Help with launch profiles for low TWR Spaceplanes

[FyunchClick]

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.

But it's also a lot heavier and not particularly useful for the purpose of SSTO due to the abysmal TWR compared to it's own weight. All my SSTO's now rely on rockets to get from about mach 3-something to orbital speed, and the LV-N just contributes about 200-250 m/s to that when rockets flame out after they pushed my apoapsis up to 60-65 km or so and have plenty of time-to-apoapsis to burn into a comfy LKO. For the most part they're there because they get me to Minmus with ease. If I'd replace their weight with rocket fuel I'd make orbit even easier.

Edited October 16, 2015 by FyunchClick
Ninjad

[kStrout]

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 sure but they're so big and heavy with the same thrust that it only makes the TWR issue worse. I am considering using them if I ever need to make a spaceplane to go beyond LKO, but as this plane is a LKO crew supplier, the low mass but relatively high ISP of the terriers is working out great. On I side note I build a similar sized cargo vehicle but decided instead of having 2 terriers I was going to use 1 swivel. It flew amazingly, but it used a little more fuel, but not much, more testing to ensue later.

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

Well that's the thing I am getting the idea I will have to change, my new strategy is to flatten out to 5° at 10km and then keep tangent to Kerbin until orbit, this usually means my nose is at 15°-20° at 20km, but I don't think my prograde marker ever passes 10° the entire flight. However I'll have to check this afternoon.

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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)

I see what you are saying, that shouldn't be hard, In my current flight plan 16km is about when I pass 1km/s. I'm just a little worried about pulling up too hard and stalling, but I'll try it out.

[Val]

Well that's the thing I am getting the idea I will have to change, my new strategy is to flatten out to 5° at 10km and then keep tangent to Kerbin until orbit, this usually means my nose is at 15°-20° at 20km, but I don't think my prograde marker ever passes 10° the entire flight. However I'll have to check this afternoon.

Stoney3k great advice about wing incidence will help pulling the prograde marker up, and reduce drag at the same time.

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.

If you hadn't considered using wing incidence, here's an explanation of why it is so great.

Like most of us, you were probably taught that lift works like this.

Pictures showing lift without Angle of Attack. It's not wrong, but it's also not the full picture. On top of that KSP defaults to wings mounted that way. Which means we all have a tendency to mount wings with zero Incidence. When really we shouldn't.

At least 2 things are missing in relation to KSP.

1. Camber in wing profiles only provide part of the lift, in most flight regimes.
   
2. KSP does not model cambered wing profiles.
   

Which means wings in KSP always need Angle of Attack to provide lift.

If you mount wings with no Angle of Incidence, then the whole craft needs to be pointed up for the wings to lift it.

When wings are mounted with Incidence, the nose can be lowered closer to the direction of movement (prograde), reducing drag from the fuselage. Drag from the wings will of course remain unchanged, but that is much less than drag from fuselage.

In real life even cambered wings are mounted with incidence, for the same reason.

"Wings are typically mounted at a small positive angle of incidence, to allow the fuselage to have a low angle with the airflow in cruising flight. Angles of incidence of about 6° are common on most general aviation designs."

Source

The amount of incidence you need depends on mass and wing area. I usually go for 5° Incidence (because that's easy to do in stock) and a wing area 1/8^th to 1/6^th of the mass in t. (not counting wing area from Mk2 fuselage)

This will result in the craft being able to fly completely level with the horizon at sea level (below 500 m), without losing or gaining altitude, at or just above Mach 1 (350 m/s) which minimizes drag at transonic speeds, where drag is very high and thrust is still low. This is more relevant for RAPIER powered planes that have less thrust than Whiplashes, until speed is above 400-something m/s.

TLDR; But it also means that if you fly level at 10 km, then the prograde marker will start to pull above the nose around Mach 3 (900 m/s), and you can start following it up, to get a good angle for the transition to ballistic flight, with very little pointing away from prograde.

Edited October 16, 2015 by Val
Added TLDR

[ajburges]

Well sure but they're so big and heavy with the same thrust that it only makes the TWR issue worse. I am considering using them if I ever need to make a spaceplane to go beyond LKO, but as this plane is a LKO crew supplier, the low mass but relatively high ISP of the terriers is working out great. On I side note I build a similar sized cargo vehicle but decided instead of having 2 terriers I was going to use 1 swivel. It flew amazingly, but it used a little more fuel, but not much, more testing to ensue later.

