Jet Engines First Stage

[zarakon]

because you cant lift a 900 ton first stage on jets alone....

http://i.imgur.com/P60wob9.png

But if you did use jets, your first stage simply wouldn't be 900 tons! Most of that 900 is your first-stage fuel, which you would need far less of if using jets

[Eric S]

...I forgot to mention that, yeah. I assume it can't calculate their Delta-V because their thrust and efficiency vary with speed as well as altitude, which it can't predict.

I think at least part of the problem KER and MJ have computing the delta-v of jet-based rockets comes from the very limited amount of IntakeAir they start with. I've definitely seen times where MJ was computing the amount of delta-v based on a few seconds of burn time.

[Unistrut]

I used a jet first stage for most of my launches back in 0.18 (haven't had as much time to play since then, baby started crawling) and I avoided the problems with the following adjustments:

1 - Spool up - First stage, jets light. Second stage, launch clamps release. Light 'em, wait until they get up to speed, then away!

2 - Flameout - Dump 'em before that happens. Now, if you dump them while running you will have more excitement than you originally planned for, but it's simple enough to set an action group that closes all intakes and shuts off all jet engines, so you can then eject them all and light your rockets.

So flights with these were Engine ignite, wait until fuel use increased to 1.8 (IIRC), launch clamps release. Watch intake air until it drops to 0.2 then press "3" to kill the jets and immediately follow it with spacebar to light up the rocket stage. Proceed into orbit.

[Lanfranc]

The major problem with using jet engines is that you will have less rockets rather than more, as any right-thinking Kerbal would want. This is obviously not ideal.

[Bothersome]

It's an old trick I've used many times before and still have some larger craft that I still use occasionally when the mission needs that equipment.

I still use some Jet assisted designes even in my modern equipment lineup.

Just a couple to look at...

[roosterr]

My deal is realism. And for whatever reason (probably lack of thrust, relative) I have never seen a land launched rocket with jets for the first stage. So i dont build em.

[Themohawkninja]

Jets can run out of air, which means that your total possible altitude with a jet stage is limited by the amount of intakes you have on it (which increases weight and part count), and you can't asparagus stage the atmosphere.

[Tiron]

My deal is realism. And for whatever reason (probably lack of thrust, relative) I have never seen a land launched rocket with jets for the first stage. So i dont build em.

As someone pointed out, expense. Jet engines are much more complicated and expensive than a rocket engine, and you can't really cheapen it up much to make it single use (like you can with a rocket). This pretty much limits jets to reusable designs, and there HAVE been several proposals along these lines.

The trick being you need some kind of rocket to finish your ascent, and carrying two sets of engines, one of which is ALWAYS going to be dead weight, sharply increases the empty weight of your craft. Keep in mind that while Earth has the same gravity and sealevel pressure as Kerbin, Earth is MUCH larger and its atmosphere extends MUCH further out. Orbital Velocity at 200km (still well inside the atmosphere, although it's so thin that it doesn't matter much short term) for Earth is about 7.8 KM/s, over THREE TIMES the Orbital Velocity at 70km for Kerbin (which is outside the atmosphere and therefore equivalent to a MUCH higher Earth Orbit).

For a real world SSTO, you need to have around 90% of your launch mass as fuel. It's extremely tricky to build a craft that has that and can carry any significant payload. Adding Airbreathing engines you can use partway up reduces the amount of oxidizer you need to carry (-weight), which could improve it...except then you have dead weight engines sitting there all the time. Unless someone comes up with some kind of hybrid jet/rocket engine (they're working on it), but even then, one of the biggest things that jacks up weight on jet engines is the intake ducting (one of the reasons airliners have theirs in nacelles). So there's a lot of debate and question about if having air-breathing engines partway up could increase the payload fraction or not. Nobody's tried it yet because, well, frankly an all-rocket approach is easier to design.

