Help with launch profiles for low TWR Spaceplanes

[kStrout]

As the name might suggest, I am currently having issues getting low TWR SSTOs into orbit, my current design has 2 turboramjets and 2 terriers, It flies great in atmospheric mode, but when I switch to the rockets my TWR of 0.5 makes it so I have to maintain a 45° AOA to avoid falling back into the atmosphere, and that has severe consequences on my deltaV. I see lots of people making SSTO's larger than mine with less rocket thrust so I want to know, is there a standard ascent profile for low TWR SSTO's? Here is a picture of mine if it is of any help.

note: Technically this is a modded craft, but the only mods I am using that add parts are TAC life support and Kerbal engineer, which do not affect the performance of the plane.

Edited October 17, 2015 by kStrout

[FyunchClick]

Why not replace the whiplashes with rapier for a bit of extra impulse? They'll also get you higher in airbreathing mode.

[kStrout]

That's the thing, this is in career mode, I don't have RAPIERs yet, I have made at least a dozen RAPIER powered planes, this is new for me.

[Alshain]

This is why I feel Spaceplane SSTO's are broken in 1.0. A lot people will jump in here and claim they can build one but the the truth is more likely that at least half those people don't realize they can take the wings off what they built and launch it vertically. It isn't really a plane at all, it's a rocket with wings.

OP, simply put. A low TWR spaceplane is overly difficult in the currents state of the game because all jet engines, including the RAPIER are just too weak and consume way too much fuel.

But hey it's realistic, so the fact that it ruins the fun of the game should be fine, right?

Edited October 15, 2015 by Alshain

[Stoney3K]

To begin with, you need more wing mounted at an angle of incidence so you can keep your nose prograde when accelerating to hypersonic speeds.

Climb above 10K then accelerate with your jets until you get up to about Mach 3.5-Mach 4.

When you want to engage the rockets is entirely up to you but for one thing: DO NOT pitch high up since this will induce a ton of drag and slow your plane back down. If your apoapsis is low, pitch up slightly (to about 15 degrees over the horizon, the rest of the climb is already caused by the curvature of Kerbin).

Those two LV-909's should be enough to get that bird into orbit. Looks like you burned way too much LF on the initial jet ascent, try climbing slower in the lower atmosphere so you're not wasting fuel on drag.

Edited October 15, 2015 by Stoney3K

[kStrout]

Thanks, that might be exactly what I was doing, I was pitching up high in order to keep my time to Apoapsis increasing(you might be able to tell I'm more used to rockets). That is a very interesting tip about the wings, I definitely will try that. However I didn't burn that much fuel on ascent it just looks that way because I drained the LF tanks to 2/3 to conserve mass, but I will try climbing slower.

[Scarecrow88]

I tried building the same craft based on what I can see from your picture, and wasn't able to achieve orbit. The best I could manage, following a shallow ascent profile once above 10,000m, was an apo of 75k, but I ran out of oxidizer when trying to circularise with the peri still at 24k. I tried swapping the liquid fuel tanks for rocket fuel tanks to give more oxidiser but, predictably, ran out of fuel.

That was with an empty cargo bay, so it is safe to assume that if there was a payload, adding more mass to the craft, there would be even less chance of obtaining orbit. On that basis, I rebuilt the craft without the cargo bay, but as this only saved .25 mass, I still wasn't successful. It's possible that if you removed the passenger cabin instead of the cargo bay, and added two toroidal fuel tanks somewhere, you might just succeed with the slightly lower overall mass and additional fuel/oxidiser, but I didn't actually try that configuration.

ETA : I finally tried the configuration outlined in my last paragraph, and actually achieved orbit, with enough fuel left for a de-orbit burn. However, this was again with an empty cargo bay, so I am not sure how successful it would be with the additional mass of a payload.

Edited October 15, 2015 by Scarecrow88

[Van Disaster]

More wing. Lots more wing. You're up in thin air & really don't want to be pitching up so much, so more wing should help - at that sort of height it shouldn't cause much drag ( and also you can float up to altitude easier too ). I'm a bit dubious about 0.5 rocket TWR, but I've been a FAR guy for years and that is a bit stock-specific.

[Empiro]

From my experience, the drag in 1.0.4 is still quite high as soon as your AoA increases, causing tons of losses due to drag.

I've noticed from some Scott Manley's videos that he maintains a fairly high pitch in the low atmosphere so that he's climbing quickly when he switches to rocket engines (at 1100-1200 m/s). This lets him keep a minimum angle of attack while still climbing upwards.

I usually play with FAR, which I find to have much lower drag and actually easier to make a mixed jet/rocket space plane in.