Well that's the thing I am getting the idea I will have to change, my new strategy is to flatten out to 5° at 10km and then keep tangent to Kerbin until orbit, this usually means my nose is at 15°-20° at 20km, but I don't think my prograde marker ever passes 10° the entire flight. However I'll have to check this afternoon.

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I see what you are saying, that shouldn't be hard, In my current flight plan 16km is about when I pass 1km/s. I'm just a little worried about pulling up too hard and stalling, but I'll try it out.

Unless your craft has non-orthogonal lift vector's, 30° AoA is about the critical AoA. You have enough speed that stalling is less a concern than losing said speed. Try to keep constant speed during the pitch up.

Remember: airspeed, altitude, or brains; you always need at least two.

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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.

That's not efficiency that's just sand bagging dV.

I caution against LV-N without RAPIERs. They are too heavy and/or too weak for efficient ascent on their own. Once you have RAPIERs, you can use the LOX in the adapter parts for a quick burst of thrust via rocket mode. That thrust will will push Ap out and give the nukes plenty of time to circularize.

[AeroGav]

Rocket guys care a lot about thrust-weight ratio, but space planes it's not so important, just takes longer to get there and gives you more time to think. What planes it's all about drag vs thrust. So long as thrust wins, you're getting closer to space with every second that passes.

I've heard that bicouplers are very draggy, so i'd be tempted to ditch that and use only a single rocket. Once you're in rocket mode the atmosphere is already very thin and a combination of orbital effect and residual lift are nullifying gravity. However i'd also use an inline cockpit and put an intake on the front - since you've no option but to take the drag hit from a fuselage and two nacelles, might as well get more air from the fuse.

Finally, I like more wing. In this game, wing surfaces do not create parasite drag, only lift induced drag - ie, drag is proportional to angle of attack. More wing = lower alpha, therefore less drag. I pack as much wing as aesthetic considerations, structural rigidity and aerodynamic stability allow.

The profile i use -

1. Try to maintain around 5 degrees angle of attack during climb, where drag is lowest in most aircraft.

2. As we climb, the airspeed required to maintain 5deg will increase. When it reaches 240 i start pitching up to stop it rising more, since you're nearing the sound barrier and drag is high in that region.

3. At some point, my plane will stop gaining altitude or airspeed. Or i'll start exceeding 10 deg AoA, which is where drag gets too high for my liking. Or i find myself running out of pitch authority, with huge red lines coming from stalled control surfaces. At this point crossing the sound barrier is the lesser of two evils.

4. I let my AoA drop to 5 deg or less to accelerate through the high drag zone as fast as possible. 430 m/sec is in the clear, you're over the drag spike. I start a gentle pull-up to break out of the dive at around 350 however.

5. There is probably no time for this unless you have a really low TWR plane. I pitch up to avoid exceeding 500 m/s below 16,000 when drag starts to rise again after clearing sound barrier. really there's no time though, because

6. 18 -19k is best altitude for your top speed run. I need to start pitching down at 16k so not to bust this.

7. At 18-20k, accelerate to highest poss speed in level flight, then start a slow climb.

8. At some intake air value, i switch on the rockets rather than let my hard won momentum dissipate.

9. I pitch up to 10 AoA until 35-40k when i stop getting signnifant lift, then prograde. Get what you can from those wings while you still have a bit of atmosphere, and out of the soupy stuff where your rocket is less efficient.

10. the final part of the journey is now under little or no time pressure.

[DancesWithSquirrels]

More wing, give the wing a bit of AOA (rotate it up ever so slightly), and replace your two Terriers with a single Swivel for more thrust, TWR over 1.0.

Wing angle and area should be such that you're flying with an essentially level fuselage at Mach 1 - perhaps not perfect, but that's a good starting point.

[Red Iron Crown]

Rocket guys care a lot about thrust-weight ratio, but space planes it's not so important, just takes longer to get there and gives you more time to think. What planes it's all about drag vs thrust. So long as thrust wins, you're getting closer to space with every second that passes.