[Itchylol]

Adding air intakes and places to actually put the jet engines is a mess and make for way too many parts. Id rather just put some giant fuel inefficient rockets than extremely fuel efficient jets that are a struggle to put on and lag alot.

[Synapse]

What about using a rocket at a tiny amount of throttle with the jets providing the main thrust? And a computer system that slowly adjusts from one to the other?

[Tiron]

What about using a rocket at a tiny amount of throttle with the jets providing the main thrust? And a computer system that slowly adjusts from one to the other?

There's no way to independently throttle the engines at present. Except for SRBs, which very realistically go at full throttle from activation until they run out of fuel.

Jets on rockets, when you're not just flying a rocket like a spaceplane, seem to work best when used as boosters on designs that are going for extreme Delta-V efficiency. Otherwise you're probably better off with a spaceplane or a regular ol' rocket.

[Levelord]

Unless someone comes up with some kind of hybrid jet/rocket engine (they're working on it), but even then, one of the biggest things that jacks up weight on jet engines is the intake ducting (one of the reasons airliners have theirs in nacelles). So there's a lot of debate and question about if having air-breathing engines partway up could increase the payload fraction or not. Nobody's tried it yet because, well, frankly an all-rocket approach is easier to design.

Actually what we currently have is a rocket/jet hybrid engine called the SABRE.

"The new engine is able to compress the air before it is injected in the combustion chamber and then cooled with a revolutionary system that they are keeping secret for the time being. The company also said the new engine will have the capability to move a plane at five times the speed of sound and once that speed is reached the outer atmosphere, it would switch to its rocket mode in order to reach orbit. What is interesting about this concept is that it stores its own oxygen directly from the atmosphere and then uses that for its rocket propulsion technology."

Read more: http://vr-zone.com/articles/first-of-its-kind-sabre-engine-travels-through-air-and-space/18110.html#ixzz2anuL4l9P

[Jason_25]

As someone pointed out, expense. Jet engines are much more complicated and expensive than a rocket engine, and you can't really cheapen it up much to make it single use (like you can with a rocket). This pretty much limits jets to reusable designs, and there HAVE been several proposals along these lines.

I looked this up and wanted to add some additional information.

Rocketdyne SSME - unit cost $50,000,000 / sea level thrust 418000 lb/f

General Electric GE90 (largest jet engine in the world) - unit cost $24,000,000 / thrust 115300 lb/f

So you would need about four GE90s to equal the sea level thrust of one SSME. That is going to be one large and awkward rocket.

As far as complexity though, in theory a simple rocket engine can be not very complex, but it will be less efficient and reliable than a throttle-able, computer controlled, cryogenically cooled engine like the SSME. The SSME is very complex, more so than the big GE engine I should think. Take a look at this onboard computer controller:

http://en.wikipedia.org/wiki/File:99723290_SSME_Blk_II_Controller.jpg

[Sof]

IMO, using jets as a first stage just leads to forming bad habits. It only works because the current drag model doesn't work. As soon as there is a realistic representation of drag in the game, jet engine first stages won't work.

[Skorpychan]

I tried a hybrid rocket. It was very efficient, but very slow, and a pain in the ass to dump engines on the way up as air ran out, and still only lofted a small payload into orbit.

On the other hand, a bunch of mainsails lifts every time, no asymmetric flameouts, no worrying about intake air.

Really, a spaceplane carrying the rocket as a payload would be the most efficient system. Get up high efficiently, build up speed, then make a zoom climb, set the rocket off, and kill the engines for a safe return to kerbin. However, then you'd have to control the rocket AND the spaceplane.

[numerobis]

IMO, using jets as a first stage just leads to forming bad habits. It only works because the current drag model doesn't work. As soon as there is a realistic representation of drag in the game, jet engine first stages won't work.

How is the drag model relevant here?

[Themohawkninja]

How is the drag model relevant here?

