Jet engines on rockets; viable?

[numerobis]

Switch to turbojets, and remove the radial air intakes. You will build up a lot more speed.

[Skorpychan]

Swapped to turbojets, ram intakes on the four side-mounted ones as well as some radial ones, but I want as much air as I can suck in, don't I?

Also, radial intakes are the only way to get any air whatsoever into the two mounted on the bottom; I needed more thrust, more fuel needed burning, so why the hell not?

Downside, they only get me up to about 150k or so until the air gives out and I risk flameouts. Also, the whole jet stage exploded as soon as I jettisoned it and I fired the upper stage engine. Was that the unburned jet fuel?

[numerobis]

At high altitude, above about 1500 m/s, the air a radial intake provides allows you to increase throttle -- but not enough to compensate the drag they impose on your spacecraft.

IntakeAir is available for all engines, regardless of any realism. You can put an intake on a cube strut on a long stick out back and use that air to feed engines at the front of your plane if you want. One intake can provide air for as many engines as you want; conversely, you can pack as many intakes as you want and have them all feed the same engine.

[numerobis]

10x the drag

My tests indicate you actually have more than 10x the drag at low speeds. The drag of an intake varies with area and speed; the more area, the higher the drag coefficient at any given speed. So when you scale up your air intake, you scale up both mass and drag coefficient, which is a double-whammy. It maxes out at 2 though, so it pretty quickly doesn't matter anymore.

[HoY]

I didn't actually do any testing I simply moved the decimal over 2 places >.< the same launch profile and mission was done with both versions. Launched vertically off the pad with a 3t payload on the nose, transfer and aero capture at Laythe, skipping jool's atmosphere completely. Dropped the payload off in orbit of Laythe, landed then took off again on Laythe then returned to Kerbin and parachuted to KSC. Both had roughly the same amount of jet fuel remaining after the initial launch to LKO, but having to launch by hand means that they weren't both identical launches so its a little skewed. For all intents and purposes they work as hoped

[numerobis]

My test was that I made a 4x intake, a 12x intake, and a 10,000x intake -- that is, copy the ramAirIntake part and modify the mass, area, amount, and maxAmount by those multipliers. The bigger the multiplier, the lower the speed at which we get to 2.0 drag.

The 10,000x intake comes in at 100t dry, with 2,000 units of air inside -- that's 10 tonnes of air! And you get 2.0 drag while basically at a standstill. Trying to move that is a challenge.

[Skorpychan]

So, I should take off most of the radial intakes, and just spam more engines and ram intakes with tri- and bi-couplers?

Also, I was having issues controlling it with the engines up high-ish on the jet fusalage and control surfaces down low. It was too stable, and wouldn't respond to the gravity turn as well as my rockets do. Is that an issue that's solvable by putting the engines lower and aero control surfaces up higher?

Also also, it exploded when I jettisoned the jet fuel fuselage and engines, and ignited the upper stage's rocket. Is that because of the unburned fuel?

[numerobis]

Take off all the radial intakes, and compensate with nothing. Your high-speed performance will improve. Radial intakes are a net negative on their own above ~1500 m/s. Nacelles are even worse.

Jet engines are vectoring, so you want them below the center of mass because the control algorithms in KSP don't take the center of mass into account -- above the center of mass, they gimbal the opposite direction that they should. The further down, the more torque, which goes for any control element. The further up above, the more torque, also -- bad for gimbaling engines that go the wrong direction, but perfectly OK for aerodynamic control surfaces. But keep in mind that the higher you go, the less your control surfaces matter (since there's not much air left).

What exploded? If what exploded is what you jettisoned, then this seems like a non-problem. If what exploded is your upper stage, is it because your jet stage was still on, and caught up to your upper stage? If so, then the solution is to use the jets longer.

[HoY]

When you jettison the jet stage do it with an action group that closes the intakes, shuts down the engines, and activates the decpuplers all at the same time. That'll stop an explosion caused by the engines still thrusting when dropped!

[K^2]

In my experience, the action groups trip the jetison before they shut down engines, at which point, they can't shut down the engines.

Personally, when I need to drop stages with running engines, I hit X, jetison, and throttle up again. It takes less than a second, so you don't lose much in terms of speed or climb rate.

[HoY]

I've never had that happen,, plus when you close the intakes it kills 100% of the engine thrust regardless. No spooling down, no uneven flameouts. Just dead. Never had it misfire on me since I started doing it that way, tho I haven't used a hybrid since .19.1 except ssto's that don't drop engines simply toggle them.

[numerobis]

I typically run until the fuel runs out. Barring that, I run until the TWR of the spent jet stage alone (after decoupling) is well below that of the rocket stage above. Barring that, one action key closes the intakes, then I stage.

[Skorpychan]

The jettisoned bit exploded, but it was close enough to nearly throw the upper stage off course. It's set to decouple from the upper stage and decouple all the engines from the fuel tank as well at the same time. No need to close the intakes, because the engines were nearly flaming out there anyway.