Gotta say I disagree with this. TWR is fairly critical in the transition from air-breathing flight to closed cycle, the craft must be able to push Ap far enough ahead to have time to finish raising Ap, which is the trouble the OP was facing. If you pass Ap in a low-TWR ship it will be very difficult to achieve orbit.

Finally, I like more wing. In this game, wing surfaces do not create parasite drag, only lift induced drag - ie, drag is proportional to angle of attack. More wing = lower alpha, therefore less drag. I pack as much wing as aesthetic considerations, structural rigidity and aerodynamic stability allow.

Lift and drag are exactly proportional in KSP. A small wing at high alpha producing the same lift as a big wing at low alpha will produce the same drag as that big wing. There's no difference aerodynamically, but the larger wing is more mass and thus reduces TWR and delta-V so the best thing to do is use as little wing as required to get off the runway. Another important thing is to have an angle of incidence on the wings, so that the fuselage can present its lowest drag profile into the airstream and the engines push in the most useful direction while the wings still produce lift.

[GoSlash27]

More wing, give the wing a bit of AOA (rotate it up ever so slightly), and replace your two Terriers with a single Swivel for more thrust, TWR over 1.0.

Wing angle and area should be such that you're flying with an essentially level fuselage at Mach 1 - perhaps not perfect, but that's a good starting point.

*SMH*

What is with this sudden fascination with the LV-T45??

Scott Manley uses one and all of a sudden people think it's a great engine...

The Swivel is an *okay* engine for this job. It is not ideal, though. The best engine is whatever will do the job with the least combined mass of engine, oxidizer, and fuel. The Terrier and Poodle are both much better than the Swivel for this job.

Sorry, not venting at you personally. I just keep seeing this everywhere lately and it's bad info.

Best,

-Slashy

[kStrout]

All right well I took advice from everyone, and removing the extra tanks I used to get to orbit(barely) last time, I tried your ideas out on my craft, and it worked beautifully. After a few attempts I got the ascent nearly perfect and got to a 200 x 200 km orbit with enough fuel to come back. (again barely, after dropping by periapsis for re-entry I had 2.8 oxidizer remaining) The perfect ascent seems to be this: go as steep as you can initially while still building speed, around 10km level out (with a gravity turn preferably) and start building speed. Maximum air-breathing speed should be ~15km and 1200m/s, traveling almost horizontally. (you may notice in my attempt I was too high and going up too fast), then SLOWLY, but not too slowly, pull up into a climb, keeping your AoA around 10°-15° the whole time. Speed will drop, for me just below 1100m/s, but keep going until your prograde is about 15° or higher and your nose is a bit above that. Sometime around the end of this turn, fire the rockets. When the engines cut out I was able to get my apoapsis to 35km and a time to apoapsis of 45s, the latter will decrease but if you stay as steep as possible without stalling it shouldn't go negative(mine did however get to 6s). When it starts to increase go as close to prograde as possible while still making in increase.

I have tons of great pictures, but imgur isn't letting me upload them because... reasons. I'm getting errors.

I also added slight wing incidence and it does work wonders.

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*SMH*

What is with this sudden fascination with the LV-T45??

Scott Manley uses one and all of a sudden people think it's a great engine...

The Swivel is an *okay* engine for this job. It is not ideal, though. The best engine is whatever will do the job with the least combined mass of engine, oxidizer, and fuel. The Terrier and Poodle are both much better than the Swivel for this job.

Sorry, not venting at you personally. I just keep seeing this everywhere lately and it's bad info.

Best,

-Slashy

Yes they are better but they are much more difficult to effectively use(note I started a whole thread on the subject:sticktongue:) but the swivel is easy, powerful, reasonably efficient and not too heavy. It's easier to stick on the back of something you made in 5mins and launch it to orbit. I'm not disagreeing with you, I actually had a version of my plane before that used a swivel, but it needed more fuel and didn't look as nice, so I switched to my current version.

[GoSlash27]

Gotta say I disagree with this. TWR is fairly critical in the transition from air-breathing flight to closed cycle, the craft must be able to push Ap far enough ahead to have time to finish raising Ap, which is the trouble the OP was facing. If you pass Ap in a low-TWR ship it will be very difficult to achieve orbit.

