In-atmosphere flight of SSTO spaceplanes...

[PanzerAce]

So I've got a basic SSTO Spaceplane that mostly works. I could get into space in .90 with NEAR fairly easily. However, I had a problem then (that I ignored) but figure I should ask about now: I can get to ~20km with no issues. However, once I'm up there, I only have a few options:

1) already be on a ballistic arc, and fire my rockets.

2) have my engines flame out, slide down to ~15k, re-light them

3) have my engines not flame out by coasting up slower, but have zero control authority, slide down to 17k, repeat.

It *seems* like I should be able to get higher, but I either am going to slow and my engines burn out, or I completely lose control authority. Any recommendations? (If it matters, I'm running a canard setup)..

Edited May 2, 2015 by PanzerAce

[Blu_C]

What type of engines are you using, out of curosity?

Different types of engines now perform significantly differently at different speeds and altitudes. You will notice that standard jet engines are MUCH weaker when going at mach speeds or at high altitude. With Turbo Jets you need to be going fast before they really kick in, and to operate above ~18k above sea level requires a fair number of air intakes (I suggest at least 2 ram air intakes per turbojet, if you can swing that).

From there a lot of it seems to be a problem of climb angle. If you are too shallow then you pick up too much speed in the lower atmosphere and will blow up. Too steep and you don't have enough time to pick up speed, not getting the full benefit of your air breathing engines. The sweet spot depends on the exact plane, but try experimenting with something between 30-55 degrees. If you get it right you should be able to easily reach around 1,000m/s before flaming out with an apoapsis of over 60km (and with some experience getting sub-orbital is pretty easy).

No matter how many air intakes I put on the engines always seem to flame out somewhere between 30-40km (though they lose a significant amount of thrust as you climb above 18km) so try to account for that. You'll probably want to activate your rockets before the turbojets completely die.

[eddiew]

The only approach that's worked for me so far is:

- get to 15km asap

- controlled ascent to 20km, doing 900-1000m/s when you get there

- haul the nose up hard

Air-breathing rapiers fizzle about 27km, and your apoapsis should be around 35, which is a reasonable curve for your rockets to take over from Basically you won't get a stable flight much above 20km any more with nuStock aero, but you can use it as a launching ramp for the best ballistic trajectory you can get with jets.

And yeah, intake stacking is now pointless. Jet thrust relates to external air density, not internal air supply.

[PanzerAce]

What type of engines are you using, out of curosity?

Different types of engines now perform significantly differently at different speeds and altitudes. You will notice that standard jet engines are MUCH weaker when going at mach speeds or at high altitude. With Turbo Jets you need to be going fast before they really kick in, and to operate above ~18k above sea level requires a fair number of air intakes (I suggest at least 2 ram air intakes per turbojet, if you can swing that).

From there a lot of it seems to be a problem of climb angle. If you are too shallow then you pick up too much speed in the lower atmosphere and will blow up. Too steep and you don't have enough time to pick up speed, not getting the full benefit of your air breathing engines. The sweet spot depends on the exact plane, but try experimenting with something between 30-55 degrees. If you get it right you should be able to easily reach around 1,000m/s before flaming out with an apoapsis of over 60km (and with some experience getting sub-orbital is pretty easy).

No matter how many air intakes I put on the engines always seem to flame out somewhere between 30-40km (though they lose a significant amount of thrust as you climb above 18km) so try to account for that. You'll probably want to activate your rockets before the turbojets completely die.

Pair of turbo jets. I've got a ram air and two pre-coolers for each (I'm building these things to atleast *look* like a real plane. I'm trying not to just do a pure zoom-climb, since there's got to be a more efficient way to do it.

Eddiew: I'd love to do that, but I don't have any control authority at 20km. doesn't matter how much I pull back, that nose won't climb. Annoyingly, it'll go DOWN no problem....

[PanzerAce]

Played around with various setup of jets, rapiers, wings, etc. At one point I hit what I think is ram-jet mode at ~25k. Engine noise changed and the thrust jumped all of a sudden. Problem was at that point my AOA was ~25 degrees, I was basically just sliding through the air :/

[capi3101]

Eddiew: I'd love to do that, but I don't have any control authority at 20km. doesn't matter how much I pull back, that nose won't climb. Annoyingly, it'll go DOWN no problem....

