how to build a large working SSTO for 1.0.2 - 1.0.4 ?

[scavenger]

hi dear experts

I see many SSTO howtos on youtube but it's for old versions, and small SSTOs.

Is there any tutorial for a working larg SSTO with clear explanations for turning points during ascent?

from what I saw on youtube here is the curves I drawn:

But when I try to apply any of these for 1.0.2 then 1.0.4 now, I still don't get past the 19,000m. Once I flight past the 700m/s at these altitudes, the plane becomes uncontrolable then dismantle.

I tried like 15 or 20 different versions but no chance.

Here is the latest version I am working on, which is really strong and stable, and the center of thrust is aligned with the mass.

Still it cannot pass the 19-23km altitude:

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please HELP, I really do not know what to do

Edited July 6, 2015 by scavenger

[yorshee]

What I do is pitch up as much as I can without my thrust decreasing (for larger clunkier crafts it's normally at around 15-20 degrees) and I keep at that angle until I get to around 10km, pitching down slightly if I see the thrust decreasing.

Once it gets to 10k metres, I pitch down and maintain level flight until I get to around 1000m/s, which is when the thrust on my jet engines starts to decrease a lot, and once the thrust gets lower than 150 kilonewtons I switch to my rocket engine of choice and pitch up as much as I can without losing speed and stay at that angle until I get an apoapsis of around 75km, making sure to throttle down if I'm gaining too much speed - I want to get my apoapsis above 70km when I'm at around 55km.

This always works for me and I've never had any trouble with it. Try pumping your fuel forwards so your centre of mass is as far forwards as it can be.

[DeMatt]

RAPIER engines are an SSTO designer's best friend. Use 'em instead of turbojets unless you're trying to do a liquid-fuel-only SSTO.

Check the F3 log to see what actually breaks first. It vaguely sounds like your plane is running out of IntakeAir and thus getting an asymmetrical thrust flameout.

[rofltehcat]

Is this with FAR or in stock aero? That breaking apart looks strange.

Other than that the initial climb curves are too steep for the new jet engines. They don't have that much power until they are above 10k and have some speed. Try a 30-35° climb instead. And how heavy is that thing?

[scavenger]

i deactivated FAR for the moment, as I must realize it with stock first. THEN I will switch to FAR.

mass is 92t.

What I do is thrusting at 45-42 up to 5000m then decrease by 2 degrees each time my acceleration drops bellow 1m/s. Thanks to smartass from MJ2. Thus I eliminate the problems from my poor pilot skills (and the imprecision of the joystick)

I am trying again right now and let you know if yorshee has the point

[RuBisCO]

Just going to pimp my ride, launch regime is in the description.

http://forum.kerbalspaceprogram.com/threads/123442-Xylol-5-All-stock-space-plane-proven-to-lift-59-tons-cargo-to-orbit

[Mic_n]

MOAR JETS!

The thing is that just like with rockets, SSTO weight increases exponentially.. Heavier payloads need more fuel to maintain the same dV. More fuel means a heavier craft, and on it goes. You need to add thrust accordingly. My "medium" lifter (Mk2, 3x cargo bays to carry a payload up to around 13t) uses 6 jets to get to orbit. Your craft there looks like it's quite considerably heavier. Put a couple more jets on it (at least) and see how it fares then.

If you can't "find a flight profile", you don't have enough thrust. If dropping to ~10 degrees at 10km doesn't get you into the positive feedback acceleration loop, your TWR is just too low, it's as simple as that.

(EDIT: 92t?? Yeah, not going to work with 7 engines. I'll take a look at mine later on to compare, but I'd suggest you'll be after an awful lot more than that.)

I like to be pushing 900m/sec at the 20km mark, with my prograde marker at around 20-25 degrees.. time to apoapsis of around a minute or so, but it varies with the TWR you have when you're on rockets. The faster you're going initially, and the faster you can accelerate, the better you can hold your Ap out in front of you, and that's the key to it.

I'd really like some jet engines suitable for Mk3 parts though. I'm tempted to just .cfg edit and rescale up a copy with appropriate mass & thrust changes to fit the 2.5m parts (1.25 -> 2.5m rescaleFactor = 2, thrust & mass increase 2^3 = 8 times). The engine can't really handle the part counts required to make heavy SSTO spaceplanes properly.