[Nothalogh]

I usually play with FAR, which I find to have much lower drag and actually easier to make a mixed jet/rocket space plane in.

This is true, FAR makes better spaceplanes

[Destroyer713]

I can't say I've ever experimented with non-Rapier SSTOs, so any advice I might be able to give may be sketchy at best.

One of the biggest roadblocks to any winged SSTO is drag. One big source of drag comes from having a high angle of attack, because a lot of air will be hitting the underside of your plane. Increasing the angle of incidence (basically rotating the wing or canard so the front is higher than the back) on all of the wings and canards can help decrease your needed AoA to around zero once you get up to speed, greatly decreasing drag.

You probably don't need more wing than you already have. As long as you can lift off the runway reasonably enough, you should be fine. Adding additional and unneeded lift will only increase drag.

Beyond that, the only thing I can think of is adding more fuel and/or bigger non-airbreathing engines. Until you do unlock Rapiers, the fuel-efficiency of a spaceplane won't be as high as it could possibly be.

Edited October 15, 2015 by Destroyer713

[Van Disaster]

More wing = lower wing loading = less AoA at high altitude = less parasitic drag from that. You get a bit more from having more wing, just have to find the sweet spot.

[legoclone09]

Maybe switch to a single Swivel? TWR would increase a lot with that, but the engine might hit the ground, or it's too heavy. If it's too heavy a Reliant might do the job.

[GoSlash27]

kstrout,

You really want to maintain a shallower climb once you hit 18km altitude or so. The plane will climb on it's own so long as you have sufficient speed.

You also want adequate wing lift so that you're not trying to fly the whole spaceplane at high AoA. That generates a lot more drag than lift and also causes high cosine losses.

Basically... don't try to throw it into orbit with thrust, fly it into orbit with speed.

Best,

-Slashy

Edited October 15, 2015 by GoSlash27

[ajburges]

Low anaerobic thrust will be (comparatively) very costly to get to orbit.

The anaerobic thrust stage can be conceptualized as the question of how do you reach orbital velocity by Ap? The speed building stage of aerobic flight leaves you with a horizonal prograde (near Ap). You want to use your wings and cheap thrust at the end of aerobic mode to alter trajectory to ballistic and push time to Ap out far enough that your gravity turn will reach orbital speed. Low TWR designs need a lot of time to accelerate that needed 1.2 km/s.

If you have RAPIERs, the oxidizer you can store on just the adapter parts can really push that Ap to the future and give the OMS time to finish circularizing (RAPIERs can also give you more aerobic speed in jet mode, but LOX is cheap). An alternative thrust solution is an incorporated (or staged) SRB. SRBs have a ton of thrust for their weight and cost. The final solution is more anaerobic engines, but there is little comparative advantage in disabling orbital insertion engines once in orbit.

Once you pass 24 km wings generate too little lift. Almost all of what keeps you up at that range is momentum.

- - - Updated - - -

TLDR:

For efficient orbital insertion: aerobic mode should terminate into the start of a gravity turn. You want enough anaerobic thrust so that you reach orbital speed before Ap while thrusting near prograde.

[Red Iron Crown]

This is why I feel Spaceplane SSTO's are broken in 1.0. A lot people will jump in here and claim they can build one but the the truth is more likely that at least half those people don't realize they can take the wings off what they built and launch it vertically. It isn't really a plane at all, it's a rocket with wings.

OP, simply put. A low TWR spaceplane is overly difficult in the currents state of the game because all jet engines, including the RAPIER are just too weak and consume way too much fuel.

But hey it's realistic, so the fact that it ruins the fun of the game should be fine, right?

Hardly. Check out the airbreathing entries in the Payload Fraction Challenge linked in my sig, many have <1 initial TWR (sometimes much, much lower) and have payload fractions that put pure rockets to shame. I'm not even much of a spaceplane person and I've had success with designs with less than 0.5 initial TWR, and my heavier ones don't exceed a TWR of 1 until they are up over 10km. I get that you don't like the changes to spaceplanes in 1.0+, but please quit spewing these falsehoods about them being broken or impossible when there is so much evidence to the contrary if you'd care to look.

OP: My advice would be to light your rockets sooner (when the jets are still running) and point just a bit above prograde, try to keep your time to Ap steadily climbing. Once it is 1 minute or more away you can safely point prograde to complete raising Ap to the desired altitude.

[GoSlash27]

This is why I feel Spaceplane SSTO's are broken in 1.0. A lot people will jump in here and claim they can build one but the the truth is more likely that at least half those people don't realize they can take the wings off what they built and launch it vertically. It isn't really a plane at all, it's a rocket with wings.

