1.whatever SSTO flight profile

[Captain Vlad]

So despite starting my career over and being nowhere near some of the wings and engines I like to put on SSTOs, I couldn't resist the temptation and started a sandbox game just to make some spaceplanes. Remade an old one that worked okay in .90 but I feel it has more potential in 1.02. If by potential you immediately think 'wow that's some good acceleration' then you and I probably think alike, but I'm running into a problem.

I'd heard Rapiers didn't accelerate so well at higher altitudes, and that the new semi-standard SSTO ascent profile is to gain speed at a lower altitude before pitching up. Buuuut...doing so, the Mk1 to Mk2 adapter I was using as a portion of the fuselage...overheated and blew up.

I started my first big acceleration around 10,000 meters, and a second attempt at 15,000 with the same results. Am I just starting too low or what?

[Teilnehmer]

I believe you can play with your pitch angle.

The Rapier’s ceiling is about 28 000 m, and the top speed is about 1500 m/s. The pitch angle should be so steep that you reach the top speed at the ceiling height where the atmospheric pressure is much lower.

Edited June 3, 2015 by Teilnehmer

[ajburges]

I think optimal accent for SSTO is still being worked on.

ISP does not appear to alter with jet thrust. That means jet engines are real gas guzzlers in their power band (due to higher thrust). For that reason I avoid going through the transonic speeds (250-500 m/s) until 10+ km where the air thins out. Avoid cruising in the transonic range the drag forces are the highest there and you'll just waste fuel fighting drag. RAPIER power plants may need to climb higher than 17km for a gravity assist to beat trans-sonic drag. Their thrust profile it's not as well aligned to that wall as turbo jets.

Once you pass the sonic barrier, you want full acceleration remember you guzzle jet fuel very fast at this point. "Reentry" effects are a suggestion to pitch up, but you want to be close to them to build speed faster. Chemical rockets still consume fuel faster than jets. Some profiles actually don't manage convection heat, but simply soak it up and get out before damage occurs. You want velocity to be at least 1 km/s 30 degrees above horizon when your jets cut (angle is inversely proportional to the TWR of your anaerobic engines, but if you are significantly off on this angle you either have too much engine hanging out as dead weight or not enough and you will suffer gravity losses). 1 km/s is a minimum. Any amount you add here is less anaerobic dV needed.

If you have asymmetric flame-out issues, you don't have enough intakes or they are poorly assigned. Your height/speed should kill their throat before they starve.

[KerikBalm]

I prefer to climb 30-45 degrees past 8km, then start following prograde to exceed 400 m/s, and generally want to be climbing at 20 degrees as I pass 15km.

At 20km, I want to be going 15-20 degrees up, ideally I'lld reach over 1,200 m/s surface speed with an apoapsis of above 30km.

With rapiers I tend to get 1,250-1,350 and a higher apoapsis before switching to rocket mode.

When I only get to 1250 m/s, it is typically because I climbed too fast, and I'll have an apoapsis of around 40km... which may med up being better, I'm not sure.

A good basic rule would be 20 degrees at 20km, I guess

[KerbMav]

Would building the wings with a higher angle of attack help in getting to higher speed, jets' thrust pushing more forward than up?

[ajburges]

1 km/s is less than half orbital speed. While you want as much horizontal speed as possible, you will still need to pitch up for a gravity turn on anaerobic thrust. Fighting the gravity turn produces plenty of lift.

It is definitely worth an experiment, but I imagine the higher induced drag would make it a net loss. You also complicate your lift vs AoA profile.

[Vanamonde]

How-to question moved to the how-to section.

[Red Iron Crown]

My best results for spaceplane ascent in 1.0.2 have been from:

1. Build plane with RAPIERs, 1 intake per engine, and about 1.3ish initial TWR.

2. Climb out at full throttle and 45 degree AoA, generally staying at less than 350m/s.

3. At 10km begin gradual pitch down until hitting 12-15 degrees by 12km.

4. Accelerate! Pitch up a bit if overheating gets scary, pitch down if climbing too fast.

5. Should reach 1200-1400 m/s by a bit over 20km.

6. Keep riding the airbreathing mode as long as possible, up to about 28km most times.

7. After flameout, switch to rocket mode, aim a few degrees above prograde and burn until Ap is 75km+ and several minutes away.

8. Point prograde and coast to Ap, circularize for <200m/s.

I'm by no means a spaceplane expert like some around here (they may have different ideas), but this method works well enough for me.

[ajburges]

My best results for spaceplane ascent in 1.0.2 have been from:

2. Climb out at full throttle and 45 degree AoA, generally staying at less than 350m/s.

Crown, unless drag changed from the observations I saw, you may want to limit yourself to 250 m/s on initial climb. 350 m/s is around the maxima of drag vs airspeed. You want to be below 270 or above 480 to minimize drag losses.