RIC,

Ahh, but this is why it's so important to not pass Ap in a low t/w spaceplane

You have to be careful about how much you pop up at the transition so you don't wind up in that predicament.

I've played around with some ridiculously underpowered spaceplanes (as low as .34 t/w on rockets) and gotten them to orbit. This does incur a penalty to drag losses, though, and I like to keep it at at least .5.

Best,

-Slashy

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All right well I took advice from everyone, and removing the extra tanks I used to get to orbit(barely) last time, I tried your ideas out on my craft, and it worked beautifully. After a few attempts I got the ascent nearly perfect and got to a 200 x 200 km orbit with enough fuel to come back. (again barely, after dropping by periapsis for re-entry I had 2.8 oxidizer remaining) The perfect ascent seems to be this: go as steep as you can initially while still building speed, around 10km level out (with a gravity turn preferably) and start building speed. Maximum air-breathing speed should be ~15km and 1200m/s, traveling almost horizontally. (you may notice in my attempt I was too high and going up too fast), then SLOWLY, but not too slowly, pull up into a climb, keeping your AoA around 10°-15° the whole time. Speed will drop, for me just below 1100m/s, but keep going until your prograde is about 15° or higher and your nose is a bit above that. Sometime around the end of this turn, fire the rockets. When the engines cut out I was able to get my apoapsis to 35km and a time to apoapsis of 45s, the latter will decrease but if you stay as steep as possible without stalling it shouldn't go negative(mine did however get to 6s). When it starts to increase go as close to prograde as possible while still making in increase.

I have tons of great pictures, but imgur isn't letting me upload them because... reasons. I'm getting errors.

I also added slight wing incidence and it does work wonders.

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Yes they are better but they are much more difficult to effectively use(note I started a whole thread on the subject:sticktongue:) but the swivel is easy, powerful, reasonably efficient and not too heavy. It's easier to stick on the back of something you made in 5mins and launch it to orbit. I'm not disagreeing with you, I actually had a version of my plane before that used a swivel, but it needed more fuel and didn't look as nice, so I switched to my current version.

kStrout,

Glad to hear you made it and yeah, that looks about like my standard profile.

Yeah, it's easier to circularize with more t/w, but that's a by- product of using more engine than you need rather than a property of the Swivel itself. As you point out, it needs more fuel and oxidizer to do the same job. This is why the spaceplane gurus don't use them; they're all about getting the payload to orbit as cheaply as possible.

Best,

-Slashy

[Nothalogh]

All right well I took advice from everyone, and removing the extra tanks I used to get to orbit(barely) last time, I tried your ideas out on my craft, and it worked beautifully. After a few attempts I got the ascent nearly perfect and got to a 200 x 200 km orbit with enough fuel to come back. (again barely, after dropping by periapsis for re-entry I had 2.8 oxidizer remaining) The perfect ascent seems to be this: go as steep as you can initially while still building speed, around 10km level out (with a gravity turn preferably) and start building speed. Maximum air-breathing speed should be ~15km and 1200m/s, traveling almost horizontally. (you may notice in my attempt I was too high and going up too fast), then SLOWLY, but not too slowly, pull up into a climb, keeping your AoA around 10°-15° the whole time. Speed will drop, for me just below 1100m/s, but keep going until your prograde is about 15° or higher and your nose is a bit above that. Sometime around the end of this turn, fire the rockets. When the engines cut out I was able to get my apoapsis to 35km and a time to apoapsis of 45s, the latter will decrease but if you stay as steep as possible without stalling it shouldn't go negative(mine did however get to 6s). When it starts to increase go as close to prograde as possible while still making in increase.

Yep, that's how I do it

[ajburges]

*SMH*

What is with this sudden fascination with the LV-T45??

Scott Manley uses one and all of a sudden people think it's a great engine...

The Swivel is an *okay* engine for this job. It is not ideal, though. The best engine is whatever will do the job with the least combined mass of engine, oxidizer, and fuel. The Terrier and Poodle are both much better than the Swivel for this job.

Sorry, not venting at you personally. I just keep seeing this everywhere lately and it's bad info.

