Silly jet engines are not for planes anymore

[Joe_Bender]

So I got bored yesterday and came up with a stupid idea of using jet engines to launch a rocket.

I built a two stage rocket with LV-T45 motors and then attached three jet engines radially around the bottom.

I used structural pylons so I can dump the engines when they became useless. I also had to run a fuel line FROM the fuel tank TO the engines.

As far as staging goes, I had the jet engines do the lifting with the T45 not firing at lift off.

So when I reached around 20k in elevation, I drop the jets and ignite the rocket motor.

benefits? I only used half the fuel if I went all rocket engines.

Worth it? Probably not, but just for giggles, it was something different.

[Mr. Scruffy]

Yepp. This was the last thing i did in 0.21 - playing around with jets as lifter-stage. I used one or two seperate jet-fuel tanks, though, with 4-8 engines. The engines where usually below the rocket in the central column, while the fuel was attached radially (one fuel tank per engine is just too much). The ascend curve and stage-timing took a bit of experimentating, but lifting ratio of something like 3 (starting mass) to 1 (mass in orbit) is a league of its own, when compared to rocket-only-approaches.

[Lohan2008]

Nod/ Most of us have played with the jet assist. Not much of a difference but it is worth the giggles

[Crush]

I also experimented with first-stage lifters using jet engines.

The result was that it is a viable approach, but it uses much more parts than just using mainsails and X-32 fuel tanks.

It wouldn't even be cheaper with the current part costs. One Turbojet Engine costs almost as much as a Mainsail, but has only one sixth of the thrust. Jet fuel is cheaper than rocket fuel, but you also need intakes, and these are also quite expensive. But when money is ever added, part prices will need a large rebalancing anyway.

[Andersenman]

Well known also from

, available on Spaceport. Edited October 20, 2013 by Andersenman

[Godot]

I also had Jet Engines as support of several of my rockets rom, I think, 0.20 onwards ... usually attached to the Main Rockets via radial decouplers

[Athywren]

I almost always use jets to reach the 10-20k mark now. As long as you have enough of them to put enough go behind your ship, they're a really reliable way to reach those altitudes and set off your rockets with a far more fuel efficient starting point. Hopefully the high cost of the jets will be counterbalanced by allowing recovery of de-orbited parts (assuming they don't 'splode on landing (parachutes, yay!)) when finances are introduced.

[Sof]

Yeah, this isn't anything new.

I preferably don't use jet engines on rockets because of how outrageously overpowered they are in comparison. It is entirely possible to get a jet rocket into space without even using rocket engines. Intake spam doesn't help it either.

Overall jet engines have such a huge ISP compared to rockets that I consider it cheating to use them in vertical take off spacecraft.

Second only to mechjeb.

[EatVacuum]

Jet engines are useful for regular rockets, not just space planes, but you have to consider the gains vs. the losses. For a "straight up" rocket, they are maybe less useful than they are for something that spends it's time in atmosphere, but you have to consider a couple of things other than raw thrust. Take a look at a couple of stats comparing the jet engine with a close equivalent, say the LV-T45 engine (data taken from their configs);

Jet Engine - maxThrust = 150

PROPELLANT

{

name = LiquidFuel

ratio = 1

DrawGauge = True

}

PROPELLANT

{

name = IntakeAir

ratio = 15

}

atmosphereCurve

{

key = 0

1000

key = 0.3

1800

key = 1

2000

}

LV-T45 - maxThrust = 200

PROPELLANT

{

name = LiquidFuel

ratio = 0.9

DrawGauge = True

}

PROPELLANT

{

name = Oxidizer

ratio = 1.1

}

atmosphereCurve

{

key =

0 370

key =

1 320

}

The LV-T45 does have 1/3 more thrust (200 vs. 150), but look at the numbers for atmosphere curve - the numbers that are bolded are the Isp at various altitudes (well atmospheric pressures really). For those not familiar with the config files, the first number after "key = " is the atmospheric pressure, the second is the Isp. So at sea level the jet has an Isp of 2000 vs. the rocket's 320. At zero pressure, the rocket has moved up to 370, and the jet is (theoretically) down to 1000. Theoretically because of course the jet will flameout before it gets to vacuum.

Since Isp is your "miles per gallon" it would seem that the jet engine gets somewhere between approximately 2.5 and 6.5 times as far on a tank of gas. But, it's actually way better than that - the rocket engine burns fuel and liquid oxygen in about a 9 to 11 ratio, and it has to carry both of them. The jet burns fuel and intake air - and in a 1 to 15 ratio, and it only has to carry the fuel - the air is free (well, you have to have an intake or two). So at sea level for the expenditure of 1 fuel and 15 free intake it generates (1+15 times 2000 =) 32,000 units of thrust. Burning the same amount of carried combined fuel and oxidizer (0.9 fuel + 1.1 liquid oxygen, halved) you get 160 units of thrust. At the (theoretical for the jet) vacuum pressure, the numbers would be 16,000 vs. a whopping 185 units of thrust. So the weight saved using jet fuel to get you your first 600m/s of velocity and 17-18km of altitude (doable with a couple of intakes per engine) is not inconsiderable.

The jet fuel expenditure is so low, you probably only need one tank of jet fuel for a handful of jets used as boosters for your rocket. In a "straight up" flight profile, what you need to do is decide if the weight of fuel saved is worth the extra parts - the weight of the jet engine isn't a concern if you are jettisoning it like a used booster. For a space plane or some other form of SSTO, the weight is a consideration. But for a space plane, your ascent profile is designed (or it should be) to get you to 20km and 1000m/s before you switch over to rockets, and you will spend a lot more time getting there, so the fuel savings are greater.

But, what if you could keep using the same engine in both air breathing (i.e. jet) and closed cycle (rocket) modes and get the advantages of both? That is the idea behind the Reaction Engines Sabre engine that is in development (http://www.reactionengines.co.uk/sabre_howworks.html). At this point I would put a shameless plugin for an addon I'm involved with, but since I'm not sure when it (if?) it will get done, I'll instead direct you to the B9 Aerospace pack. Buried in the dozens of other parts and pieces are a 1.25m and a 2.5m Sabre jet engine, so if you are one for playing with add ons, it could be the best of both worlds for you.