Jet Engines First Stage

[Synapse]

Is it just me or are rockets redundant for a first stage? One canister of jet fuel can fuel 4 jet engines using fuel lines, so they are much more weight-efficient for getting up to 20,000m. Plus there isn’t as much of a penalty for gravity turning earlier, especially if you add wings.

See an example:

http://forum.kerbalspaceprogram.com/showthread.php/43909-Stock-Hermes-Jet-Delivery-System-Duna-on-80-tonnes

Am I missing something? Why use rockets over jet engines?

[Maxwell Fern]

I was wondering the same thing. Why bother bringing canned oxidizer along with you when you can scoop it directly in the lower atmosphere ?

[stupid_chris]

I mainly use rockets because it's much faster and makes a little difference. Yes your rocket is more massive and is bigger, but it's much easier and faster, and fuel costs nothing for now, that's good for me.

[Synapse]

I mainly use rockets because it's much faster and makes a little difference. Yes your rocket is more massive and is bigger, but it's much easier and faster, and fuel costs nothing for now, that's good for me.

Try using jets... I can easily get 1000m/s and then push my velocity up to 3500m/s using a single rocket and 4 jets on the vehicle link I posted. Using the structural fuselage for 2 of the jets means that I can run 4 jets from 2 jet fuel cannisters, still giving me insane amounts of fuel left over at 20,000 meters. In fact you could probably run 6 or 8 jet engines from 2 fuel cannisters. Intakes weigh absolutely next to nothing. I really don't see a case for using rockets, as the system stands.

[stupid_chris]

What I meant is the total time to get to orbit. I don't see any downsides to using rockets, so I use them because rockets, that's why.

[Synapse]

The downside is that they weigh a lot more?

[Tiron]

What I meant is the total time to get to orbit. I don't see any downsides to using rockets, so I use them because rockets, that's why.

I have one rocket that uses jet engines as liquid boosters. It flies a standard rocket trajectory, but doesn't even use the main rocket engines until I start the gravity turn at 10,000m. There's twelve engines with twelve intakes and 2 mk1 fuselages for fuel. I start cutting the engines off four at a time as it starts to run out of intakeair around 20km, and cut them all lose shortly thereafter when the last four can't get enough air.

For all 12 engines there's only four decouplers, and only two fuel tanks, so the total weight it adds is amazingly low considering.

Takes a little more work to manage but it's not particularly any longer than a normal rocket launch and adds a LOT of Delta-V.

Edit: I'll put it this way.

Without the jet Engines, Mechjeb says it has a total of 17999 m/s worth of Vacuum Delta-V. I currently have one of the probes it launches up, it's in a 80,978 KM orbit around Kerbin at the moment after having fully mapped Kerbin, the Mun, and Minmus, with 9727 m/s Vacuum Delta-V Remaining, awaiting a Transfer window to Duna.

Edited August 1, 2013 by Tiron

[stupid_chris]

Not really a problem. If a mainsail a jumbo 64 and a nose cone can get me to space and that a jet engine a jet fuel tank and an intake can get me there too, both give me the same part count. Weight is just a number and doesn't influence much things. I see jet engines to be slower to get to orbit and more dangerous due to the possibility of asymmetric flameout, so yeah, that's why I use rockets, regardless of the weight. Both can be done pretty well and both have their advantages and disadvantages. I just prefer rockets.

[Synapse]

I prefer rockets too, however I am pointing out that in KSP physics, jet engines are better in every way for your 0-20,000m stage.

[Tiron]

Not really a problem. If a mainsail a jumbo 64 and a nose cone can get me to space and that a jet engine a jet fuel tank and an intake can get me there too, both give me the same part count. Weight is just a number and doesn't influence much things. I see jet engines to be slower to get to orbit and more dangerous due to the possibility of asymmetric flameout, so yeah, that's why I use rockets, regardless of the weight. Both can be done pretty well and both have their advantages and disadvantages. I just prefer rockets.

Higher Weight = Lower Delta-V, means you have to burn more fuel to accomplish a given maneuver, which means less ability to maneuver with a given quantity of fuel, which means fewer places you can go.

On a rocket you can minimize the problem of asymmetric flameouts pretty easily. They're so heavy they don't turn much anyway, so with Mechjeb's slightly inept 'prevent flameouts', an action group to cut some of the engines, and a little attentiveness, and it's not much of an issue.

That particular design is my mapping probe, so it was designed to maximize Delta-V at literally any cost. Almost the entire thing is either fuel or engines.

The whole point is to be able to map as many places as possible with a single probe, since you can only map with the focused craft, having multiple up at once doesn't help much (though you can do other stuff while you transit). I'm praying it's got enough Delta-V for a single probe to do the Entire Jool System, but I'm not counting on it, and keep getting my saves broken before I can find out.

[PwnedDuck]

I always side mount jet engines: they work like fuel efficient, extremely light SRBs. The only problem in that mechjeb can't work out their Dv additions and gets confused by them into not detaching stages, so I need to do it manually.

[Tiron]

I always side mount jet engines: they work like fuel efficient, extremely light SRBs. The only problem in that mechjeb can't work out their Dv additions and gets confused by them into not detaching stages, so I need to do it manually.

...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.

[Synapse]

For the fuel usage involved, this is amazing efficiency. I had loads of jet fuel left, as well.