[Red Iron Crown]

Hmm, I may have to revise my ascent with that info. The RAPIERs don't have the oomph to go >480 m/s down low, my planes have been going from 150m/s just after takeoff to near 350m/s by 10km, I thought I was enough below the sound barrier to be avoiding the worst of the drag. More testing required, maybe I can save some fuel by throttling back during the climbout.

[gilflo]

Here is my spaceplane, tested with FAR, so it must work without FAR. http://kerbalx.com/gilflo/FAR-Avenger-3

There's explanations how to use it and accelerate before switching on rockets.

Some pictures here: http://forum.kerbalspaceprogram.com/threads/121176-Official-FAR-Craft-Repository?p=1991998#post1991998

[kStrout]

My issue is that if you are going to slow when you gain altitude, you aren't going fast enough to get enough air to go faster, I usually flatten out at around 1-2 km until 400m\s and then start climbing because otherwise I just lose speed as I climb even at full throttle.

[ajburges]

My issue is that if you are going to slow when you gain altitude, you aren't going fast enough to get enough air to go faster, I usually flatten out at around 1-2 km until 400m\s and then start climbing because otherwise I just lose speed as I climb even at full throttle.

I solve this with a (near) gravity turn and avoiding WEP for my initial accent. Flatten out invokes the wrong imagery and using full throttle leaves you no reserve when altitude robs your power.

I ascend to 10 km with 250 m/s at the steepest angle (at most 70) I can maintain speed at 70% throttle. Having some afterburner is important so you have more oomph when you get high and turn. Around 10 km I max throttle and gradually level out being careful to avoid much negative wing lift (negative AoA). Negative wing lift during this just creates induced drag losses. I pick up speed through the turn which is relatively quick due to the low speed.

Lower aerobic TWR craft will need a shallower accent, but they typically need gravity assists to enter the super sonic regime anyway so they need to fly to 18 km in initial accent.

This only works because mach 1 does not change WRT pressure. If that flaw is fixed, the rules will change. I don't know the equations, but I imagine going Mach 1.5 at sea level will never be efficient regardless.

[Captain Vlad]

I'll give some of these suggestions a go when I get home and let ya' know how it works out. Thanks in advance for the help.

[NathanKell]

Speed of sound is a function of temperature.

As for ascent, climb to over 10km subsonic, dive slightly to break through to supersonic, climb up to about 18-21km gradually lowering pitch angle so you milk as much air-breathing speed as you can. If you're going supersonic in level or climbing flight you're probably over-engined.

[GoSlash27]

Speed of sound is a function of temperature.

As for ascent, climb to over 10km subsonic, dive slightly to break through to supersonic, climb up to about 18-21km gradually lowering pitch angle so you milk as much air-breathing speed as you can. If you're going supersonic in level or climbing flight you're probably over-engined.

The generic recommendations here work well for me.

Not much to add except that RAPIER designs can build speed at around 20 km altitude while TJ hybrids like to build speed around 15-17 km.

Good luck!

-Slashy

[ajburges]

Speed of sound is a function of temperature.

Is that the KSP implementation or are you inferring that's real life? Speed of sound is proportionate with bulk modulus (stiffness) and inversely proportionate with density of the medium. The only effect temperature has on it is on density.

Slashy is right about preferred altitudes.

Turbo and RAPIER thrust curves WRT altitude see a major drop in the first derivative at around .06 and .03 atm respectively. That is roughly 15 and 18 km. Flying higher allows better thermal management though.

Turbos have maximum thrust WRT speed at 1 km/s. RAPIERs at 1.3 km/s. Going much above that means you have probably over engineered your craft.

On an interesting note, RAPIERs have an unique drop in the first derivative of thrust WRT pressure at .3 atm (6-7 km). It's slight, but that is where turbo jets really diverge from RAPIER performance until they intersect at .04 atm and RAPIERs beat turbos. So there is numbers to support breaking the sound barrier at 7-10 km on RAPIERs. (You lack lift to fly well at 18 km subsonic without over engineering)

Edited June 4, 2015 by ajburges

[Captain Vlad]

Welp, she made it. Not efficiently and with just enough fuel left over to de-orbit and set down on dry land. Climbed to about 15K then followed the tip that you need to be close to reentry effects...trailed flame most of the way out, but the light feathery flame, so no damage incurred. I made it to 28,000 meters before I had to switch modes and attained a 118x101 km orbit (or thereabouts). Need some serious flight profile work, but nice to no I could do it, especially with this odd looking beast.

[antbin]

Congrats!

In my limited experience with max cargo fraction, the most important measure of my ascent profile wasn't angle or altitude, it was climb rate. I tried to maintain at least 150 m/s vertical climb speed, which means gravity turning as you accelerate in the 8-18km powerband, but never letting your prograde level out at less than 5-10 degrees above the horizon.

Milking the last bit of airbreathing speed at >20km is important, but not at the expense of losing your climb rate. You'll just have to spend more (heavy) rocket fuel raising your apoapsis again, while bleeding drag all the time. When you go for orbit, go for it!