Can the first stage use jet engines only?

[Valley]

Tried using jet engines as my first stage and not going well. I was wondering if it is possible or if I should just give up (or maybe use them to HELP the first stage - replacing the solid fuel rockets).

My idea was that the fuel was lighter and lasted longer but the engines don't seem to be powerful enough to help launch a payload (at least not at the tons I need).

They don't even get off the ground.

Edited March 3, 2015 by Valley

[LordFjord]

Jets as first stage work very well. In fact the jetpowered wingless lifters are the ones with the highest payload ratios. If you show us a pic then folks around here won't hesitate to give you advice

[LethalDose]

Tried using jet engines as my first stage and not going well. I was wondering if it is possible or if I should just give up (or maybe use them to HELP the first stage - replacing the solid fuel rockets).

My idea was that the fuel was lighter and lasted longer but the engines don't seem to be powerful enough to help launch a payload (at least not at the tons I need).

They don't even get off the ground.

First off, yes, air-breathing boosters can work.

Secondly, like with everything else in this game, it's all about your design.

IMO, Basic jet engines kinda suck for anything other than subsonic low-altitude (<12 km) flight. If you're using the basic jet instead of turbo's, you're going to have to limit yourself to smaller payloads or use a bunch of engines. This is how every lift off works: TWR > 1

Also remember that jets need time to 'spool up' to full thrust. You need to let your engines run for a bit before they're going to lift anything; they're not like LF/Ox engines that go to full power in practically no time.

If you want to provide pics we can try to help you.

[Xannari Ferrows]

Probably not adding anything new here, but what are you trying to lift, exactly?

[Pecan]

...They don't even get off the ground.

Stationary, on the ground, jets do not deliver their maximum thrust, even after 'spooling up'.

Test A jet engine to see what you can launch (at all) with it, then you'll know how many engines you'll need for your target mass. They aren't high-powered, but they are very fuel-efficient. Until version 1.0, possibly.

[Valley]

Thanks folks. I think I am trying to life too much with too little. I am going to go test out some designs. (I was thinking of using jets to help delivery fuel into orbit but I am thinking I should switch to transporting crew but let me try a few more designs).

[Aethon]

Scott Manley uses a jet powered first stage in his old, reusable space program. The design gets modified throughout the series.

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[Valley]

I just finished designing, building, and launching a one-crew delivery system and I am surprised how little fuel the first stage used and how long it lasted before the lack of oxygen forced me to release it and go to the second stage. Why don't more YouTubers' use jet engine first stages? I am AMAZED on how effective it seems to be. It could have easily of climbed four or five times the height (or more) with the fuel it had. I am shocked I have never tried this before. Thanks for the advice folks - I learned to warm up the engines before releasing the clamps.

[FlyingPete]

With the right design, you can build a vertical SSTO using jets for the initial climb. Comes in handy for delivering bulky payloads, plus you get to recover your expensive engines rather than drop them into the sea.

[magnemoe]

With the right design, you can build a vertical SSTO using jets for the initial climb. Comes in handy for delivering bulky payloads, plus you get to recover your expensive engines rather than drop them into the sea.

Yes, I have an tail sitter VTOL SSTO, its basically an small spaceplane with two rapiers. Wing design is lots like an star wars x-wing with tail fins on wing edge and landing legs on them.

Works well, landing is a bit complicated, fly slow, use hotkey too switch to two 24-77 in nose to bring the plane pretty vertical, pop parachutes switch back to rapiers and use them to land slowly.

Can also be used on Laythe, capable of water landings and pretty uneven terrain.

[FlyingPete]

Works well, landing is a bit complicated, fly slow, use hotkey too switch to two 24-77 in nose to bring the plane pretty vertical, pop parachutes switch back to rapiers and use them to land slowly.

You'll get more control by using a couple of the Vernor engines to increase the pitch moment. That way you can just keep pitching up until it stalls. Set the hotkey to enable/disable these thrusters.

Unless you're trying to get the plane to 'hang' vertically from the nose-mounted engines- this doesn't work

[cantab]

Which jets are you using?

The basic jet gives full thrust at no speed, and drops off as you accelerate. You can get high but expect a fairly slow climb.

The turbojet on the other hand can take you nearly to orbital speed, but does not give full thrust at zero speed. Use extra jets, or rocket boosters.

Either jet takes time to reach full thrust, so hold the rocket on the launch clamps while they start up. And of course all jets need intakes!

[MarvinKitFox]

Yes, the first stage can use jet engines!

Jet engines deliver some very, VERY good fuel efficiency, but they tend to be a bit heavy and only operate while in at least some density of atmosphere.

Jet engines also tend to have somewhat low thrust-to-mass ratios.

Thus, you want to have them operate as long as possible (to make up for the low TWR) and to stay low enough to have access to the needed air for intakes.

"Brilliant Idea" says I.

Lets use a much more horizontal flight profile!

Now of course it is a bit hard to fly your rocket on a very horizontal flight while it is picking up speed, that pesky thing "gravity" keep on trying to pull you down.

How about using some of this air that is whoosing by?

If I put a flat thingummy in the wind, and angle it just right, it might help me fight gravity.

A flat thing that uses the Wind.. hmmm.. a wind thing.... lets call it a "wing" for short.

Now as all this is some very plain straightforward thinking, let's call the whole contraption just that. A Plain!

Oh no, a plain is a flat piece of earth...

Lets call it a Plane!

[FlyingPete]

Yes, the first stage can use jet engines!

Jet engines deliver some very, VERY good fuel efficiency, but they tend to be a bit heavy and only operate while in at least some density of atmosphere.

Jet engines also tend to have somewhat low thrust-to-mass ratios.

Thus, you want to have them operate as long as possible (to make up for the low TWR) and to stay low enough to have access to the needed air for intakes.