Edited August 1, 2013 by Synapse

[Umlüx]

but you would need loads of jet engines to lift very heavy payloads. i think i'll stay with rockets for now.

[annallia]

I do use them now and then, mostly in my ultra light lifters (sub 5 ton payload) because they are efficient and small. Anything with a larger payload though I find diminishing returns on my rockets, rather than build banks and banks of rocket engines I would rather just use a large fuel tank and a mainsail.

[el_coyoto]

Jet engines can be very efficient first stages, I've seen very some smart and compact jet engine designs do the same job than more traditional rocket stacks. (I think there is a video of this from a guy whose name starts with 'S' and ends with 'cott Manley' )

But I don't use them for 2 reasons :

• they need time to spool up on lift-off (ok, it's not really a problem, but it bothers me)
  
• I think that IRL rocket engines are cheaper than jet engines because they are simpler to build and have less moving parts [Citation needed].
  

It's a matter of personal preference, but I would use them only on SSTO boosters. Even if that means going to space with jet engines attached to my rocket...

[Motokid600]

This thread is making me rethink the deign of my Laythe return craft. Thing needs to get to a circular 65km orbit and still have enough fuel to dock with the CM.

[-RanZ-]

I use rockets... I just love semi-controlled explosions propelling Jeb to orbit

[Tiron]

but you would need loads of jet engines to lift very heavy payloads. i think i'll stay with rockets for now.

Not as many as you'd think:

The trick is that I'm using Turbofans rather than Turbojets. They develop a LOT more thrust at low altitudes (12 turbojets can barely get it off the ground), and I found in early testing (with an admittedly slightly different flight profile) that flying a rocket flight profile rather than a spaceplane profile, it never got enough speed built up to really get the Turbojets to develop more thrust than the Turbofans, even at altitude. Turbojets have a MAJOR speed factor in their thrust determination, so on a relatively low TWR lifter like this one, they actually don't work as well as Turbofans do. The fact the Turbofans are more efficient doesn't hurt, but it's mostly a bonus.

Didn't take it to orbit, because I had my flight path set for a higher TWR rocket, and this one didn't like it. Didn't make it crash but it did waste a lot of fuel before I figured it out, and I'm about to go to bed, so I'll launch one to orbit later.

I'm actually debating mentally if I could improve the efficiency a bit with some changes(mainly engine changes), but it'd take a LOT of testing to work it out.

Edit: Also, try not to use any intakes except the ram intakes, but especially not the radials or double-especially the engine nacelles. They all have intakeair quantities and drag determined by the size of the intake, except that there's a cap on how high the drag will rise...and the cap's the same on all the different types of intakes. They all max out at '2'. Not sure if that's the drag coefficient or what(If it is, the Nacelle and the Radial are both far heavier as well, so that'd be EVEN WORSE!) Anyway, what happens is once you get some speed up, all the intakes end up with the drag capped, and yet they all produce less intakeair than the ram intakes. The radials don't produce much, and the engine nacelles barely produce any at all, and yet end up with massive drag anyway.

Edited August 1, 2013 by Tiron

[rryy]

Like this?

The payload for this alone is 75 tons and because I could not make a decent lifter to get it into orbit, I decided to spam jets and intakes. The jets I have are a bit overpowered compared to the stock ones because a mod that fixes the Isp of rocket engines doesn't go well with jets, but I didn't want to uninstall it. Even still, the thing barely gets into orbit with the fuel it has and I had to send up another craft to refuel it so it can dock with a station.

Edited August 1, 2013 by rryy

[automcdonough]

there is a logical progression towards spaceplanes happening in this thread. lol

[Nuke]

the reason why air breathing engines arent used on rockets is simple, they are much much more expensive. thats going to be a huge issue for skylon, even if it works as planned. i have a feeling the engine maintenance will be the big number in the cost of a refit. and thats aside from the fact that you wouldnt want a bird to get sucked into that engine on takeoff.

[Tw1]

I find it's true up to a certain size.

Smaller craft are easily launched with a should-be-recoverable jet engine stage, but real heavy lifting is more practical with rockets. (part count, less lag, and cheaper.)

Things of that size could be lifted in a big SSTO (I've not mastered them yet), or shuttle, which could be cheaper if re-usable.

Edited August 1, 2013 by Tw1

[zarakon]

the reason why air breathing engines arent used on rockets is simple, they are much much more expensive. thats going to be a huge issue for skylon, even if it works as planned.

Cost is less of an issue for reusable craft.

But since most rockets are considered expendable, it would be hugely wasteful to put jet engines on them.

Another major factor is that jet engines in KSP are simply overpowered. Rocket engines in KSP have scaled-down thrust:weight ratios compared to real rockets (KSP range is generally between 10:1 and 25:1, real-life range is generally between 30:1 and 150:1) to make up for the smaller planet and lower orbital speeds. Jet engines in KSP though have higher thrust:weight ratios than real jet engines (KSP jets are about 10:1 and 12:1, while real engines are between 3:1 and 8:1).

So in real life, jet engines have about 1/10th the thrust:weight ratio of rocket engines. In KSP they're about 1/2 instead.

Total thrust on real jet engines is also just too low compared to rockets. You would need over 30 Concorde jet engines to match the thrust of the Falcon 9 first stage, or over 200 to match the thrust of the Saturn V first stage.

[zekes]

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