Edited June 27, 2015 by Mic_n

[scavenger]

thank you rubisco, i will give it a try when i will unlock the rapier. for the moment I want to do it with the X4 engine only, as I see many people doing it.

Well, it is much better now: once i got to 18,000m, i made a quicksave and tried different things.

- stayed at 10deg up to 1200m/s then rocket then AP goes to 100km but i do not have any oxidizer left to circularize

- tried different degrees between 5 and 20, AP do not goes up 75km

- tried ascent autopilot too, but then AoA enter into action, and I played with the dynamic pressure fadeout to raise it to 7000Pa and AoA to 12deg but still not enough oxidizer to orbit.

Then decided to land instead, and successfully landed after the 4th attempt.

will change the design now

[Mic_n]

PS I don't really agree with those curves. If you're going to do the "level off then climb" profile, level off and gain your speed lower in the atmosphere.. certainly below 15km. That gives you more time to pitch up at a lower rate (thereby not bleeding off as much speed) and really pull not just your nose but your actual flight path up, giving you a higher, further Ap that maximizes your ability to just burn straight prograde to push that Ap up out of the atmosphere and minimize steering losses while burning that expensive rocket fuel.

There's no one-size-fits-all flight profile though. It's a matter of thinking about what you're trying to achieve through each step. Climb to altitude, accelerate to maximum speed, turn your orbit, raise your Ap, then circularize... all with a minimum of fuel use. While adding more engines means more weight and burning fuel faster, it also means you accelerate faster and can progress through those stages faster, losing less dV to drag in the process. There are an awful lot of variables and it is a relatively complex task.. much moreso than the alpha releases.

[Mic_n]

Oh.. I should probably clarify: that's pre-rapiers.. ie ramjets and LVT-45s, aerospikes or even LV-909s. I haven't messed much with rapiers since I haven't unlocked them in my career yet, but that all gets moved up a little.. they have a higher and faster ceiling, so you can push them a bit more. Still the same concepts, though.

[Old Foxboy]

What's with this Jet in the vertical stabilizer? Doesn't it push down the nose or did you adjust it's angle to thrust through the COG?

[Mic_n]

Was wondering about that myself... I get the feeling it was an aesthetic "Trijet" kinda idea and figured with the 6 others right along the CoM line it's not such a big deal, it wouldn't throw out the CoT by a huge amount.

But yes, it's certainly less than ideal and if you're having troubles with "the basics" of getting the thing to orbit is probably not a complication you should be adding. (The original poster, not Foxboy)

[rofltehcat]

92 t with 7 turbojets puts you at 13,14 t/engine. Before the patch a turbojet could (from my experience) work up to 14 t but since the patch the stuff I build that actually gets to orbit is more around 12 t/engine. Try slapping on 1-2 more engines and see how fast you get up there.

[Old Foxboy]

BTW: Does anybody know if the intake air resource distribution bug between air breathing engines has been resolved? Because "my craft starts to lose control at 700 m/s at xx altitude" sound a lot like one engine losing thrust and the craft yawing out of control.

[scavenger]

lateral jets are lower then the center of mass, thus the vertical stabilizer jet re-equilibrate the center of thrust. May sounds silly but it really does.

And for intake air resource distribution how can you know? All I see is all of the jets stops at the same time by 22-23km.

[Crazy_Redd]

*follow comment* I'm realy looking for something that can bring my stations in orbit without wasting a lot of money on rockets.

[Kagame]

[Dr. Jet]

Correction: not 10000 feet, but 10000 meters.

[Mic_n]

Because "my craft starts to lose control at 700 m/s at xx altitude" sound a lot like one engine losing thrust and the craft yawing out of control.

It does a bit.. For reference: if that's happening you should hear the flameout sound and your craft will start to yaw to one side. That'll likely be followed by more flameouts and an uncontrollable flat spin, spinning one side to the other as your intakes turn 'off axis' and get less air incoming and more engines cut out.

From the looks of it though, you've got a ram intake per engine there, so that really shouldn't be a problem.

It could also be CoM shifting behind CoL with fuel use. Less of a danger now with even fuel drain, but if it's relying on a full tank up front to balance the engine weight, that'll cause "sudden mid-flight dramas." The craft there looks basically ok, but there is a lot of weight along the back there, and those Mk3 lifting bodies themselves generate a fair chunk of lift.

