Help with Mk3 SSTO

[ScottyDoesKnow]

Having a lot of trouble with this. I stole the wing design from here: http://forum.kerbalspaceprogram.com/threads/121328-Mk3-VTOL-SSTO-with-rotating-engines

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(Last picture is of old version, HyperEdit'd into orbit)

But I just can't get anywhere with this thing. I looked up how to build a plane, and moved the wings back so lift was behind CoM. But no matter how much or how little I tilt up the same thing happens:

1. Quickly get up to ~300m/s

2. Rise slowly to ~10km, slowing down all the time

3. Start losing altitude

I removed a lot of the fuel, emptying the back tank, removing half the oxidizer from the main tank, removing all oxidizer from the wing tanks. I made all the horizontal controls not do yaw and the tails only do yaw. Don't know what I'm doing wrong.

Download: https://dl.dropboxusercontent.com/u/16399537/ksp/SSTO%20Mk%2001.craft

Edited May 22, 2015 by ScottyDoesKnow

[archnem]

Usually when I have the exact problem you're describing it's one of two things:

1) Not enough lift. Add moar wings!!

or

2) Not enough thrust. Add moar engines!!

Or you know, both.

Just from eyeballing your design, I'm guessing more wings should help.

[cybersol]

More engines would fix it, but you can probably nurse it to orbit. Start by rising as fast as you can while keeping speed below 260 m/s (because drag is much higher in the region above there). When you get up to 13 km, level off to +5* or even less if needed and accelerate through transonic to 450 m/s. Then raise up again an gain speed until you the air-breathers don't accelerate anymore.

[ScottyDoesKnow]

More engines would fix it, but you can probably nurse it to orbit. Start by rising as fast as you can while keeping speed below 260 m/s (because drag is much higher in the region above there). When you get up to 13 km, level off to +5* or even less if needed and accelerate through transonic to 450 m/s. Then raise up again an gain speed until you the air-breathers don't accelerate anymore.

Switched the adapter to use 4 jets instead of 2 in the middle. Seems like I'm doing something wrong though. Going straight to 45 degrees gets me up to around 13km, I gain speed like crazy (can easily get up to over 1000 if I want to), but as soon as I pitch back up the engines flame out and I lose all my speed. Do I just need more air intakes so I can use the jets longer?

[Chris_2]

Hi, I've debugged your craft, and 2 small changes are needed:

Disable the pitch control on the airbrakes

Change the angle of the canards a little bit upwards to provide a pitch up torque:

With those 2 changes It'll break the sound barrier very easily, and you'll prevent a -lot- of drag by using the canards instead of the airbrakes for pitching

Also, your craft has a problem of asymmetric flameout, and the turbojets flame out before the rapiers do. You'll need to rebuild the craft so the sequence of engines and intakes is symmetric, and the turbojets are 'first in the list' to receive air.

So, You'll first want to add the intakes on one side, then -one- jet engine (on the same side), then the rapiers (on that same side), than the intakes on the other side of the craft, then the second jet engine, and finally the last 2 rapiers.

Sequence of adding parts to the craft:

Intakes (left side)

Jet (left side)

Rapiers (left side)

Intakes (right side)

Jet engine (right side)

Rapiers (right side)

This way, the turbojets keep running, even when the rapiers have changed to rocket mode (and you'll have maximum thrust at all times), and everything flames out symmetrically.

Edited May 21, 2015 by Chris_2

[Rune]

Hi, I've debugged your craft, and 2 small changes are needed:

Disable the pitch control on the airbrakes

Change the angle of the canards a little bit upwards to provide a pitch up torque:

http://static.tweakers.net/ext/f/grf4RWXPCa6ITWjiCETVO53V/full.png

With those 2 changes It'll break the sound barrier very easily, and you'll prevent a -lot- of drag by using the canards instead of the airbrakes for pitching

Also, your craft has a problem of asymmetric flameout, and the turbojets flame out before the rapiers do. You'll need to rebuild the craft so the sequence of engines and intakes is symmetric, and the turbojets are 'first in the list' to receive air.

