Rocket Section of an SSTO: How do I do it?

[Starwhip]

I can post pictures in a while, but first, listen to my problem:

I can easily make high-speed, high-altitude aircraft, capable of hypersonic speeds. Now, when I try to make SSTOs, my problem is the rocket stage. How much fuel is necessary? I never have enough, either due to a piloting error or a building error. So how much fuel is necessary? 1 FL-T200 (the one with the "go faster" stripes, if the part # is incorrect) is not enough for me. And how much jet fuel is needed?

In terms of engines, I usually have 2 or 4 turbojets, and 1-3 Aerospikes.

Orbital characteristics: just before jet flameout, I am up to about 25,000 meters (apoapsis ~30,000) and around 1700 m/s. Just after rocket stage runs dry, I have an apoapsis of around 69-71,000 meters.

Any help?

Edited January 30, 2014 by Starwhip

[Taki117]

What's your periapsis (If any) at that altitude? IF you have a periapsis above 20km you are doing wonderfully, but you shouldn't need Aerospikes, try less fuel with a smaller engine, like the 48-7S or a set of 24-77s. This will reduce vessel mass and give you the short boost you need to get into orbit.

[Silverchain]

If you can get your speed high enough that your apoapsis is well outside the atmosphere then you'll need very little rocket fuel to circularise the orbit (and deorbit); some of my smaller planes use only a few of the tiny grey tanks and a couple of 48-7S engines.

[capi3101]

Can you talk a bit about your typical flight profile? You know, what elevation are you going to when you take off, when do you start leveling out, when do you flame out, how fast you're going, etc? That could be part of the issue there...a pair of FL-T200s and a 48-7S engines get me into orbit, but only if I've handled the rest of the ascent properly.

Before you flame out, try throttling back to about 2/3. Watch your thrust levels from your engines; you want to keep them roughly even - throttle back if you start noticing one engine with a substantially higher thrust than the rest so you don't go into a flat spin. You want to try to get that extra 300 m/s (up to 2000 or so) before you kick in the rockets.

This may or may not be helpful or give you ideas; I too am relatively new to the wonderful world of spaceplanes:

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[Starwhip]

Flight profile? Ok, here:

From runway, ascend at 45 degrees to around 10,000 meters. Level out (about 10 degrees) at around 11,500 meters. Continue on jets at full power until flameout. Kick up the rockets from this point, point again at 45 degrees until apoapsis is about 70,000 meters... and then run out of rocket fuel.

EDIT:

Something QUITE interesting I've noticed is this: before flameout, the engines (jets) are still operating even though the resource called "IntakeAir" is and has been at zero for some time.

Edited January 30, 2014 by Starwhip

[Pecan]

[spaceplane newb writes] My ascent profile: 45 degrees to 10km, from there pitch down so that speed has time to build - 100m/s per 1km/alt over 10km (=100m/s @ 11km, 200m/s @ 12km, etc.). Usually 20-30 degrees to 15km (500m/s), 15 - 20 to 20km (1km/s), 10 - 12 degrees to 25km (1.5km/s) and then tease it up to 30km and 2km/s. (Sometimes I get bored around 27 or 28km and light the rockets early).

I'd think that if you pitch down to 10 degrees at 11.5km you're spending too much time - and fuel - in the still relatively thick atmosphere below 20km. The higher you go the faster you can go but, conversely, you can't continue to climb too quickly or your jets will starve.

[Edit: Oh - and the ONE thing that changed my 100% failure rate to (more or less) 100% success was throttling-back the jets when they start to run out of intake air. As long as the speed is still building I'll go to 80%, 60% and even down to 30% - possibly with a bit of rocket-assist at that point; they're not using too much fuel at 30% throttle, after all).

Edited January 30, 2014 by Pecan

[Starwhip]

Ok I noticed this: I am NOT going high enough nor fast enough. I'll add more intakes, I have only 4 radial ram intakes and 2 of the circular ram ones.

CURRENT AIRCRAFT CONFIGURATION:

12 RAM intakes (radial)

2 RAM intakes (stack)

2 Turbojet engines

1 Aerospike Engine

2 FL-T200's

2 Jet Fuselage

Action Groups for INTAKES, INTAKES & ENGINES, and AEROSPIKE

Ascent Profile:

45 degrees (30-ish prograde marker) to 14,000 meters

Down to 30 degrees (20-is prograde marker) to 17,000 meters

Down to 15 degrees (5-10 prograde marker) to 30,000 meters

Throttle down to keep jet thrust stable and airspeed accelerating

(At 30,000 the jets failed to provide sufficient thrust; 48-7S not enough.

Added AEROSPIKE, testing now.

It goes up to about 30,000 meters on jets, reaching a top speed of 1,600 m/s. Do I need more jets?

