Best Ascent Profile for SSTO?

[inigma]

I just can't figure it out. What is the optimal ascent profile for a standard small SSTO spaceplane?

Edited September 4, 2013 by inigma

[SnappingTurtle]

Get as high and as fast as you can on jets. It's so efficient I don't think it matters especially how long you stay on jets. Just before you're about to flame out and you've gotten as high and as fast as you can switch to rocket engines. It's more efficient to burn along the prograde vector, but you'll be in atmosphere longer but that doesn't matter much. If you switch to rocket engines at about 25-30 km and get an apoapsis of 100 you'll only lose a few km off that from drag.

Edited July 22, 2013 by SnappingTurtle

[nhnifong]

The same trajectory as a rocket.

[allmappedout]

The same trajectory as a rocket.

No.

You need to fly up fairly quickly to around 8-10km, then flatten out very shallow, and raise your altitude slowly whilst accelerating forward. As you get higher, the air is thinner so you go faster, but you get to a point where your intakes aren't enough to accelerate any more (make sure you throttle down slowly to keep from flaming out), in which case you then engage rockets to get to orbit by pointing upward to raise your apoapsis quickly.

[glacierre]

No.

You need to fly up fairly quickly to around 8-10km, then flatten out very shallow, and raise your altitude slowly whilst accelerating forward. As you get higher, the air is thinner so you go faster, but you get to a point where your intakes aren't enough to accelerate any more (make sure you throttle down slowly to keep from flaming out), in which case you then engage rockets to get to orbit by pointing upward to raise your apoapsis quickly.

I tend to agree, although you can climb a bit faster (20 km maybe) if you have enough lift to support it.

[phatm0n]

http://www.youtube.com/watch?v=s32TQWQZfEk

This video's fairly helpful and the guy is a decent presenter. You should check it out if you want to try your hand at an SSTO.

[Cranium]

It generally depends on the design of your plane. I generally like to nose up to 45 degrees immediately on takeoff, and as soon as I seem my air intake dropping, I nose down to 15 degrees. Once my air intake drops to about 25%, I like to level off as much as I can, gaining speed over altitude. I try to stay at the threshold of flaming out for as long as possible, and lowering the throttle until I no longer have any thrust. If you did it right, you should see that your AP has exited outside of Kerbin's atmosphere. Wait until that point, and then burn prograde with your rocket engines.

For larger planes, you may not be able to nose up to 45 degrees right away, so an ascent angle of 30 degrees is also acceptable.

I generally do nothing with the air intakes, because they generate a negligible amount of drag compared to the amounts of thrust generated by the jet engine(s), so leaving them open is definitely a viable strategy. You definitely want to get 2-4 ram air intakes per engine at a minimum. This should ensure that you have enough air to get into the upper atmosphere.

[inigma]

My SSTO gets to 22km without flaming out, and can climb to 90 degrees from runway easily. I just wonder at which point or angle do I ignite my space engines - do i just treat the plane like a rocket at that point?

[Rhomphaia]

No.

You need to fly up fairly quickly to around 8-10km, then flatten out very shallow, and raise your altitude slowly whilst accelerating forward. As you get higher, the air is thinner so you go faster, but you get to a point where your intakes aren't enough to accelerate any more (make sure you throttle down slowly to keep from flaming out), in which case you then engage rockets to get to orbit by pointing upward to raise your apoapsis quickly.

SSTO != spaceplane

[allmappedout]

SSTO != spaceplane

Point conceded, but context implied spaceplane, given that any rocket based SSTO has an identical ascent profile to a rocket.

Edited July 22, 2013 by allmappedout

[inigma]

i updated op to say SSTO spaceplane.

[carazvan]

Think your answer really depends on the number of air intakes per engine and TWR your SSTO has.

You want to circularize your orbit as high up as possible. The lower you are the lower the terminal speed you can achieve.

With 8 air intakes/jet engine I generally go up and hold just above 30k until I gain 2000+ speed then I let the spaceplane slowly pitch up (just 5-10*) and slowly decrease throttle until the thrust of the jets is lower than the thrust of my rocket engine.

At that point I close off air intakes and switch over.

My record so far is only needing 50deltaV to circularize

[inigma]

Does anyone else have suggestions to answer my OP? I'm trying to figure out what best methods to achieve orbit with a generic SSTO most people have come across.

[capi3101]

I imagine flying a spaceplane to orbit would be much like flying an SSTO rocket, which is just like flying any other rocket except that you really need less and less thrust as you go higher, so watching your throttle is important (in particular keeping the gee-meter in the green is very important). As vertical as you can manage to 10k without stalling on course 090, 45 degrees at 10k, switch to rockets at flame out, 20 degrees at 30k if you're at least 30 seconds to apoapsis, burn along the horizon at 50k and kill it once your apoapsis is in orbit.

Thing about spaceplanes - once you're up high enough, they're either rockets or bricks...