OP, simply put. A low TWR spaceplane is overly difficult in the currents state of the game because all jet engines, including the RAPIER are just too weak and consume way too much fuel.

But hey it's realistic, so the fact that it ruins the fun of the game should be fine, right?

Alshain,

If there is even one person here who is making low TWR spaceplanes, then they are by definition not "broken", but rather simply more difficult than they were before.

And of course, there are several of us who are doing this. Not only doing it, but doing it at mass efficiencies that conventional rockets can't hope to touch.

If we can do it, then so can you. Instead of complaining that they're "broken", I suggest upping your game

Best,

-Slashy

[Right]

Replace you're ram scoop with a shock cone if you have it unlocked.

Mount the wings at a 5 degree incline.

Keep your CoL slightly behind your CoM to optimize craft stability + pitch authority. (Be careful of where the dry CoM is)

Try to keep your angle of attack between 0 and 5 (10 at the most).

[kStrout]

Well it turns out Van was right, more wing does the trick, I still had to add a bit more fuel, and the margins were so tight that I am going to add a bit more, but I was able to achieve a 200 x 200km orbit. No pictures right now, because imgur doesn't like me at the moment, but that's the trick: more wing, and living life on the edge with only 10 seconds and decreasing time to apoapsis.

- - - Updated - - -

Low anaerobic thrust will be (comparatively) very costly to get to orbit.

The anac thrusterobi stage can be conceptualized as the question of how do you reach orbital velocity by Ap? The speed building stage of aerobic flight leaves you with a horizonal prograde (near Ap). You want to use your wings and cheap thrust at the end of aerobic mode to alter trajectory to ballistic and push time to Ap out far enough that your gravity turn will reach orbital speed. Low TWR designs need a lot of time to accelerate that needed 1.2 km/s.

If you have RAPIERs, the oxidizer you can store on just the adapter parts can really push that Ap to the future and give the OMS time to finish circularizing (RAPIERs can also give you more aerobic speed in jet mode, but LOX is cheap). An alternative thrust solution is an incorporated (or staged) SRB. SRBs have a ton of thrust for their weight and cost. The final solution is more anaerobic engines, but there is little comparative advantage in disabling orbital insertion engines once in orbit.

Once you pass 24 km wings generate too little lift. Almost all of what keeps you up at that range is momentum.

- - - Updated - - -

TLDR:

For efficient orbital insertion: aerobic mode should terminate into the start of a gravity turn. You want enough anaerobic thrust so that you reach orbital speed before Ap while thrusting near prograde.

So are you suggesting that I try to be going upwards at a steeper angle before I turn on the rockets? You clearly know what you are talking about, but it seems unclear exactly what you are suggesting I do differently.

- - - Updated - - -

OP: My advice would be to light your rockets sooner (when the jets are still running) and point just a bit above prograde, try to keep your time to Ap steadily climbing. Once it is 1 minute or more away you can safely point prograde to complete raising Ap to the desired altitude.

I normally start my rockets when my jets drop below the thrust that my rockets would produce, usually around 20km and 1200m/s, the problem is that as soon as the jet's cut out my apoapsis is consistently around 30s and to keep it increasing I would have to assume a 45° AOA (this proven in practice), how are you getting your time to apoapsis up to 1m with just air-breathing engines? Are you going steeper, but then how do you get the necessary speed?

[Nothalogh]

I find these troubleshooting threads better than challenge threads.

I've always shied away from Jet-Poodle of Jet-Terrier designs, but this made me want to try.

So, I present the Hotdog

Javascript is disabled. View full album

[ajburges]

Well it turns out Van was right, more wing does the trick, I still had to add a bit more fuel, and the margins were so tight that I am going to add a bit more, but I was able to achieve a 200 x 200km orbit. No pictures right now, because imgur doesn't like me at the moment, but that's the trick: more wing, and living life on the edge with only 10 seconds and decreasing time to apoapsis.

- - - Updated - - -

So are you suggesting that I try to be going upwards at a steeper angle before I turn on the rockets? You clearly know what you are talking about, but it seems unclear exactly what you are suggesting I do differently.

- - - Updated - - -

I normally start my rockets when my jets drop below the thrust that my rockets would produce, usually around 20km and 1200m/s, the problem is that as soon as the jet's cut out my apoapsis is consistently around 30s and to keep it increasing I would have to assume a 45° AOA (this proven in practice), how are you getting your time to apoapsis up to 1m with just air-breathing engines? Are you going steeper, but then how do you get the necessary speed?

