Spaceplane Fuel/Weight Ratios

[Colonel_Camel]

Spaceplanes are STILL the nut that I can't crack (Grrr!)

ATM, I'm trying to get a space shuttle-esque spaceplane in to orbit, but it just isn't making the grade. I'm not sure whether my problem is poor ship design, or whether I'm just not flying it right.

I'd be curious to know what the initial weights and fuel loadings are for *successful* spaceplanes launched by other other players.

My stats are:
Weight: 83.776t
Fuel: 6350
Oxodizer: 2750

Weird thing is, I got an experimental shuttle similar to my currant design in to orbit ONE time, with plenty of fuel/oxodizer to spare, but I've never come close to replicating that flight since.

BTW, can anyone reccomend any decent spaceplane-to-orbit tutorials that are based on RAPIER powered craft? All the ones I've seen thus far seem to predate the RAPIER's inclusion in the game and thus use differant flight mechanics.

[AoD_lexandro]

Im a rapier jock. All my ships use them. My Mk3 SSTO uses 6 rapiers and can make 1200m/s in the atmosphere its that bloody fast. Its 40.5T dry weight with 49T of Liquid and Oxidizer in an even load.

Launch tecnique is super simple. Take off, fly level, go to mach 2.5 ASL, pull back to 30 degrees of pitch and hold till 22k ALT, then switch rapier modes and burn prograde to pre-orbit at 80K ALT. Circularize at AP.

I have enough fuel to tank up my Mun/Mimus base modules in the cargo bay and then launch them in orbit for flight & landing on Mun/Minmus & do a retro burn for a powered landing at KSC.

[AoD_lexandro]

If you mean me, my Mk3 hits 1300+m/s @20k at about 15-20 degrees on the navball before I switch to prograde and closed cycle. I used the closed mode on low burn levels for a more sustained burn up to 2000+ms before circularising. Personally I find 850m/s ASL and 28 degrees climb angle just about "right".

[Colonel_Camel]

Originally posted by AoD_lexandro:

Launch tecnique is super simple. Take off, fly level, go to mach 2.5 ASL, pull back to 30 degrees of pitch and hold till 22k ALT, then switch rapier modes and burn prograde to pre-orbit at 80K ALT. Circularize at AP.

This sounds like my first problem - I've been climbing to 10k altitude before I begin trying to build up serious speeds. I'd seen a tutorial indicating this was the right way to do it.

Originally posted by RoofCatA:

What is your speed and angle on NavBall at 20km altitude?

Speed: 1000-1300+m/s
Angle: Varies, as I've been doing a lot of trial and erroring trying to figure it out. Sometimes I've built up speed beyond 1400 at 13-15k altitude, but then I try to lift the angle to 45deg and the speed drops down to just over 1000. Other times, I've kept the angle closer to 15deg, and the speed doesn't drop much below 1300.


Both of your advice seems to be helping, as I just got to an "orbit" with a periapsis of 66k before my oxodizer gave out. I know this isn't a resounding success, but it's the furthest I've gotten so far. ;)

[Toastie Buns]

Uhh...okay. I boost above the cloud cover and start there. Attack runs gain 1500m/s (before everything starts to toast it's own buns) then ease back into a 40 degree AoA and just cruise upwards (engines need closed cycle mid way through the climb) I'll cut the engines when my apoapsis hits about 68,000 and the lift from the atmosphere takes it to 70,000. If it doesn't, a quick spritz fixes that. Circulization costs a mere 300m/s or occasionally 400m/s depending on how heavy the craft is.

Horses for courses, eh? My planes aren't giants, they're only using 300 liquid fuels to do this shared over 3 rapiers.

[Toastie Buns]

...I may be too baked to understand this. Traditionally, the fighter jets at Edwards would pretend to be spacecraft by gaining as much horizontal momentum as possible then trading horizontal momentum for vertical lift, thus they 'popped' outside the atmosphere for a little bit. Of course, KSP doesn't mirror this well without aero mods, but the principle should be identical.

So then, shouldn't the attack run gain as much horizontal velocity as possible? This isn't possible at sea level, because drag compounds you....

[Toastie Buns]

Right, but Rapier get all their performance boosts at Mach 3.5 (ish?) where they get 8x the thrust. Going at sea level would net you 3x thrust. Eventually you reach a point where air friction will sizzle your low temp wings. So then the bulk of your speed run would be gained through going to thinner air (climbing), thus seems to negate the reason for a sea level attack run in the first place, when you could just quickly ascend to cloud cover and begin there.

Needless to say, I'm still confused.

I get the principle....but it seems to me you'd lose less boosting above cloud cover at lower speeds then beginning the run there. At 300 - 400m/s, you're not losing THAT much by shifting the prograde vector down and you reach 1500m/s VERY quickly. It's not like a vertical launch shifting to horizontal, it's more like a 10 degree pitch down.