Sounds like a pitch authority issue. Can we see an screenie of one of the planes with which you are having issues please? Side profile in the SPH with the CoM and CoL markers turned on would be most helpful. I'm wagering that your pitching surfaces are either too small, too close to the CoM to be effective and/or in-line with the main wing (which reduces their effectiveness).

[StainX]

So what i do now in 1.0 (still experimenting of course):

For SSTO`s i use a ram inlet,pre cooler and rapier.

Look at the built craft dry and full state for COM/COL balance purposes... Hint: Place Payload close to COM.

Ascent Profile as i handle it now:

!!Always keep a good look at mach or heat effects!!

< 9km : 30-45° pitch

< 20km: 10-20° pitch

On low thrust, switch to rocket motor/mode and pitch up to 30-40°, you should be going around 900-1000 m/s at that point.

Descent Profile (easy thing):

Lower Periapsis to ~40km (depending on craft) above space center. (I always plant a flag at the end of the runway to have something to aim at)

Keep the Pitch up so your vertical speed isn't too low, not too much or you loose speed that you still need to get close to KSC.

If you feel nervous about heat... activate Airbrakes a few seconds.

When KSC is in Range, Airbrakes on, and do the usual landing stuff.

Edited April 30, 2015 by StainX

[PanzerAce]

Sounds like a pitch authority issue. Can we see an screenie of one of the planes with which you are having issues please? Side profile in the SPH with the CoM and CoL markers turned on would be most helpful. I'm wagering that your pitching surfaces are either too small, too close to the CoM to be effective and/or in-line with the main wing (which reduces their effectiveness).

I'll get screenies of all the basic designs I've been working on when I'm back on the computer I've got KSP on.

Too small of control surfaces? do the stock canards not cut it in conjunction with rear surfaces?

[Blu_C]

I'll get screenies of all the basic designs I've been working on when I'm back on the computer I've got KSP on.

Too small of control surfaces? do the stock canards not cut it in conjunction with rear surfaces?

That is really hard to answer without seeing the craft. One very real possibility is that the rocket is getting excessively nose heavy or that you have a huge amount of drag on the tail which can make it difficult to deviate from the prograde vector.

[PanzerAce]

Oooooooooooook, worked some stuff out. Ditched the oldder designs that were giving me trouble and started from a clean slate. Built the following delta space plane.

Fuselage parts, front to back, are: Mk2 cockpit, drone core, crew cabin, docking port, Mk2 liquid fuel tank (short), Mk2 rocket fuel tank (Long), Bicoupler, RAPIER engines.

Side pod parts, front to back, are: Ram intake, FLT800 fuel tank, pre-cooler (do these actually do anything?), RAPIER.

Using the Big-S delta wings (empty of fuel) and elevons (Big-S elevons make it WAY too twitchy though, and I've got more than enough authority now, so prob going to switch those for smaller elevons).

I can get this sucker into space using either a straight up zoom climb (unable to orbit), or a more gradual, lower throttle climb to ~18k before flooring it (Also unable to orbit, but a good bit closer).

The issues I'm having are:

-1050m/s over the ground seems to be the upper limit before the cockpit explodes from over heating (below ~22km).

-At that altitude, even if I pitch up, I don't have the lift to climb anymore, and start to "slide"

-keyboard controls are SUPER twitchy, even with precision controls activated. Would it actually help to setup my X52 Pro to fly this in-atmo?

As a side note, on the subject of engines/intakes: a pair of RAPIERs is a necessity at this weight (34 tons). the other two can be swapped for turbo-ramjets (again, do these actually do anything over the rapiers?), but I can't swap in a set of, say, aero-spikes, unless I'm satisfied with sub-mach-1 speeds in atmo.

My thoughts right now are to try swapping the FTL800s for a series of Pre-coolers or Mk1 Fuselage intakes. I don't think I'll have the oxidizer at that point to actually reach orbit (fuel hasn't been the issue yet), but I might be able to run on jets a bit longer/higher. Still not going to solve the lift issues at high speeds....

Edits below as I test:

Test 1: Swapped the FTLs to 3 precoolers (for a total of 4 behind the ram intake), and the outer rapiers to turbo-rams. Was able to get to ~24k under ramjet? power and ~1200m/s surface speed. The problem? with SAS on or off, the plane started to see-saw. Further testing on this setup shows that while I can reliably get sub-orbital at this point (~80k Apo) in a gradual climb method (Rapiers to rocket power at ~20k, turbos run till about 25k), I consistently fall about 900m/s short in terms of delta-v.

