Number of RAPIER engines per ton of payload?

[Soda Popinski]

I've very new to making space planes / SSTO vehicles. I built a KSC to LKO orbital bus to bring ferry my crew to an orbitally assembed interplanetary ship. So I wanted to bring up 2 Hitchhiker storage containers along with a Cockpit mk I. So 6.25 tons. Also, I'm a horrible pilot, so I put on enough wings to give me a stall speed of about 70 m/s.

Since I'm horrible at this, I put together quite a monstrosity of a space plane (but it works). I suspect I wasn't being very efficient, as I have 6 RAPIER engines (pretty sure I can go down to 4), an X200-32 fuel tank, a pair of mk I fuselages, and a LOT of intakes (4 rams, probably 14 radials). I'm running FAR, so I don't like sticking intakes onto wings without a fuselage or some realistic attachment point.

My ascent profile has been, shut off most intakes, take off, pitch up to 60 degrees, full burn until ~10km, flatten out to 20 degrees, slowly climbing and building speed to about 26-27k, opening up intakes along the way, and shutting down 4 of 6 engines. At this point, I'm doing about 1800-1900 m/s, then I pitch up to 60 degrees, go closed cycle, and burn until my apo hits 90 km or so. Standard circularization and orbital rendezvous after that.

Do I really need this many engines? I suspect not. I like having a relatively quick ascent profile, but wondering if I've crossed the point of diminishing returns.

Edited December 24, 2013 by Soda Popinski
Got answered.

[Hodo]

You can attach intakes to wings of craft, just don't go crazy, I can find a number of real life examples of aircraft with intakes ON the wings. But I use the B9 parts to get this look, the stock parts are horrible for that.

On to your question...

Do you have any pictures of the craft in the SPH hanger with the CoM, CoL, and CoT icons on?

You don't need that many engines, but it is nice to have them. If you are shutting down engines then they become dead weight, and that is a problem, so my solution would be get rid of the engines I shutdown. If I can still achieve orbit without them and use less fuel I have improved the design, but if I can't then I add back a single engine at a time. My goal is not to shutdown engines until it is absolutely needed.

With one of my fastest SSTOs the SP-103 Jackal it has 4 B9 turbofan engines and a Poodle booster. I run all 4 Turbofans until 28km, at about 1900m/s then I switch over to the Poodle and finish the burn to orbit, which takes about 20seconds. A common mistake in KSP and with everything is we have come to believe more is better. Fact is more is just that more.

[Soda Popinski]

I'm a bit embarrassed, as this is my first SSTO / spaceplane. Its ugly as sin. Maybe I should have started with something smaller.

[Sirine]

I'm a bit embarrassed, as this is my first SSTO / spaceplane. Its ugly as sin. Maybe I should have started with something smaller.

http://i.imgur.com/XkZNB3b.png

Honestly...your craft look nice...1st time ever see a plane cockpit to big command pod...fit nicely.

[Hodo]

I'm a bit embarrassed, as this is my first SSTO / spaceplane. Its ugly as sin. Maybe I should have started with something smaller.

http://i.imgur.com/XkZNB3b.png

It looks good. Now that I see the craft, you don't need that many engines, you could dump to RAPIERs and still achieve orbit. It may take a bit longer but you can do it. The only other thing I see is a FAR related issue. The amount of radial intakes you are using is uneven and will cause a larger amount of drag on the top of your craft, pulling the nose up. You could ditch that top most radial intake and still have airflow enough to meet your target numbers.

[maccollo]

I'm a bit embarrassed, as this is my first SSTO / spaceplane. Its ugly as sin. Maybe I should have started with something smaller.

http://i.imgur.com/XkZNB3b.png

It looks way better than the flying bricks one often sees in KSP =P

[Dispatcher]

...Do I really need this many engines? I suspect not. I like having a relatively quick ascent profile, but wondering if I've crossed the point of diminishing returns.

With a plane, there is lift to consider, as well as dV. For vertical rockets, there is a thrust to weight ratio for each engine of course; the RAPIER being one of them. For it, I've approximated it on a chart. See my sig line for the link.

Edit: revised the TWR of the RAPIER modes on my chart.