[Toastie Buns]

The scientific number for 'cloud height' is 'just above the first atmospheric bar'

(I forget I run mods sometimes)

And I get the 8x (8.5 actually) from the velocity curve you spoke of.

velCurve
		{
			key = 0 1 0 0.08333334
			key = 0.2 0.98 0.42074 0.42074
			key = 0.7 1.8 2.290406 2.290406
			key = 1.4 4.00 3.887193 3.887193
			key = 3.75 8.5 0 0
			key = 4.5 7.3 -2.831749 -2.831749
			key = 5.5 3 -5.260566 -5.260566
			key = 6 0 -0.02420209 0
		}
		atmCurve
		{
			// higher thrust at altitude than even TRJ
			key = 0 0 0 0
			key = 0.018 0.09 7.914787 7.914787
			key = 0.08 0.3 1.051923 1.051923
			key = 0.35 0.5 0.3927226 0.3927226
			key = 1 1 1.055097 0
		}

As you see, at Mach 3.75, the engine produces 8.5x stationary thrust
Then we factor in atmosphere, drag etc and you get how fast the engine can go before drag overwhelms it, resulting in the 3x thrust at sea level I spoke of before.

Ergo, looking at this from a PURELY GAMEPLAY stance, it seems to me that you're better off reaching the correct altitude as quickly as possible to take advantage of the rapier's main selling point: higher thrust at higher speeds at higher altitudes. (compared to a whiplash, which gets its best multiplyer 3.63 at Mach 2.5 and physically cannot produce additional thrust at Mach 4)

Hope this clears up my massive confusion, since you don't appear to be saving a great deal with a sea level attack run.

[Toastie Buns]

Originally posted by RoofCatA:

Again, what is cloud altitude?

Again, JUST ABOVE THE FIRST ATMOSPHERIC BAR. I mean, I can fire the game up and find out, but I'm sure you're familiar with the first ♥♥♥♥♥♥♥ bar on your altimeter.

[Toastie Buns]

It's just a frame of reference. I assume more people are familiar with 'the first bar on their altimeter' than they are if I were to say '8000m'

And according the playing the game; my clouds finish at 7600m

[Toastie Buns]

AoA from sea level is ~20 degrees until ~8000m
AoA at 8000m pitches down to ~5 degrees, maintained until Mach 5 or flameout, whichever comes first
~20,000m AoA pitches up to ~40 degrees, at which point we're trading horizontal speed for vertical speed

No, I'm not going to get out my protractor and give fine measurements, ballpark figures work fine.

[Toastie Buns]

No, because I'm still climbing. I still have that 5 degree incline, remember? Due to how fast I'm going, I'm ROCKETING through the atmosphere, not attempting to hug the horizon. I *could* hug the horizon, but I find at Mach 5 I start to explode, so I need the higher atmosphere to allow for that...less explody virtue it offers.
If I don't pay attention and allow myself a 10 degree AoA, I'm flaming out at Mach 4 punching through the last layer of atmosphere.

I'll *probably* be reaching Mach 4 at ~14,000m. I'd have to actually pay attention, since I've never had to have this conversation with anyone.

Edit: I may just record it.

[Toastie Buns]

Originally posted by RoofCatA:

oh, I got. You pitch down from 20° to 5°. I misinterpreted (my bad english probably) you are going -5°. If you can get so much speed there, your ship is OP already.

Yeah. I tend to cubic strut 2 extra rapiers at the side of one placed with node attachment, then allow them 100 liquid fuels each. The rest is kept snugly inside its switched off tanks, waiting for flameout. I'll have those tanks right clicked and ready to just switch on liquid fuel and hit abort (backspace) to close the intakes and fire closed cycle (I haven't upgraded the SPH to get numbered action groups)

It gets to space SO ♥♥♥♥♥♥♥ QUICKLY that I wondered if there really was any advantage to sea level attack runs as opposed to cloud cover attack runs (which we've now established as 8000m) that's all.

Guess we got a bit sidetracked, but whatever. I *could* have just tested it, but wheres the fun in that?

Right?

Guys?

[Toastie Buns]

RIGHT. Okay. I thought 'diving' simply meant the transition from say, a 20 degree AoA to a 0 degree AoA. I now realise what you mean is diving to achieve more speed, following a prograde vector that's below the horizon to then push back it back up using the lift of the lower atmosphere.....right?

As to why I push to 40, I use the lift co-efficient (what's left of it) to raise the apoapsis. The closed cycle rapier basically does very little work, it just sprizes its fuel a little and prepares for the big push once we get to within 3-4 seconds of our time to apoapsis. I'll try my hardest to get 70km, because I want that extra fuel to be lazy and dock with whatever the first time around (usually needs a substantially larger orbit and of course dV to slow back down)

After 50km however, lift just buggers off. At that point if you're not at 70km, it requires another spritz of fuel

EDIT: So my question was really: Is a 5 degree AoA from sea level (including attack run, of course) better than lofting yourself to 8000m to begin the attack run?