Best,

-Slashy

The LV-T45 offers a nice compromise of thrust, Isp, node size, drag, and tech availability.

Terriers tend to be light on the thrust for simple ascent burns. You either need a tuned ascent or more engines. Flying efficiently with low thrust is not trivial.

The LV-T30 has less Isp and that is a poor trade for the greater thrust. The LV-T40 has plenty of thrust.

The Poodle is a much nicer engine, but the node size adds drag (or does it?)

The aerospike is better for practical use, but that is late tech tree.

[GoSlash27]

The LV-T45 offers a nice compromise of thrust, Isp, node size, drag, and tech availability.

Terriers tend to be light on the thrust for simple ascent burns. You either need a tuned ascent or more engines. Flying efficiently with low thrust is not trivial.

The LV-T30 has less Isp and that is a poor trade for the greater thrust. The LV-T40 has plenty of thrust.

The Poodle is a much nicer engine, but the node size adds drag (or does it?)

The aerospike is better for practical use, but that is late tech tree.

ajburges,

No, not particularly. The Poodle is just plain all- around a better option in installations where the Swivel would be considered. And often people will opt for higher t/w for designs that don't really warrant it because it's easier. Even in those cases, the Terrier can usually still out- perform the Swivel in the same application.

It's fine if you just want to make a working spaceplane and don't care about payload fraction, but as far as that goes you can use pretty much anything for an anaerobic stage in a spaceplane. The folks who are building efficient and economical spaceplanes don't use swivels and there's a good reason for that.

Best,

-Slashy

[Van Disaster]

Lift and drag are exactly proportional in KSP. A small wing at high alpha producing the same lift as a big wing at low alpha will produce the same drag as that big wing. There's no difference aerodynamically, but the larger wing is more mass and thus reduces TWR and delta-V so the best thing to do is use as little wing as required to get off the runway. Another important thing is to have an angle of incidence on the wings, so that the fuselage can present its lowest drag profile into the airstream and the engines push in the most useful direction while the wings still produce lift.

New stock "aero" is more of an atrocity than I thought, then. At least the old one was comical. Why replace something daft with something slightly less daft.

If you start digging spaceplanes, do yourself a favour & install FAR. You can turn off aero failures & some of the more complicated stuff while you get the hang of it, but everything will make sense.

[AeroGav]

Lift and drag are exactly proportional in KSP. A small wing at high alpha producing the same lift as a big wing at low alpha will produce the same drag as that big wing. There's no difference aerodynamically, but the larger wing is more mass and thus reduces TWR and delta-V so the best thing to do is use as little wing as required to get off the runway. Another important thing is to have an angle of incidence on the wings, so that the fuselage can present its lowest drag profile into the airstream and the engines push in the most useful direction while the wings still produce lift.

Damn that's a bit lame. At least the optimum should be a wing that gives around 5 degrees alpha to be somewhat closer to real world aero. Obviously how you'd size it would still depend on what flight reigime you want to perform best in but hey..

During the early rocket powered part of the ascent though, i'd thought it's still better to have a larger wing at low AoA than a small one at high AoA because you'll be thrusting closer to prograde. As regards using incidence, a mk2 fuselage also makes lift so you want that at similar angle to wings as well?

[ajburges]

ajburges,

No, not particularly. The Poodle is just plain all- around a better option in installations where the Swivel would be considered. And often people will opt for higher t/w for designs that don't really warrant it because it's easier. Even in those cases, the Terrier can usually still out- perform the Swivel in the same application.

It's fine if you just want to make a working spaceplane and don't care about payload fraction, but as far as that goes you can use pretty much anything for an anaerobic stage in a spaceplane. The folks who are building efficient and economical spaceplanes don't use swivels and there's a good reason for that.

Best,

-Slashy

I said the Poodle was a better performer, but it still requires that 2.5m node. How does the drag of the Poodle + adapter compare to the Swivel? I would assume the 1-2 Swivel does better on drag.

You need a touch over 3 Terriers to match the thrust of one Swivel. Again, fewer parallel stacks should translate to less drag (assuming you need more thrust than the Terrier gives to begin with).

