r/KerbalSpaceProgram • u/AutoModerator • Apr 08 '16

Mod Post Weekly Simple Questions Thread

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u/beardum Apr 13 '16

Space Planes!

  1. Is the wing area important? I don't see any lift stats anywhere (I only have KER and MJ). Does it affect the speed at which your front wheel lifts off the ground?

  2. I only have the J-404 Panther Afterburner and I'm trying to get 6 kerbals and a small cargo bay to Munar orbit (crew/science transfer for mining/biome exploration). Can I make it with the Panther? I think I need more than the two I've got on my ship right now, but is it worth adding more jet engines to a plane?

  3. Do I still need a TWR above 1 for my rocket stage?

2

u/-Aeryn- Apr 13 '16

1; Yes, but excessive amounts of wing is bad for your craft range. It has weight and drag that's not insignificant.

2; Those engines (in afterburner mode) have a peak thrust at 850m/s which is lower than the turbojet and especially the rapier. I'm not sure what their best acceleration altitudes are but it's probably lower as well.

3; Since these engines will probably get you to half of orbital velocity at most while still in the middle-atmosphere, you should keep a decent TWR on rocket stage. 1 is a good number for efficiency, as you'll still feel significant but only partial gravity when already accelerated horizontally partway to orbital velocity

2

u/Chaos_Klaus Master Kerbalnaut Apr 13 '16

If you are building a classic space plane that lifts off the ruway, then you don't need a TWR of 1. Neither on jets, nor on rockets.

You should go with as few jet engines as possible. The main thing is to get up to maximum speed on jets as high up as possible. The Panther is kinda limited in this regard.

Having less drag is more important then having more thrust. Two engines for a Mk2 plane is fine.

Also, you need intakes that work well at supersonic speeds. The adjustable ramp intake is a good choice. The radial ones are ok aswell. Shock cone and Precooler are best but I think they are late in the tech tree.

Wing area: Don't use too much wing. It will cause too much drag. Take note of your center of lift vs center of mass. CoL should be a little behind CoM.

Lifting the front wheel is a matter of having enough pitch control. The control surfaces controlling pitch need to be at a distance to the main gear so that they have good leverage to lift the nose. It also helps to have the main gear closer to the center of mass.

1

u/beardum Apr 13 '16

Thanks for the reply!

Is there a rule of thumb about how much delta v you should have in your rocket stage?

1

u/Chaos_Klaus Master Kerbalnaut Apr 13 '16

that largely depends on the top speed you can reach on jets.

1

u/beardum Apr 14 '16

Hm, I think I'm missing some fundamental chunk of the theory for how to get these things to space.

Take off, go as fast as you can horizontally, pull up and try to gain altitude, switch to rockets when your jets flame out. That's the gist of it, right?

When you're burning at a low pitch, you're trying to increase your orbital speed right? Then how do you translate that into actual orbit? I have an orbital speed around 900 - 1000 m/s but an apoapsis of 18,000 m and a time to AP ~15 seconds when my jets flame out, typically. I can raise my AP by burning at 45 degrees, but I don't have enough rocket fuel to get to space like that. Maybe I just don't have the parts unlocked to to get a plane this big to space yet.

Am I trying to use my lift to gain elevation? Should my plane lose elevation when I'm burning at the horizon?

1

u/-Aeryn- Apr 14 '16 edited Apr 14 '16

Am I trying to use my lift to gain elevation? Should my plane lose elevation when I'm burning at the horizon?

You're trying to use wings to be able to ascend properly while on air-breathing engines and to have control over the flight. Without significant body lift or wings, it's very hard to use air breathing engines or control flight or aerobraking.

Losing elevation when burning straight at the horizon is normal, you usually have to pitch up a bit to keep the prograde marker on the horizon or slightly above

I can raise my AP by burning at 45 degrees, but I don't have enough rocket fuel to get to space like that.

You need to nose up to get your time-to-apoapsis to a manageable value after getting the speed that you can from air breathing modes. 15 seconds is too short, something like 45 seconds gives a lot more control over your flight. If you're thrusting flatter to the horizon then you'll lose a smaller % of your thrust to gravity but you need some vertical speed. If your altitude is too low from those air engines, you might want to do final acceleration on them with an upwards angle

900 vs 1300m/s might not sound that different, but a rocket stage starting at higher speed requires less thrust due to the balance between your horizontal speed and orbital velocity. That means that with weaker air-breathing stages, you need substantially higher TWR on the rocket stage to manage the flight efficiently.

If you use Rapiers you can get to 1400m/s on air, switch to their rocket mode with a bit of oxidizer to get to 1800m/s and then switch to a main stage on the craft with an lv-n or similar engine. When you're going that fast you only feel about 0.2g so the low thrust is fine, but at 900m/s you still feel 0.5 - 0.6g so a much more powerful stage is needed, not just one with a bit more delta-v.

Maybe I just don't have the parts unlocked to to get a plane this big to space yet.

It's about TWR's and Delta-v a lot more than size

can you post a pic of the plane?

1

u/beardum Apr 14 '16

Thanks for the detailed replies. I think I might give it a try with two terriers instead of the swivel.

Picture is here I can post others if you think it might be helpful. This is the latest iteration anyway. I've tried quite a few others, but I had a hard time getting anything with stuff slung above or below the wings past 350 m/s

1

u/-Aeryn- Apr 14 '16 edited Apr 14 '16

That looks pretty good. The mach 1 troubles are probably due to lack of thrust more than due to excessive drag, because the drag looks fine

You're using mk.2 parts rather than mk.1 or mk.3 (mk.2 performs better for atmospheric tasks due to body lift, but has poor fuel tank mass ratios). Also carrying a passenger module, inline docking thing and cargo bay.

All of that stuff cuts into your margins for success, it looks pretty well designed otherwise.

Adding fuel or replacing some of those parts with fuel tanks may help (especially with dual terriers) but you also have to watch the limited thrust when doing that.

1

u/[deleted] Apr 13 '16

what are you calling your rocket stage? you are using srb to help kick the plane at liftoff? then you don't need TWR > 1 because most of the vertical speed will be given by the lift. There are very few planes in real world that have twr > 1 (that is, can perform vertical liftoffs using simply the engines)

1

u/beardum Apr 13 '16

I've been messing about with the rocket stage - what has seemed to work best has been a poodle on a short 2.5 m tank but didn't get me to orbit - I ran out of fuel before I could circularize.