[sweetpezak]

Yeah it really depends on a bunch of different factors, the number of intakes/number of jet engines ratio, the mass of the ship/how much rocket fuel you're bringing, the number of rockets and the TWR, but for some of the simpler SSTO's where you aren't spamming massive amounts of intakes you likely aren't going to get over 25-30km in height; what i generally do is go 45 degrees from take-off to 10km (can go steeper but depending on your craft you might lose too much speed/intake air)... then at 9-10km pitch down to 20-25 degrees (again depends on intake air/speed you have if you can pitch higher/lower than this), then at 18-20km pitch down again to 10 degrees or less to gain enough speed above 20km to start the rockets, keeping the jets on as long as possible until right before the jet engines can burnout. Then after starting the rockets and gaining a good amount of speed begin to pitch back up to 45 degrees or a little steeper if you can to climb through the rest of the atmosphere quickly. Finally just get into orbit as you normally would... Meant to be just a general guideline, if you are passing your apoapsis and dropping height then you will have to pitch up more to keep it in front of you, but if you pitch up too high, your intake air will drop fast and you won't be gaining speed at the rate you need to... So it is a balancing act and will vary with every SSTO you make; having something like flight engineer redux to see your time to apoapsis info helps a bunch in flying these as you can adjust your pitch in pretty much real-time to keep the apoapsis in front of you.

Edited September 4, 2013 by sweetpezak

[Scarecrow88]

As a general rule of thumb, I climb fairly steeply initially to get out of the thickest part of the atmosphere to reduce drag. I will then flatten out to allow speed to build up without climbing too quickly in order to avoid the jet engines flaming out due to lack of air. When it gets to the point that I got as much speed from jet power as I am likely to get and the jet engines are about to die anyway from lack of air, I switch to rocket power and continue the ascent as per a normal rocket launch.

[ComradeGoat]

If you have enough TWR on the jets and enough intake air to run at full power at 28-30k, you can get up to 2000 m/s at that sort of height in straight and level flight. This is almost orbital velocity. Problem is, you'll lose most of it to friction if you cut the power, but you don't want to be carrying high TWR rockets because they're heavy. Here's what I do:

45º up to 10K and about Mach 1 (330 m/s), pitch down to about 30º. Watch engineer redux - assuming I have a full power ceiling at 29-30K, when my apoapsis hits 28K level off - this will stop apoapsis from climbing.

Make fine adjustments at 28K to stay at that altitude. Let the plane accelerate to close to top speed (2000 m/s or higher if it'll do it: my current SSTO shuttle will do 2100 m/s at 29K on one turbojet).

Once your acceleration drops right off, pitch up as steeply as you can without going into a flat spin or flaming out. Having only a single engine here helps because it stops flameouts themselves making you spin.

The apoapsis will climb sharply away, As it gets a decent time i front of you (say 20 seconds), pitch down again. Reduce power slowly to avoid a flameout. If your rocket engine is a NERVA, it should be lit at this point, otherwise stay on just jets.

By the time your apoapsis is at 50K or more, pitch down to horizontal. Keep reducing power to avoid a flameout. If you only have one jet engine, you can let it flameout and then reduce the power until it restarts. If you have more than one, avoid a flameout at all costs, so be more conservative.

By 40K, I'm on a trickle of power, but my apoapsis and periapsis are still climbing. If you've done this right, your apoapsis will be in space.

Once the engine won't even idle, cut it and close your intakes. If you are well above 40K and your apoapsis is above 80K, you can glide into space before circularising from this point. If your apoapsis is still below 70K, or close to it, you'll need to use your rockets to lift it into space.

Good luck!

[Blaster]

Blaster's Brute Force Space Method, Used for getting the KermaJet series planes into space. Full power from runway, minimal intakes open. Pull up at 80 m/s and get airborne.Pitch up as high as the plane allows, climbing rapidly to flameout threshold altitude and opening intakes as needed (varies depending on plane, need 0.1 air per engine to go full throttle), then level out and go fast as you can.Once your acceleration is slower than 1m/s/s, start climbing and throttling down, keeping in mind the 0.1 air/engine at full throttle rule. Right-click the jet to monitor its stats.Once the jet engine thrust is less than rocket thrust, hotkey to rockets and pull up. Get the AP into space, close intakes, and round out the orbit. Yay space. Also, Forum isn't recognizing my line breaks (press Enter), on Chromium Browser in Lubuntu Linux.

[Hodo]

My profile on almost all of my Space planes (SSTO) is the same.

-Climb at 30-45deg to 12km.

-level off at 12km accelerate to 1300m/s

-climb to 28-30km at 10-15deg (which ever the engines will maintain power without flameout) then level again

-accelerate till she will go no faster generally around 2000m/s for most of my designs.

-turn on rockets 100% throttle nose up 20-25deg, shut down jets.

-watch Apolapse, when it reaches desired altitude cut engines.

-Set orbital pattern

Done.

It works for every one of my space planes, even my monster 250ton Bison.