SSTO spaceplane won't gain enough speed without losing altitude

[Hoovy123]

I am getting the hang of building and flying atmospheric aircraft, but I have trouble with SSTOs.

I follow the ascent profile that nearly everyone does, I pitch up to 40 degrees and when I'm at about 10 km, I smooth out to 10 or 15 degrees.

The problem is when I'm gaining speed while preparing to switch to rockets, I lose altitude. I pitch my nose up to prevent this, and then I'm either losing speed or gaining speed at a really slow rate. What can I do to gain speed quickly without losing altitude?

My plane:

[bsalis]

It seems strange to be hitting the ceiling before getting an engine flameout.

Anyhows... any plane you build will have an operational ceiling, where you cannot go any higher or faster. Once you are there, you either engage the rockets (or burn up some more fuel before engaging the rockets). If you don't get to orbit you need to make design changes.

Perhaps if you link the craft file i can have a look.

[lammatt]

you have 1700m/s of rocket dv...

i am pretty sure you can basically switch to rocket at any speed >1000m/s wrt the surface. (since a 75kmX75km orbit is ~2300m/s and a surface speed of 1km/s is ~1200m/s orbital speed)

[boomerdog2000]

Have you tried switching the engines around?

1 909 in the center and 2 turbojets on the outside.

While this would net you less thrust in orbit, you'd have more thrust in atmo.

[Vlk]

I dont really like exploiting the air intakes too much, so my SSTOs usually start at around 1400m/s. But this way, i need stronger thrusters. Like the LV-T45 - it has a slightly worse ISP, but it gets you out of the atmospehere faster.

[numerobis]

The top speed I calculate for your plane on jets alone is 1140 m/s at 23 km. That's a long way from orbital velocity, so you'll need to pitch up rather a lot to fight gravity.

Your rocket engines have TWR about 0.65, so at best you can go 80% of terminal velocity off them alone at full throttle. The lowest altitude where they could push against drag hard enough to reach orbital velocity is just over 33 km; below that altitude, you need to be pitched up to fight gravity, which reduces your speed.

Somehow you need to bridge the gap between 23km at 1140 m/s and 33km at 2200 m/s. If it's possible, my best guess is that you'll need to fly your jet at a steep angle to push apoapsis up high (say, 38km, where your top rocket speed is well above orbit velocity). Then you use your time on a ballistic trajectory to try to gain enough horizontal speed for your low-TWR rocket to reach orbit.

But it might not be possible, in which case the answer is either more rocket thrust or more intakes -- or, perversely, bring up less rocket fuel to reduce your mass. Halve the mass and you can ride jets up to about 24km at 1550 m/s, whereas your rockets can push you to 12% faster than terminal velocity, so you can reach orbital velocity just above 30km. There's still a gap to bridge, but it's much smaller.

Edited October 11, 2013 by numerobis

[Hoovy123]

Thanks for the help everybody. I replaced the one turbojet with 2 turbojets, one on each side. I also replaced the 2 LV909s with an aerospike and made the top layer of wings swept wings. It actually accelerated pretty fast while gaining altitude and made it into orbit with the help of the aerospike, though it had to deorbit with RCS since it ran out of fuel.

Edited October 12, 2013 by Hoovy123

[numerobis]

One thing to note: more jets means faster speed at low altitude where there's lots of air, but it doesn't help your top speed. The only way to get higher speed off jets at high altitude is by adding intakes.

Another thing: you can fly in hybrid mode, which helps bridge gaps between all-jet and all-rocket. Start the rockets, but keep riding the jets up (and throttling back when the air gets thin). Eventually, running the jets is no longer worth the extra drag from the intakes, so you turn off the jets and close the intakes.

[Starglider]

Its been my experience that too much wings slows you down too much, yet you may need them due to having too much weight. From the looks of it, have you tried less fuel, and less rockets? I think all you need for rockets, is a pair of Rockomax radials. There's other things I see as well; do you really need all those RCS thruster blocks (can you make do, placing the bare minimum at your CoM, and relying on SAS for rotation)? Can you make do with less RSC fuel?

Weight is not your friend.

