Using Jet Engines as First Stage Boosters?

[sardia]

Would using Jet Engines as a first stage yield benefits beyond what a SSTO would, or does it suffer from the same problems as SSTO? It would definitely take longer, and the savings wouldn't be as good. Has anyone tried it recently?

Edited October 13, 2015 by sardia

[Wolfox]

The biggest flaw is that air-breathing engine´s thrust is directly dependent on the intake air speed, so while sitting on the launchpad, they will barely provide enough power to lift themselves. The only viable usage are usually (not taking in account various challenges on this topic and experimentals) for small payloads and using aerospikes, but those burn through the fuel in a very similar fashion to conventional SRB´s.

EDIT: But they might be interesting boost for mid-stages, but this would add deadweight to first stage. Worth some testing tho.

[Snark]

I've tried it before, and it works okay for small to medium sized ships. Other than playing with it to verify that it works, though, I don't use them. If you jettison them like a spent booster, they're not super cost effective, and they're more tedious to engineer and launch than a "traditional" rocket. So it's like getting the extra work of a spaceplane SSTO without any of the advantages.

[SpeedDaemon]

I have a reusable SSTO lifter like this based around an orange tank/Mainsail, with 8 Whiplashes radially mounted to it on the 1.25m 200u tanks. It can get ~18T to LKO. During ascent, you have to toggle the mainsail on/off strategically to get the TRJs to where they produce enough thrust, and the margins are pretty slim, dV-wise.

Pros:

- Simpler ascent profile than a spaceplane-type SSTO

- Lands on its tail via parachutes, so that's easier than landing a spaceplane on the runway

- Can fit things under fairings that won't fit in spaceplane cargo bays

Cons:

- Hard to pinpoint the landing, so never get the full 100% refund, which matters, because:

- It's expensive (~70k for the re-usable part)

- The different kinds of fuel flow (jets vs. rockets) are difficult to manage without TAC fuel balancer (the jets will steal LF from the whole craft unless you actively do something to stop them)

[Van Disaster]

We used to do this a fair bit ages ago before some rebalancing; the biggest problem I see is if you're doing a rocket ascent you're not actually using jets for very long - 20km comes up rather quickly - so either you have to lug some extremely heavy useless jet engines the rest of the way to space if you're building a SSTO lifter, or you have to abandon a bunch of expensive jet engines if you're abandoning the stage - and if you're abandoning it the extra mass of the engines is probably going to outweigh the reduction in fuel mass.

What is worth trying is Rapier powered vertical lifters, although even those might need a SRB kick off the pad unless you do the liftoff in rocket mode. Actually I think I'll try one now. I've made B9 SABRE powered ones in the past that worked pretty well, lots of changes since then though.

Edit: OK yep, Rapiers & SRB boost stage to get it supersonic works pretty well, fly it up like a spaceplane. Of course it has to look something like a spaceplane...

Edited October 12, 2015 by Van Disaster

[Foxster]

If you are building a light 'rocket' lifter (i.e. vertical take-off) then jets can make a light first stage. This because even though you need a number of engines, you need very little fuel, perhaps as little as 80 LF to reach about 25km when you will throw that stage away.

[sardia]

Say we went with a light payload, like a small sat. What do we gain over ditching just engines into the ocean vs a srb?

[Mastikator]

Back in 0.25 days I tried using a cluster of turbojets as the initial stage, they didn't provide much lift but they could get into a pretty decent altitude on a single fuel tank. It enabled me to basically start the real lift off at some 30km into the air.

In 1.0+ not so much

[Van Disaster]

Say we went with a light payload, like a small sat. What do we gain over ditching just engines into the ocean vs a srb?

Might as well just build a little conventional rocket & add a bit of fuel to deorbit it, and a couple of chutes so you can land it, presuming a small sat is a light 1.25m payload. I think where you might gain is slightly larger craft: this was my test launcher which is utterly terrible & I'd no way build like that again, but it worked ok. One of the worst parts was getting enough air to the jets because you arrive at high altitudes moving fairly slowly.

Long ago I did a comparison between similar SSTO launchers - one was a winged rocket, one was a spaceplane. The rocket's upfront cost was way lower, but after ~6 launches or so the fuel cost had caught up with the spaceplane's higher purchase cost and massively lower fuel bills. I'm not sure what sticking a big B9 sabre on the rocket would have done, I guess that's something else to try too...

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Slightly newer version of the same spaceplane - not sure if the B9 pwings costs had been sorted, originally it cost about 58k sans payload I think.

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So what you're looking at is the upfront cost of aircraft engines with the fuel consumption and TWR needs of vtol perhaps save the idea for things that just can't fit in a spaceplane?

[Clipperride]

I would imagine you'd need some SRBs to get up to speed and, if you are using those you might as well just attach larger ones and do without the jets. I launch most of my Space Planes using a SRB powered rocket sledge to get the engines working more efficiently by the end of the runway. Although, in science mode you don't have to be so (well, at all!) mindful of the cost . I just like to be over ~420m/s asap

But it's certainly a twist on the traditional way of doing things, so I shall be trying it later - cheers for the idea.

[sardia]

What if we used wings and srb to push us into a fast gravity turn? Then we could be at 20000 and 1000m/s+

[GoSlash27]

Sardia,

The way I figure it, jet engines are only useful as a first stage and first stages are considered disposable in a stock career game.

So even though they use very little fuel and therefore get you more payload fraction, they aren't cost-effective. Cost- effectiveness is always the primary concern in my first stages, so I tend to go with cheap liquid fuel boosters in early career and SRBs once I unlock the BACC and Kickback. They weigh more, but they slash the launch cost.

Best,

-Slashy

- - - Updated - - -

What if we used wings and srb to push us into a fast gravity turn? Then we could be at 20000 and 1000m/s+

Sardia,

SRBs have limited DV due to their poor Isp. Putting wings on them and "flying" them increases the DV budget, which puts them at more of a disadvantage than they would otherwise be.

Best way to use an SRB IMO is to employ it as a first stage in a conventional ballistic launch.

Best,

-Slashy