At this point I need one of the following it seems:

1) A way to cram more fuel on board without losing intake air. This is....not my preferred method, since adding fuel doesn't increase my speed, and I have to start firing the rockets lower to make up the weight.

2) A way to have the Mk2 cockpit survive higher speeds than the ~1000m/s speed it currently can. If I could run it at 15-20k and 2000m/s, I'm fairly certain this would be simple (since, for one thing, I could run the jets even higher with the extra air-flow)

3) A way to run the jets higher. I'm generally finding that it's best to switch the rapiers to rocket mode at about 20k. Higher and the turbos choke faster from lack of air, and the rapiers auto switch anyways at that point, and lower I'm just wasting air.

4) RATO (rocket-asist-to-orbit) drop pod rockets. Again, I don't want to do this. This is supposed to be an SSTO for Kod's sake.

This is the current "best" setup I've gotten to (yes, I'm missing a set of rear RCS. I'm not motivated enough to add them back in every time I swap something) (also, yay! 7.4 tons lighter!):

Edited May 1, 2015 by PanzerAce

[eddiew]

I suspect that you can get that plane to orbit by a change of piloting tactic Four rapiers is far and away enough thrust for the job, and you might well manage with just three - although I'd be inclined to add the 'missing' control surfaces to the rear wing, and have the outer pair for roll and the inner pair for pitch.

What you might want to try is this:

Turns out the key is to follow the Mach effects, without blowing up. Basically, the closer you are to Mach effect territory, the more air you will have for your engines and the higher their thrust will get (the effect is quite dramatic, actually). If you go too slow, they won't give you enough oomph, so pull down AoA again until you get mach effects and a decent acceleration, then go up basically as fast as you can. But, once you get close to the thermal barrier (about 1km/s at 20kms, less if lower) you have to pull up to go higher, faster and thus avoid burning up.

You're basically dead right that you can't exceed about 1km/s below jet altitude and survive. The best flight plans I've found are reactive to the mach effect visuals, using the last big gasp of jet thrust to pull up hard and push the AP ahead of them. With rapiers, you can get 'useful' thrust in air mode up to about 25km, and they'll die at 27 whatever you do; in an ideal flight, you'll have an AP well over 30 at this point, and I've managed 35 on occasion.

If the controls are twitchy; use smaller control surfaces ^^; In fact you probably don't need to be using the big delta wings, I think the old triangles would do... might have less drag

[capi3101]

Okay...so yeah, your pitching surfaces are both pretty close to your CoM and in-line with the main wing (the second of which, since you're going with a tail-less delta design, is to be expected). I might suggest moving the wings back a little and compensating for the shift in CoL with a pair of canards; those would help your pitch issues anyway since they'd be well forward of the CoM. Maybe slightly above the plane of the main wing if you can manage it.

Speaking of the main wing, it looks a little on the small side to me. Ordinarily I'd diagnose issues there by take-off speed, but you've got an excess of thrust available to you at takeoff (myself, I'd launch a 40-tonne plane with two turbojets and two RAPIERs), so that's out. The only other way I know to go about figuring out how much wing you need is to calculate the wing area for a reasonable wing loading, which A) is probably something that's not necessary in the new stock aero and something I haven't figured out how to do yet with stock parts...

Definitely would not be using an Elevon 1 for a plane that size as a main pitching surface, IMHO (unless it's working for you).

[PanzerAce]

I suspect that you can get that plane to orbit by a change of piloting tactic Four rapiers is far and away enough thrust for the job, and you might well manage with just three - although I'd be inclined to add the 'missing' control surfaces to the rear wing, and have the outer pair for roll and the inner pair for pitch.

What you might want to try is this:

You're basically dead right that you can't exceed about 1km/s below jet altitude and survive. The best flight plans I've found are reactive to the mach effect visuals, using the last big gasp of jet thrust to pull up hard and push the AP ahead of them. With rapiers, you can get 'useful' thrust in air mode up to about 25km, and they'll die at 27 whatever you do; in an ideal flight, you'll have an AP well over 30 at this point, and I've managed 35 on occasion.