Edited January 30, 2014 by Starwhip

[Claw]

The amount of rocket fuel is also going to be dependent on the weight of your craft. If you're using a 200, then you probably don't need 3 aerospikes.

The intake air will register at 0 for a while, but there is still some intake. This means you can slowly throttle back your turbojets as they are close to flameout and milk them for a little more thrust. You should try to get up in the 30s on jet power if you don't want to carry a lot of rocket fuel. Watch out for asymmetric thrust.

Also, more intakes are better. You didn't mention how many you have, but generally between 3 to 10 intakes per engine depending on your desires. I lean toward the 3 intakes but more can help.

[Starwhip]

YES the Aerospike did it!!!

Here's the ship: AVIONSSTO

Weight: 16 tons

In orbit: 440 units of oxidizer, so docking possible.

Manned

This is the first manned SSTO craft I have ever made... although it looks a lot like the Aeris.

[capi3101]

Nicely done.

For future reference: Pecan is right - pitching down to 10 degrees below 20k is going to waste fuel.

Here's how I typically handle design: I put my intakes in various action groups, setting them to toggle on and off. I usually set them up so that one intake per engine always stays open and the rest are roughly divided into 3-4 action groups along the #3-6 keys. Jets on #1, Rockets on #2. I usually incorporate an escape system which activates on Backspace and a seperate key for toggling chutes on #0. Solar Panels on #9.

On the runway, I throttle up, set the brake and activate the jets. I then close the intakes and let off the brake. Once airborne, I go 60 degrees to 10k (that part's like a rocket flight - fastest way out of the troposphere is as straight up as you can manage), then hit 45 when I reach 10k, down to about 20-25 at 15k. I'll watch my intake air after 10k and open up a set of intakes once the intake air gets to about 0.06 units per engine. I also watch my vertical speed indicator the whole time - I want it between 10-100 m/s; more than that and I lower my pitch, less than that and I raise it. By 25k, I'm usually at the 10-15 degree mark picking up serious horizontal velocity. You don't want your vertical velocity particularly high after 25k - your priority needs to be horizontal. Keep opening intake groups as necessary. When the last group is open, begin monitoring the thrust levels out of your engines, especially if you've got more than one Turbojet. Throttle back to 2/3 when the difference gets particularly high; this will save you from flaming out. Continue to monitor it, though, and be ready to reduce again when another differential shows up. Once you're down to about 1/3 thrust and/or no longer accelerating, kick in the rockets but keep the jets lit - you want to keep them running for as long as you can manage. If you get to the point where you have to kill the jets completely, do so and treat it as a rocket launch from there. Hopefully before that happens you'll have an orbital apopasis and a speed above 2000 m/s. Note that later in the flight (after 1500 m/s or so), you'll need to be adjusting pitch to maintain your desired elevation; this is natural, a function of the planet turning underneath you - don't worry about it.

(Typed up that post, got ninja'd and the forums logged me out for some reason; figured I'd post it anyway)

Design-wise: I'd recommend Swept Wings over Deltas. They have a better lift to mass ratio. By the same token, a pair of small control surfaces is much preferable to a single large control surface as a rule. Same reason.

Still a solid SSTO spaceplane design. Well done.

[Pecan]

Congratulations - feels good, doesn't it :-)

"It goes up to about 30,000 meters on jets, reaching a top speed of 1,600 m/s. Do I need more jets?" - you've probably sorted this by now but I'd say you don't need more jets, you need to climb s-l-o-w-ly, if at all, above 25km so the speed comes up. Go into a shallow descent if you need to and/or, as capi has also said, add some rocket-thrust to the throttled-back jets. Generally, 2km/s+ is the target, not 30km altitude. Stay between 25-30km to get the last from your jets, you only care about gaining more altitude once they've done what they can. [Again, though, you don't want to sit on them too long - I tried nursing every last bit of horizontal speed out of them and was burning so much fuel for a few extra m/s it wasn't worth it].

Spaceplanes are a lot more work to fly than rockets but it's nice to think we not throwing 80%+ of the equipment away every launch. Now ... back to my launch vehicles which do exactly that ...

[Starwhip]

Alright, I got REALLY good at this:

Avion Mark Two

Maiden voyage, in a 71,000 meter orbit, with over half a tank of fuel for both of the rockets.

[Starwhip]

And I docked it:

[capi3101]

Nicely done. Took me four permutations of spaceplanes before they finally started doing something remotely useful...

[Claw]

Very nice. Congratulations!

[Andrew Hansen]

With the RAPIER engine, I find it very easy to create SSTOs - especially with the auto-switching mechanism. Just fly straight up to 15 km, slowly tilt over to around 15 degrees up (or less), and wait until the engines switch to rocket mode at around 2,000 m/s surface speed. I did it with a 1/3 tank of the fuel tank with "go-faster" stripes on it.

Like this (skip to 2:56):