The basic space plane ascent is an aerodynamic flight to the edge of the aerobic flight envelope followed by a transition to a ballistic profile (gravity turn) on anaerobic engines. The details of your engines and aerodynamics determines your profile.

The edge of the aerobic flight profile is 1-1.4 km/s depending on engines and aerodynamics. You need to double that for orbital velocity. You need to thrust with prograde to gain that speed. If you thrust above prograde, you are using thrust to fight gravity more than gaining speed (a small AoA is okay because of the nonlinear cosine decay). On the other hand, if you pass Ap before you circularize, prograde now points downwards! The key to efficient anaerobic ascent is to pull your velocity vector up so that you enter a gravity turn that will give you the time to build that orbital velocity by the time you reach Ap (and prograde matches horizon). The lower your anaerobic TWR the more burn time needed to accumulate orbital speed and the steeper your initial ballistic velocity needs to be (time to Ap is a function that is dominated by vertical velocity while suborbital).

I propose a slight adjustment to your flight profile. Accelerate to max jet speed between 16-17 km. Light rockets at 1 km/s (keep jets on to) and pitch up (20-30 degree AoA) to pull your prograde up. As your jets starve, GRADUALLY reduce AoA to 0. Once you do, engage hold prograde and see where you wind up. If you went suborbital, you need a harder pitch-up. If you overshoot, you can relax it a little. You want to keep Whiplash engines to 16-17 km while accelerating. That's about their optimal power range. 1200 m/s is also fast for them. You want to start pitch-up before they lose their thrust (you need that thrust to aid the trajectory change)

[tseitsei]

Alshain,

If there is even one person here who is making low TWR spaceplanes, then they are by definition not "broken", but rather simply more difficult than they were before.

And of course, there are several of us who are doing this. Not only doing it, but doing it at mass efficiencies that conventional rockets can't hope to touch.

http://i52.photobucket.com/albums/g13/GoSlash27/KSP/Brawndo/Brawndo12_zpsxyh21gaj.jpg

If we can do it, then so can you. Instead of complaining that they're "broken", I suggest upping your game

Best,

-Slashy

This so much.

Alshain if you can't do it it doesn't mean it's impossible. It just means you need to learn and practise more. There are many people here that do this stuff regularly.

So stop complaining and start learning

[Val]

Well it turns out Van was right, more wing does the trick, I still had to add a bit more fuel, and the margins were so tight that I am going to add a bit more, but I was able to achieve a 200 x 200km orbit.

...

I normally start my rockets when my jets drop below the thrust that my rockets would produce, usually around 20km and 1200m/s, the problem is that as soon as the jet's cut out my apoapsis is consistently around 30s and to keep it increasing I would have to assume a 45° AOA (this proven in practice), how are you getting your time to apoapsis up to 1m with just air-breathing engines? Are you going steeper, but then how do you get the necessary speed?

Great work on getting it to orbit.

I have series of craft very similar to yours, though it is a little heavier, with a bit more fuel mass, but less dry mass. It looks to have about the same wing area as yours (pic in first post). Maybe a little more. There's also a version with crew cabin like yours, but none with docking ports.

When I pass 20 km at ~1250 m/s, the craft is at 15° pitch above horizon. My prograde marker is a few degrees below that, at 12°-14° above horizon and dropping slowly. My LV-909s will have been turned on shortly before reaching 20 km, usually at 18 km.

I keep holding 15° pitch. At ~32 km it reaches AP and might start descending a little. A few 100 meters maybe, but then it starts climbing again as speed goes above 1750 m/s surface.

A little unsettling and probably not very efficient, but it works.

What is the angle of your prograde marker at when you turn on rockets at ~20 km?

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Launch profile, more info and other versions[/TD]

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Edited October 16, 2015 by Val
Typo and clarifications

[Stoney3K]

Side note: When you have unlocked the LV-N your SSTOs become a lot more efficient because you don't need to carry Oxidizer and your burns can be INSANELY long.

The nuke engine has exactly as much thrust as the LV-909 but it can keep going almost forever since it hardly uses any fuel.

[GoSlash27]

Side note: When you have unlocked the LV-N your SSTOs become a lot more efficient because you don't need to carry Oxidizer and your burns can be INSANELY long.

The nuke engine has exactly as much thrust as the LV-909 but it can keep going almost forever since it hardly uses any fuel.

Well... they can become *somewhat* more efficient if done properly, but it's not a huge difference.

The advantage of using a single fuel type and high Isp is almost completely countered by the LV-N's 3 tonne mass.

For turbojet/ rocket hybrids, I like to stick with the Terrier or Poodle. For RAPIER designs, I don't bother adding specialized rockets.

Best,

-Slashy