So, You'll first want to add the intakes on one side, then -one- jet engine (on the same side), then the rapiers (on that same side), than the intakes on the other side of the craft, then the second jet engine, and finally the last 2 rapiers.

Sequence of adding parts to the craft:

Intakes (left side)

Jet (left side)

Rapiers (left side)

Intakes (right side)

Jet engine (right side)

Rapiers (right side)

This way, the turbojets keep running, even when the rapiers have changed to rocket mode (and you'll have maximum thrust at all times), and everything flames out symmetrically.

Perfect advice. Couldn't have written it better myself! Also, watch your ascent path: you want to level out to go transonic, and only pitch up when you run the risk of burning up.

Rune. Do what these guys say, basically.

[cybersol]

Guess the craft file was added as I was replying..

Disable the pitch control on the airbrakes

So much this! While you are at it, disable all control on the airbrakes and just use them for... braking.

Also, for the asymmetric engine flameout, another option is to install the Intake Build Aid mod and have it re-order the intakes and engines for you (press F7 and you're done).

Finally, if your willing to sacrifice some drag for ease of use, higher than around 0.0166 intake air per engine will mean you will run out of thrust before you run out of air.

[ScottyDoesKnow]

Thanks for all the help, especially Chris for trying it out. I'm close, but currently at the point where I either make it through the acceleration at 13km and don't have enough fuel to circularize, or I add more fuel and just start dropping at 13km when I stop my climb.

I went back to the 2 jet engine version, so potentially I'll try out going back to 4 and see if that does it for me. But for today I'm piloted out.

Thanks again!

[cybersol]

You are likely better off with 4 rapiers and 2 jets. You will need enough LFO to give around 1700+ dv after air-breathing is done. What is you final horizontal speed before switching to rockets?

[archnem]

Guess the craft file was added as I was replying..

So much this! While you are at it, disable all control on the airbrakes and just use them for... braking.

Out of curiosity why does this make any difference at all?

[cybersol]

Out of curiosity why does this make any difference at all?

Drag and oscillations. They generate a large amount of drag for their size when active as control surfaces, and in combination with SAS they seem to cause oscillations that further increase drag inefficiencies. You can use the in game drag lines to see this effect as your plane goes transonic.

[DancesWithSquirrels]

Airbrakes are just that - brakes. They act by inducing drag, apply it unevenly and the airplane swings toward the drag while slowing down. Control surfaces, on the other hand, apply _lift_ in the desired direction - there is some induced drag, but far far less than with airbrakes or spoilers.

[Chris_2]

Thanks for all the help, especially Chris for trying it out. I'm close, but currently at the point where I either make it through the acceleration at 13km and don't have enough fuel to circularize, or I add more fuel and just start dropping at 13km when I stop my climb.

I went back to the 2 jet engine version, so potentially I'll try out going back to 4 and see if that does it for me. But for today I'm piloted out.

Thanks again!

Great! You could also try to optimise your trajectory (the part after you've switched to rocket power), if you're not doing so already. According to

http://forum.kerbalspaceprogram.com/threads/119655-Yet-another-mk3-SSTO-UPDATE?p=1935183&viewfull=1#post1935183

Good luck!

[ScottyDoesKnow]

Great! You could also try to optimise your trajectory (the part after you've switched to rocket power), if you're not doing so already. According to

http://forum.kerbalspaceprogram.com/threads/119655-Yet-another-mk3-SSTO-UPDATE?p=1935183&viewfull=1#post1935183

Good luck!

Success! It could be I'm bad at flying but in the end I just couldn't get the speed up with only 2 turbojets. 4 got me to speed easily but I ran out of air too quick. Added two more intakes and made it with almost 400 dV to spare, which is plenty to dock with the refueling depot. And that was even with forgetting to hotkey the rapiers so I had to manually swap them over. Not a big fan of how the back of the intakes look, but it's good enough.

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Thanks again to everyone for all the help!