Because I want this to be a replacement for my heavy-lift rocket, I'm just going to spam engines anyway; nacelles are a handy way of getting tri-coupled engines and ram intakes together on a pylon. More engines + more intakes = more thrust, right?

The radial intakes will go, though.

And I'm definitely going to swap the pylons with the winglets; the aero surfaces are needed to stop this thing going russian on launch. Maybe I can fit more engines if I don't need to worry about them, as well. I'm sure there's some more room for more pylons with engines on!

[HoY]

the aero surfaces are needed to stop this thing going north korean on launch

Fixed that for you

[Skorpychan]

No, it was the russian one that yawed over and disintegrated. The north korean ones just seem to explode. I'm not THAT bad at rocket design.

[HoY]

I was just making a robbaz inspired joke I don't know much about RL countries space programs

[Skorpychan]

Holy CARP.

Ten turbojets, eight ram intakes. Got up to 45k, then it span crazily as soon as I turned the RCS on. My first reaction was to hit the 'dump everything' button, pop the upper stage off, and try to stabilise that.

I was going full throttle the whole way up. Did I overdo it? Do I need to ease off because something flamed out?

Should I dump pairs of engines on the way up?

Enough residual speed for the upper stage to burn into a proper orbit AFTER STABILISING.

ASAS was no help with the crazy spin, until I was able to dial down the rocket and use RCS to make a few burns.

INCREDIBLE speed going up once I got past 10K. Dumping the radial intakes and nacelles worked too well!

[tavert]

nacelles are a handy way of getting tri-coupled engines and ram intakes together on a pylon. More engines + more intakes = more thrust, right?

Yes, but nacelles are the highest mass and lowest area of any intake part. Since drag scales with mass, the nacelles add more drag than thrust from extra intake air. You're probably better off closing the intakes on nacelles. If you're using them just for their structural convenience, you might want to use the engine body part, which is also lighter than a structural fuselage but doesn't add tons of drag like a nacelle.

Edit: Was it turning the RCS on that started the spin, or was it just coincidence that you happened to flame out at the same time? Watch your intake air, either disable or drop engines as you get close to flameout. If you have an odd number of engines, you can leave the last engine and nurse the throttle down along the edge of flameout without losing control.

Edited July 5, 2013 by tavert

[Skorpychan]

I have no idea what specifically caused it. I turned the RCS on, then it went crazy. All three axes.

I'll have to test it again, once I figure out what I was doing.

[HoY]

At 45km with less than 1 intake per engine it was an engine flameout

You can't fly this like a regular rocket if you go hybrid. Need a much shallower ascent path and more intakes than engines

Edited July 5, 2013 by HoY

[Skorpychan]

Definitely a flameout. Did it again without RCS being on at all.

Nowhere else to PUT extra intakes. Currently got four pylons with bi-coupled engines and ram air intakes, and a set of bi-coupled engines under the bottom with no intakes attached. Should I go tri on the pylons?

What sort of ascent path is good, then? I've tried steering over at about 5k, but it starts to do the re-entry effect when I get higher and start to turn to the 45 degrees I use for rockets. Then things flame out.

[HoY]

If your going that much faster than terminal velocity then you can do a couple things. Take a pair of engines off, and do your turn a little higher in thinner air. It's really hard to say exactly how to ascent as each craft is unique, but if your around in about 5 hours when I get home I can show you in a stream how I get the craft I posted earlier in the thread to orbit the same way.

[Skorpychan]

I am not gonna be around then. Have to sleep. Youtube it, maybe?

Ditching some of the engines might work, though; it's a bit fast to control as it is. Ten engines on a single mk2 fuselage. Ridiculous.

I have no idea what I'm doing with this and it's completely fething awesome. I love this game.

And I should actually get back to trying to land on the mun soon. I HAVE a heavy lift rocket that works, and I need to nail landings.

[HoY]

Lol, i can YouTube it, I'll have to cut a bunch out as it takes about 40 mins to get up the first time while its full of fuel

I do actually have a couple launches frappsed, if I haven't deleted them, they are part of my Laythe mission series that got somewhat abandoned from YouTube and has mostly been getting streamed instead.

Know what, ill stream it anyway and link the video. Twitch saves it and keeps it after the stream is down.

Edit; changed my mind. I was already planning on working on a hybrid rocket tonight on the stream anyway, so ill continue with that plan and give you the time stamp for when I actually launch it. It's a similar size, however it's not a SSTO it will ditch it's jets once they are all done being useful.

Edited July 5, 2013 by HoY

[Skorpychan]

I'm thinking that the interim solution is to ditch the radial engines on the way up, once I get high enough. That'd improve the intake/engine ratio, and prevent the differential flameouts from being an issue by staying with the axial ones.

Pretty sure it only spins because one engine or pair of engines dies. A little futzing with fuel lines, and I can just decouple engines on a whim.