To check: Turn on the CoM and CoL indicators (yellow ball and blue arrows respectively) in the SPH. The blue one needs to be (and if you can get airborne, likely is) behind the yellow. Go through fuel tank by fuel tank, right click, and empty all the fuel out of them. You'll see the yellow indicator shift around as you do. If that moves behind the blue indicator, you're going to have trouble.

If you're flying with SAS or mechjeb, keep an eye on your 'control indicators' in the lower left corner. They reflect your "stick position" so to speak.. If you have a problem with a shifting lift/mass balance, what you'll see is the pitch indicator slowly creeping right out to the very end of the indicator, as the autopilot has to apply more and more 'stick forward' to maintain its heading. Once it actually reaches the end, the system will no longer be able to hold that orientation and your nose will start lifting uncontrollably. You're pretty much boned at that point.

[scavenger]

thanks for the new schematic, I will give it a try and verify the dV of rockets and TWR

BTW I made it to orbit! I barely hade enough oxydizer for the return but I carfully followed these rules:

- take off at 30deg until 7k

- then angle 20deg until 237k where the jets shut down and switch to rocket, then back to 30deg

- at some point I made it to an AP of 100k which was the goal. But it is not efficient for the SSTO I am using since I barely have enough oxydizer to make it

[Mic_n]

Try switching around some pure LF tanks for LF+O "Rocket Fuel" tanks... it takes some tweaking to get it right. I generally launch mine to 80km, and like to have just enough oxidiser to circularise then de-orbit while leaving a little LF there for a powered landing. By rights you should also tweak your levels based on your payload if you want to be optimally efficient, but I really couldn't be bothered going that far.

[mrclucks]

On a slightly related note, is there a mod that can show CoL and CoM in flight?

It would be super helpful at balancing space planes on return to the atmosphere

[scavenger]

If you have tanks in front and rear, you can be sure the front ones will be empty, meaning you have more weight to the rear, which is good to me.

Another update: I could make it to orbit 3 times in a row with 2 different ascent curves, for this baby which looks like the Xylol5 with 3 large cargos:

The best experience I had was with the folloging curve, which I had to adapt:

Indeed, following strictly this curve gives you hell of a speed at 10,000m like 800m/s, and 1230m/s at 20,000 but then the acceleration decrease, and you do not increase the AP any further. Because there is like no real lift anymore and just some drag.

Also from 24k you do not have so much drag anymore, and you can start to decrease the thrust to 2/3, then to 1/3 at 40k or so. Then decrease it again until you still have acceleration but AP continues to increase up to 75k.

Doing this I could manage to circularize with a full Mk3 to 2.5m adapter at 75k.

I wouldn't say it is a victory since I do not even have a payload, but it is better after each flight

The tricky part is starting from 20k. I have to figure out what the best practice is after 20k, since the jets flame out by 27k but do not provide more than 400kN at 21k. It is not enough to maintain the same speed and gain more altitude. OR should I continue with jets only up to 27k? Problem is the speed can decreas as low as 888m/s:

[ajburges]

I'm having to tweak my current cargo hauler; recent version seems to have harmed it a little.

For a turbo rocket mix, I try to reach 1 km/s by 16 km. Past that in any direction sees rapid performance loss. Once I get that speed, I use my wings and jets to pitch up, keeping as much speed as possible. Lift surfaces are the most efficient way to change heading at those speeds. I hit the rockets when I start losing any speed. Once I pass 28 km, I bring heading inside of the prograde circle. At 36 km, I set SAS to follow prograde and tune throttle for optimal gravity turn.

In design, I want rocket dV of 1.8 km/s with full TWR of at least unity. Jets as needed to break the sound barrier.

[LordFjord]

BTW: Does anybody know if the intake air resource distribution bug between air breathing engines has been resolved? Because "my craft starts to lose control at 700 m/s at xx altitude" sound a lot like one engine losing thrust and the craft yawing out of control.

It might not be as bad as in 0.90 because you don't spend such long time within the atmosphere and the transition to space is quicker, but the issue is basically still there.

see here (was done in 1.0.2, but I don't think it has changed)

@scavenger: grats to get it into orbit and the nice graphs, that might help others