"Brilliant Idea" says I.

Lets use a much more horizontal flight profile!

Now of course it is a bit hard to fly your rocket on a very horizontal flight while it is picking up speed, that pesky thing "gravity" keep on trying to pull you down.

How about using some of this air that is whoosing by?

If I put a flat thingummy in the wind, and angle it just right, it might help me fight gravity.

A flat thing that uses the Wind.. hmmm.. a wind thing.... lets call it a "wing" for short.

Now as all this is some very plain straightforward thinking, let's call the whole contraption just that. A Plain!

Oh no, a plain is a flat piece of earth...

Lets call it a Plane!

This sounds like a plausible explanation for how Kerbals invented aircraft after rocketry

[Pecan]

...Lets call it a Plane!

Nicely done, I laughed :-)

The only error is in your premise that anyone might fly their 'rocket' - which is, in any case a jet - on a horizontal flight at low speed.

Analysis would follow, but it's pointless anyway since aerodynamics will all be changing in 1.0 and the engines will be rebalanced anyway. We have no idea what will work and what won't in 1.0.

[FlyingPete]

Nicely done, I laughed :-)

The only error is in your premise that anyone might fly their 'rocket' - which is, in any case a jet - on a horizontal flight at low speed.

Maybe it ended up on a lawn dart trajectory and Jeb- never one to bail out until the last second- tried to pitch up. He found that the craft was quite stable in this configuration and landed it in the grass outside KSC. Now, if only we could fit some wheels and build a large, flat surface to launch it from...

[rkman]

I am AMAZED on how effective it seems to be.

The amazing fuel efficiency of jet engines is a bug.

[Torquimedes]

As a rule of thumb, a BasicJet can lift 15 tons off the launchpad. including its 1 ton weight plus fuel. As mentioned above, put the jets in a separate stage from launch clamps and give them a bit to spool up. If you use KER watch for TWR=1.0 to release the clamps.

Try this:

1. Start with an Engine Nacelle or Radial Engine Body.
   
2. Add a tricoupler and 3 Basic Jets.
   
3. Add 8 radial intakes, because the nacelle includes one. There's enough space to do this without part clipping.
   
4. If you're not averse to mods, the Intake Build Aid really helps to balance the intake-to-engine mapping.
   

Total mass of this unit is 3.7 tons, lift capacity is an additional 41.3 tons. The 40 units of fuel in the nacelle should propel you to 450 m/s at 20km or so where you'd run out of air anyway, so there's no wasted fuel mass when it's time to jettison. A bit pricey at 15k Kerbucks but it's a handy modular first stage. There's space to attach a parachute if you're using a stage recovery mod, or AV-R8s if you need more pitch control.

I put a liquid stage on top of this (LT-30 plus 1200 units of fuel) with a radial coupler as a hybrid booster system. Total cost is about 22k Kerbucks and each hybrid booster delivers 5 tons to LKO. If you lack larger rockets and your PC can handle the part count, it's easy to slap these on radially as a modular launch system.

KerbalX link

Edited March 7, 2015 by Torquimedes

[mrclucks]

I've made an onion stage of jet engines for my rocket before. Doing a typical gravity turn always failed due to lack of speed.

What seemed to work was moving sideways as soon as possible. Basically I would tilt my rocket over slightly and while I would move even slower vertically, my horizontal speed would increase greatly and would even accelerate at a decent speed.

The idea is that even though you're accelerating at a fraction of a meter per second per second vertically, you're accelerating much much faster horizontally, and because you spend more time in the lower atmosphere, your air breathers get more life out of them.

It does take a considerable amount of time, but in my experience I'm starting to gain altitude at a decent rate and I'm already moving vertically pretty fast so I point my rocket 5-20° above the horizon and I have no problem getting into orbit even with a twr ~1.1

[WanderingKid]

I know this is considered answered, but I did a no-upgrades career 18t Manned Mun landing using Jet Engines as an initial stage. While the majority of the episode is really based around how to get around and pull it off with no conics and nodes, the liftoff stage of the design might interest you:

[PPR]

There is certainly a benefit in using them in terms of fuel efficiency, but I would only recommend it for small loads (able to be lifted with a single turbojet engine). The reason is that there are some serious problems with them that you won't have with comparable rockets:

1) Not as responsive (it takes time for them to come to full power or cut-off).

2) Only reliable below 20km. Above that they cut off unexpectedly.

3) You only want to use one engine because the engines do not cut-off at the same time, which can cause you spin out of control and lose velocity.

4) Increased part count, both because of the air intakes and partly because they have a limited amount of power. Why use three turbojet engines when you can use one Skipper engine? In my experience, the simple designs are more reliable and less likely to cause problems with your computer.

[dewin]

You can solve #3 by setting an action group to turn off your air intakes, then hitting that just before your IntakeAir zeroes out. Jet engines take awhile to shut down, but no air = no boom.

[impyre]

I've used jet engine launch stages. The issue I generally take with them is twofold: A) lack of thrust is irritating, having only 1.25m sizes exacerbates this and if you're going for maximum efficiency and using jets... it sounds like you may as well just call it a space plane and make it SSTO. Sure, it won't deliver huge amounts to orbit at once... but it's reusable, versatile, and efficient.

[PPR]

If you are using career mode, it is also cheaper to use a rocket.

[FlyingPete]

It does get expensive to throw away jet engines as a first stage, so until we have proper stage recovery it's much cheaper to stick some rocket boosters on instead. Either that or make it a SSTO. This doesn't have to mean spaceplane, and I've built a couple of vertically-launched jet/rocket hybrids before. The jets lift you out of the thicker part of the atmosphere and start building up horizontal speed, then the rockets take over to accelerate into orbit. They're useful for launching bulky but relatively light payloads to LKO.