I have a spaceplane with a similar mission profile that clocks in at under 10 tons - Kerbil Engineer says 8.124.5 kg. It has one mk1 fuselage for jet fuel, an FLT100 for rocket fuel (yes, only that much), a turbojet and two Rockomax Radials. For lift, its uses a pair of delta wings and a pair of canards - and that's it. I can consistently get it to 150km circular orbit (where my station is) with fuel to spare for re-entry and powered landing at KSC. I could do better by decoupling landing gear after takeoff and parachuting back to Kerbin (believe it or not, parachutes + decouplers to get rid of the gear actually saves weight), but I enjoy the challenge of proper landings on the runway.

[numerobis]

believe it or not, parachutes + decouplers to get rid of the gear actually saves weight

I'm going to not believe it: stock landing gear are actually massless in flight. So you should just use them.

The newer rover wheels are not massless.

[lammatt]

dont know if Starglider's a troll

what's the point of building planes if you are detaching the wheel when you got airbourne.

How can you fking land withour wheels?

what? parachute? then why would i build a plane at the first place, i'd just vertically take off with rocket.

[numerobis]

You can land without wheels by using a parachute. What's the problem with that?

[WhiteWeasel]

Also why are you using 909's? A single aerospike provides more than triple the thrust (175> 50). And has the best Isp in atmo of any engine.

With my first successful SSTO I used two turbo jets and one aerospike.

[Starglider]

The weight of each landing gear is 0.5 tons. On a small plane, the three you need total 1.5 tons. The three decouplers you'd use to get rid of that weight, plus the parachutes you'd need to float back down to Kerbin on, weigh half as much. "Weight is not your friend" is rule #1 for spaceplanes.

Having said that, I have to admit I hate to use this strategy. The first reason is, is it really an SSTO if you have to jettison anything - even the landing gear? The second reason is, I enjoy the challenge of returning to KSC and landing on the runway. But my first spaceplane used this strategy to reduce weight, making it easier to achieve orbit. After that, however, my skills had improved to the point I didn't need to do this anymore, so I've not done this since.

So it is a viable option, even if (like me), you turn your nose up at it.

Please don't call me a troll; I'm only trying to help.

[Starglider]

Re: numerobis, on my first spaceplane, pulling the gear off was the last thing I needed to do, to get it into orbit and back. I'll grant you that it was a fairly crappy design; I've since moved on. I don't remember doing anything else on that last design change, but maybe I did? It was awhile ago.

The stats in the assembly building say each gear weighs half a ton, and Kerbil Engineer adds 500kg for each one of them (but Kerbil Engineer could be looking at the same thing).

I think I'll experiment with my current design, and compare the highest apoapsis I can get for each version to see if there's a significant difference.

[numerobis]

The VAB lists stats for everything, but it's wrong in several cases, among them the mass of the small gear bay and of the struts (also, drag for most aero components, and a few other such details).

You can experiment: is there a 0.5-tonne item at the end of this lever arm?

[Tank Buddy]

Your jet engine may not have enough thrust to generate the speed necessary to create enough lift for all that rocket fuel. Try adding more wings or another Turbofan.

[Starglider]

That's a great pic. I wish I had thought of doing it that way before I launched two spaceplanes into orbit. Its much simpler than my experiment.

Now I've got to figure out what I'm doing that's making the wheel-less spaceplane fly better.

As an aside, this also means you can't deploy landing gear as an airbrake like you can ram air intakes (something I hadn't tested, but should stem from it being massless). In fact, this means the only reason to toggle the landing gear at all, is asthetics, doesn't it?

[KerbMav]

As an aside, this also means you can't deploy landing gear as an airbrake like you can ram air intakes (something I hadn't tested, but should stem from it being massless). In fact, this means the only reason to toggle the landing gear at all, is asthetics, doesn't it?

I read about the missing weight a few weeks ago and someone also posted a fix for it.

Question that remains for me is, if FAR also corrects for the missing drag? I seem to be unable to decipher FARs GUI to spot the answer.

[Madopow2110]

You need to have at least 600 m/s of vertical speed before you even think of lowering your angle of attack