IMHO there are two main classes of space plane: Lifters and interplanetary craft. Lifters just need to reach LKO with payload (and return). Extra trust allows for lower part count, more payload flexibility, and fewer parallel stacks. Interplanetary craft need the Isp efficiency, but those craft are less practical than a transportation network.

For reference, what kind of anaerobic TWR are we talking about here? I tend to aim for an anaerobic takeoff weight TWR of .7 if there are no RAPIERs. I could likely reduce thrust some more, but that then eats into my flight margin.

[GoSlash27]

During the early rocket powered part of the ascent though, i'd thought it's still better to have a larger wing at low AoA than a small one at high AoA because you'll be thrusting closer to prograde.

[Red Iron Crown]

Ahh, but this is why it's so important to not pass Ap in a low t/w spaceplane

You have to be careful about how much you pop up at the transition so you don't wind up in that predicament.

I've played around with some ridiculously underpowered spaceplanes (as low as .34 t/w on rockets) and gotten them to orbit. This does incur a penalty to drag losses, though, and I like to keep it at at least .5.

Sure it's possible, but it is much more difficult than in a higher TWR rocket stage, certainly not easy enough to claim that "TWR is not so important" in spaceplanes.

Absolutely not. Drag from wings is computed completely differently than other parts. Generating lift from the Mk2 fuselage is *very* draggy. For best results, use the wings (with incidence) to create the lift and leave the fuselage aligned with airflow.

Are you sure about that? The Mk2 parts use the same ModuleLiftingSurface module as wing parts, I believe it is calculated in the same way, they just have some different values for the variables (i.e. they produce drag even at 0 AoA). I think of Mk2 parts as draggier wet wings that can't easily have an angle of incidence.

[GoSlash27]

I said the Poodle was a better performer, but it still requires that 2.5m node. How does the drag of the Poodle + adapter compare to the Swivel? I would assume the 1-2 Swivel does better on drag.

ajburges,

At supersonic speeds, surface and tail drag are miniscule. The size1->size2 adapter is a very clean shape (.425 Cd) and it occludes the engine itself. The drag penalty is marginal in that case.

Adding parallel stacks for the Terrier can also be done cleanly, and when using TJ hybrid designs you don't have much choice in the matter anyway; you have to add nodes. I like to use 2 Terriers for 1 TJ or 1 Poodle for 2 TJs.

IMHO there are two main classes of space plane: Lifters and interplanetary craft. Lifters just need to reach LKO with payload (and return). Extra trust allows for lower part count, more payload flexibility, and fewer parallel stacks. Interplanetary craft need the Isp efficiency, but those craft are less practical than a transportation network.

I agree. In fact, I consider interplanetary spaceplanes to be a self- conflicted notion. The entire point of an SSTO spaceplane is efficiency. There aren't many concepts less efficient than lugging an entire airplane to another planet.

However, spaceplanes do need some additional orbital DV to be useful as delivery vans, which is their prime function. They need to be able to correct inclination a couple degrees, intercept and rendezvous with a station in LKO, and deorbit.

I design my ships to have at least 250m/sec on orbit.

For reference, what kind of anaerobic TWR are we talking about here? I tend to aim for an anaerobic takeoff weight TWR of .7 if there are no RAPIERs. I could likely reduce thrust some more, but that then eats into my flight margin.

My TJ hybrid designs are pretty darn weak by that measure. I'm down around .5 at takeoff and I've gone as low as .34.

Best,

-Slashy

[Val]

All right well I took advice from everyone, and removing the extra tanks I used to get to orbit(barely) last time, I tried your ideas out on my craft, and it worked beautifully.

That's great

... The perfect ascent seems to be this:...

around 10km level out (with a gravity turn preferably) and start building speed. Maximum air-breathing speed should be ~15km and 1200m/s, traveling almost horizontally. (you may notice in my attempt I was too high and going up too fast), then SLOWLY, but not too slowly, pull up into a climb, keeping your AoA around 10°-15° the whole time. Speed will drop, for me just below 1100m/s, but keep going until your prograde is about 15° or higher and your nose is a bit above that...

This part I don't agree with. A perfect ascent should not have you going at 10°-15° off prograde or losing speed.

I recommend starting the pitch up maneuver already from 10 km, with enough speed* and just enough pitch up that when you reach 15 km you're going 1100+ m/s and at 15° pitch and prograde.