Air intakes generate drag, and if aerodynamics is modeled realistically, the drag will get very high very fast, and generate a lot of heat (see SCRAM jets and SABRE engines).

[Synapse]

You spelled disclaimer wrong

[numerobis]

The main disadvantage of jets is complexity: you need to add more parts, then on launch you have to worry about intake air. MechJeb can alleviate part of that.

Rocket science is much easier.

BTW: MechJeb calculates the deltaV assuming each intake is a tank of air with 0.2 units (or 1 for radials) in it. It takes a lot of intakes to get enough stored air for 1 m/s of deltaV. I've considered doing the work to make MechJeb understand intakes. Even then it would be off due to the thrust curve.

[Tiron]

I looked this up and wanted to add some additional information.

Rocketdyne SSME - unit cost $50,000,000 / sea level thrust 418000 lb/f

General Electric GE90 (largest jet engine in the world) - unit cost $24,000,000 / thrust 115300 lb/f

So you would need about four GE90s to equal the sea level thrust of one SSME. That is going to be one large and awkward rocket.

As far as complexity though, in theory a simple rocket engine can be not very complex, but it will be less efficient and reliable than a throttle-able, computer controlled, cryogenically cooled engine like the SSME. The SSME is very complex, more so than the big GE engine I should think. Take a look at this onboard computer controller:

http://en.wikipedia.org/wiki/File:99723290_SSME_Blk_II_Controller.jpg

The 'Complexity' of the SSME is because it's old, more than likely. You have to remember, the whole space shuttle 'system' was designed in the late 70s, and while it got a number of substantial upgrades in the meantime (glass cockpits, etc), they were still using some very old tech right up until the end. Like 386DX processors. That old tech wasn't particularly compact, making things rather larger than their modern equivalents.

The other thing there is radiation: Once you're outside the ionosphere, the radiation levels jack up tremendously, especially if you go anywhere near the van allen belts, and this will screw up electronics if they're not shielding (flipping bits, etc). One of the reasons they kept using older tech for so long was that they understood how it responded to radiation very well and could deal with it better. But they still needed shielding, which bulks things up quite a bit.

The final thing to remember is that the SSMEs were particularly complicated and expensive compared to most rocket engines, because they were designed to be reusable. This entails a lot of extra complexity that you don't run into with 'disposable' engines, which also drives the price up. A better comparison for a non-reusable rocket would be the J-2X, which makes all the KSP engines look sad and pathetic.

Vacuum Thrust: 1,307 kN (294,000 lbf), Dry weight 5,450 pounds (2,470 kg). Isp (vac.) 448 seconds. Can't find the unit cost, unfortunately, but it ought to be a fair bit lower than a SSME.

What makes me sad though is that when you look at those numbers... It's putting out almost as much thrust as a Mainsail, but weighs a bit LESS than a poodle. And better vacuum efficiency than a LV-909. Whimper.

Edited August 3, 2013 by Tiron

[Synapse]

It does exist in a world 5 times bigger though...

[Tiron]

It does exist in a world 5 times bigger though...

More like 10 times, but yes.

I knew the parts we have aren't as good as real ones, because it's so much easier to get to space given the planet (and particularly its atmosphere) are so much smaller, but the magnitude of just how much this was the case hadn't really hit me till then.

[rmzalbar]

I just tried a jet first stage. Ended up with an orange tank to orbit system with a 50% mass-payload ratio, no SRB's, recovery chutes on all dropped parts and no orbital debris. Awesome!

I run up to 36k, fly 0° until out of jet fuel, then drop jet stage and fly 15° to punch atmosphere with rocket stage. Mechjeb can manage most of the tedium but you do need to tweak the ascent angle once after dropping the jet stage and then back to 0 again after a short spaceward burn.

My orbital fueling station is a full orange tank, 750 RCS fuel, 1500 ion gas (why not) small and standard clamp-o-trons. It also retains the ability to boost to a higher orbit or transfer to a munar orbit if needed there.