Edited December 29, 2013 by Dispatcher

[Soda Popinski]

Thanks for the compliments on the look of the thing. That's because you didn't see it from above, where my lack of skill on putting wing parts together is very evident. I only played will part clipping for the first time, and have some "corners" from the rectangular wing sections sticking out into the air stream at one point.

I don't get much of a problem of uneven drag, which may be because I was unable to mount the engine nacelle (RAPIER + fuselage) through the wing in a centered manner, where most of it is sticking out on the bottom.

Oh, I used the capsule because I wanted, A) a transition to the large diameter Hitchhiker containers, and provide more passenger accommodations, as this is essentially a KSC to LKO space bus. Since it uses, essentially a hybrid engine, and is ungainly, I named it "The Prius." The the real Prius is probably more aerodynamic (drag coefficient of 0.26 for my car's model year). Though, unlike my care, it takes off...well, like a rocket.

[numerobis]

I tend to use one turbojet per 15t total mass (and with careful flying you can work 20t). I haven't played much with the rapier yet; it's about 10% less powerful, so I'd expect comparably less capacity to lift your plane up. The limiting factor is how many engines you need to lift off. If you can lift off but not make orbit, remove engines: the number of engines doesn't really matter for reaching orbit, what matters is the amount of air you can collect.

Small note: the Mk3 cockpit is lighter than the big capsule.

[Soda Popinski]

I tend to use one turbojet per 15t total mass (and with careful flying you can work 20t). I haven't played much with the rapier yet; it's about 10% less powerful, so I'd expect comparably less capacity to lift your plane up. The limiting factor is how many engines you need to lift off. If you can lift off but not make orbit, remove engines: the number of engines doesn't really matter for reaching orbit, what matters is the amount of air you can collect.

Small note: the Mk3 cockpit is lighter than the big capsule.

So, maybe 2 or 3 RAPIERs would do the trick (smidge over 30 tons), based on your rule of thumb. Thanks! I ended up using the big capsule instead of the Mk3 cockpit, as it doesn't mate properly with the Hitchhiker cans. I wasn't sure what that would do with FAR.

Edited December 24, 2013 by Soda Popinski

[Murph]

it's about 10% less powerful

A little over 15% less powerful than the turbojet, before factoring in needing more fuel.

[tavert]

Same Isp in air-breathing mode. 190 kN from 1.75 tons for the air-breathing Rapier vs 225 kN from 1.2 tons for the turbojet. And the turbojet has a 200 m/s higher top speed than the air-breathing Rapier.

[numerobis]

The thrust curve for the rapier and the turbojet is the same until 2km/s, and then the rapier loses thrust much faster. So the switchover from air breathing to rocket mode is earlier. Notably, at its top speed you aren't quite in a circular orbit; you need another ~100 m/s no matter how many intakes you tack on. Nice move from Squad, I approve.

So, after some playing around with turbojets manually, it seems that the intakes and engines work mostly the same way, except that engine flame-out is much more forgiving, at least with a single engine. Before, you'd hit about 25 km with two intakes, and bam, engine flames out. Now it continues going at full throttle up to about 36 km. However, fuel usage goes down along with thrust: the engine is just throttling back automatically. Why 36km? Because the "ignition threshold" is 0.1, and pressure goes down a factor of 10 over 11km. I haven't played with multiple engines yet.

[tavert]

Remember the top speed is surface-relative, so 2200 m/s surface is pretty much orbital.

Ah, interesting that the ignition threshold seems to actually matter now. Worth noting that it's different for the Rapier, at 0.33.

[numerobis]

The zero-thrust speed is 2200, but you'll asymptote somewhat below there. Back-of-the-envelope: useful turbojet planes top out at the quarter-thrust speed (Machingbird planes not included). The equivalent would be about 2100 m/s for rapiers, aka 2275 m/s orbit, which is about 75 m/s short of a circular orbit.

The ignition threshold matters if you're flying manually. MechJeb will still be able to throttle back and get you all the way to space.

[Soda Popinski]

I ended up taking 2 RAPIERs off the back, but neglected to rebalance the wings. Turned out the lack of mass in the rear made the craft really hard to keep pitched up on landing after a successful rendezvous with a mothership and deorbit. Couldn't flare up, came down hard, broke a gear and lost a few engines. Also took longer to climb, and actually used more fuel fighting the lower atmosphere. So I'll stick to the MOAR engine version.