* (900-1100 m/s, depending on TWR and how heat resistant your craft is)

I made this really poor video to try and show it. It's just a short clip showing these steps.

• Level out at 10 km
  
• Build up speed to ~900 m/s
  
• Pitch up slow-ish to let the prograde marker try to catch up.
  
• Reach 15 km at 15° pitch and prograde with speed nearly 1200 m/s
  
• Engage LV-909s at 18 km
  

The craft used is the D-2 Scout Mk.4 which is very similar to yours.

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Are you sure about that? The Mk2 parts use the same ModuleLiftingSurface module as wing parts, I believe it is calculated in the same way, they just have some different values for the variables (i.e. they produce drag even at 0 AoA). I think of Mk2 parts as draggier wet wings that can't easily have an angle of incidence.

Yes, they are more draggier because you can't have incidence on them. Mk2 parts used for lift will always have the extra drag because they are also regular parts with regular drag, which increases a lot more than the lift drag when they're not pointed prograde.

Mk2 part lift is only good for high alpha maneuvers, such as re-entry, takeoff and landing, where the regular drag is low or not a problem.

And if they had built in incidence they'd be hard to use in single stack rockets. Maybe if they had tweakable incidence. That'd be cool.

[Mesklin]

kStrout, for craft like yours (high atmo TWR and relatively low rocket TWR ~0.5...0.6) main task is reach edge of turbojets area with enough vertical speed (>300 m/s) and enough horizontal speed (>1100 m/s) for fast transition from 20 km of altitude to 35-40 km for greatly decrease drag and prevention big AoA angles in thick atmosphere.

For your craft try to set pith after takeoff to 15 degrees and do not touch pith control until your turbojets will be suffocated, engage LV-909 at 15 km of altitude and maintain pith 15-20 degrees after stopping turbojets until your reach 2000 m/s of orbital speed, then set to prograde and start maneuvering only again after escaping of atmosphere.

Try to minimize pith maneuvering in time of atmospheric acceleration, each maneuver is lost of your final speed in atmosphere.

[ForScience6686]

I spent my weekend fighting this same problem. I have two whiplash and two nukes and just couldn't get the speed for orbit. Actually did make it once but I couldn't replicate it. I ended up adding two boosters, one to get above mach1 for thrust, and the second to add twr on the rocket stage. I found it better to fly at prograde as long as possible before angling up to build up plenty of speed. I am guessing that it could be a bit more efficient as I'm just behind the ap before bringing the nose up and it seems to eat up a lot of fuel even for nukes.

[DancesWithSquirrels]

Re: Swivels vs. Terriers and my previous suggestion:

This is why the spaceplane gurus don't use them...

Ah, that would be the problem with my suggestion. I'll freely admit I'm not a spaceplane guru - I find it far easier (and faster) to burn a disintegrating totem pole than fly efficiently.

That said, I'm envious of the skills required to fly SSTOs well.

[GoSlash27]

Re: Swivels vs. Terriers and my previous suggestion:

Ah, that would be the problem with my suggestion. I'll freely admit I'm not a spaceplane guru - I find it far easier (and faster) to burn a disintegrating totem pole than fly efficiently.

That said, I'm envious of the skills required to fly SSTOs well.

DancesWithSquirrels,

No worries, and again... my tirade wasn't directed at you personally.

Best,

-Slashy

[DancesWithSquirrels]

GoSlash27 - This post describes a rather earlier (0.90) example of mine, one of my rocket SSTOs with a little less than 8% payload fraction. Pretty inefficient, but I really find it faster in terms of gameplay to launch/land the rockets than with spaceplanes, and it did reliably lift 90 tons to LKO.

'Tho maybe I should give winged SSTOs another try, now that 1.x has something closer to actual aerodynamics than the beta versions (with orientation-dependent lift for surfaces, all lift centered on the first wing section attachment meaning I often needed to attach and adjust each little segment separately to get realistic lift distribution, and general soupiness) - and see if I can reliably get my designs to orbit with real payloads and more than a few fumes in the tanks.

The beta aero model drove me absolutely bonkers - I'm a pilot, and in the betas I couldn't build anything that flew the way it should...