If the controls are twitchy; use smaller control surfaces ^^; In fact you probably don't need to be using the big delta wings, I think the old triangles would do... might have less drag

As clarification: I've got smaller control surfaces, just don't bother to put RCS on untill I get something that's actually orbit-capable. TBH, I don't *think* flight profile is the issue. Like I said, unless I can find another ~1km/s somewhere in the current design, that's the issue. I can already get the AP up to 30/35 on jet power, it's getting it from that to ~75km and then circularizing that I fall short on. I generally keep it at ~800m/s untill I hit about 17km, then floor it. By the time I flame out, I'm up to ~1300m/s

Okay...so yeah, your pitching surfaces are both pretty close to your CoM and in-line with the main wing (the second of which, since you're going with a tail-less delta design, is to be expected). I might suggest moving the wings back a little and compensating for the shift in CoL with a pair of canards; those would help your pitch issues anyway since they'd be well forward of the CoM. Maybe slightly above the plane of the main wing if you can manage it.

Speaking of the main wing, it looks a little on the small side to me. Ordinarily I'd diagnose issues there by take-off speed, but you've got an excess of thrust available to you at takeoff (myself, I'd launch a 40-tonne plane with two turbojets and two RAPIERs), so that's out. The only other way I know to go about figuring out how much wing you need is to calculate the wing area for a reasonable wing loading, which A) is probably something that's not necessary in the new stock aero and something I haven't figured out how to do yet with stock parts...

Definitely would not be using an Elevon 1 for a plane that size as a main pitching surface, IMHO (unless it's working for you).

Not having control authority issues anymore actually, that seems to be fine using the Big-s. one thing I'm thinking of is making a larger delta wing to get more lift at altitude. I don't want to, but it's the only option I'm thinking to get a better profile.

[PanzerAce]

a....HA! Same basic setup, four rapiers, managed to get to ~35k on a VERY badly controlled climb before all four were on rocket power (manual changeover at ~20km for two of them). Managed to end up only 655m/s short from orbit. Still running into a lack of oxidizer issue....I'll update this post as I play with it for the next half hour or so.

Edit: test run #2 on the new setup:

FOURTY KOD-DAMN M/S SHORT OF ORBIT! Still with an inefficient launch.

With an optimized in-atmo portion of the flight, I got to a 84kmx86km orbit. 198 oxidizer left (no idea how that equates to delta v on this thing, all stock parts right now).

I present to you the SSTO-DELTA-51, a SSTO using only stock parts in 1.0.2 (47 parts [including two drouge chutes and four air-brakes], 32.3 tons):

The biggest change (obviously) is that I swapped the std. Mk 2 cockpit for the inline variant. Put two intakes on the front, swap the small fuel tank for a large rocket tank, put 60 fuel in each wing (not actually needed afterall), and you've got orbit. The key thing to note is that this setup, at 15-20km, can do atleast 1400m/s without anything blowing up. That's that much extra delta v that you *don't* need to use the rockets for, and can save fuel for actually bumping the apo up. I ended up switching one set of rapiers to rocket mode at ~30km, even though they were all still in air-breathing mode.

Edited May 2, 2015 by PanzerAce

[EtherDragon]

a....HA! Same basic setup, four rapiers, managed to get to ~35k on a VERY badly controlled climb before all four were on rocket power (manual changeover at ~20km for two of them). Managed to end up only 655m/s short from orbit. Still running into a lack of oxidizer issue....I'll update this post as I play with it for the next half hour or so.

Edit: test run #2 on the new setup:

FOURTY KOD-DAMN M/S SHORT OF ORBIT! Still with an inefficient launch.

Yep, you gotta be a pretty consistent, patient pilot to get an SSTO to go. Once you got it, though - it's cake. Here's my routine:

Take off from runway.

Pitch up about 20degrees.

Get to max V.

Pitch up to between 30 and 45 degrees to climb to 10km quickly (climb angle depends on craft).

Start leveling at 10km, by 12km I'm 10-15 degrees above horizon gaining speed like mad.

Use pitch to keep speed under 1km/s and climb to about 17.5km.

pull up hard, and get ready to switch over.

As soon as my speed starts falling off (at about 25km), switch over to rocket mode.

Maintain 30degrees upward pitch.

Push Apoapsis to 72km, engine cut-off.

Profit!

[PanzerAce]

Ether, check out the updated design. Not nearly the same worries about over-heating (1km/s? Pfah! Old news! 1400m/s is the new 1000m/s).

That being said, what's your setup look like? I'm interested to see how other people get it.