So far I haven't been able to get my apoapsis above about 45k. My flight profile is to climb to 10,000m, accelerate until I am about 1100 m/s and my whiplash thrust is >300KN. I then pitch up and climb at about 20 degrees activating my rocket engines when my whiplash thrust drops below 150 KN and then close the intakes and shutdown around flameout. I'm also having trouble with some roll instability all throughout and overall flight instability between about 20 and 35km. Thanks in advance.

Design

Try to change your mindset. Don't think about it as "getting your apoapsis up to 70km", think about it as "getting your horizontal velocity up to orbital speeds." The most efficient space plane designs will reach orbital velocity well inside the atmosphere. You have wings so use them, let them handle the altitude by creating lift and let your engines focus on your horizontal velocity. In other words, keep your pitch low. You should never be above 10° after reaching 15km, preferably below 5° but this will require some wing angle of incidence. 

The most glaring problem with your design is the engine mass. Your plane's total mass is 27t, and you have 9.6t dedicated to engines. That's over 1/3rd of your total mass, not even rockets need that much thrust. You could either get rid of engines or carry a lot more fuel and still have plenty of thrust to get to orbit. You'd benefit greatly from some wing incidence, this will allow the plane to have a lower pitch (less drag) but still create enough lift to stay airborne. You also have a lot of extra drag with the solar panels and RCS blocks, and the non-inline version of the cockpit is more prone to over heating which requires a less efficient ascent profile. Flawed as it is though, it should still be able to reach orbit if flown correctly.

The flight profile should look more like this:

• climb past 12km and drop pitch below 5°
• accelerate up to at least 1400m/s, preferably closer to 1500m/s as you very slowly (no more than 50m/s vertical speed) climb up to around 18km
• as you approach 1400m/s and your acceleration slows, start the Nervs
• maintain a slow, steady climb as you continue to accelerate; as long as you have more thrust than drag you will reach orbit
• this is all much easier with trim (alt+q,w,e,s) than SAS, unless you have wing incidence, then it's even easier to just lock surface prograde all the way to orbit

If you want a lot more tips like this and more indepth stuff, check out this SSTO tutorial. I use RAPIERs in that design but all the concepts are valid, just the numbers will need to be tweaked a bit since the Whiplash isn't as fast or powerful.

[Pahimarus]

Hi all,

Thanks for the advice. I did, in fact, go to space today.

It still handles like a pig until you get high into the atmosphere and I had to fly it perfectly to make it to space but it did it. Improvements and flight profile essentially following IronMaiden's advice to the best of my ability. Further suggestions on how to improve are welcome.

Redesign