Jets as Rocket Boosters

[Aegolius13]

I was playing around with a lunar-orbiter type rocket the other night, and noticed that a pretty good launch engine option was... the Whiplash!  I don't mean launching as a plane, I just mean putting a jet engine, some LF and an inline intake below  regular rocket and launching vertically from the pad.  I've seen some jet booster stages on low-mass challenges before, but did not expect it to be cost effective due to the price of the jets and intakes.  However, when using a Whiplash and the divertless supersonic intake (a very cheap intake with all the LF capacity needed), the cost was within a couple hundred funds of a conventional Swivel-based launch stage, and I believe it generated more effective delta-v (i.e., got my next stage closer to orbit).   Various other observations:

-Whiplashes seem like the best fit among the jet engines for this application.  The Panther has nice thrust off the ground, but loses that thrust quickly enough that I'd rather pay the extra $250 for the Whiplash.  Conversely, the Rapier is too expensive to be cost effective, and would really struggle to get off the ground.  The turbofans, just no.  

-Thrust was low at first but got my TWR over 5 at peak.  The jet cut out at 25km and 1400m/s, but from there a Terrier-based upper stage could easily get me to orbit, again with more delta-v left than a similarly priced Swivel rig.    

-Launching a 4 ton upper stage worked great, but 8 tons was too much.  It does not really matter if your rig picks up speed slowly, since burning a minute of extra LF adds only a negligible amount of cost.  But there's a point where the jet needs to accelerate fast enough to pick up more thrust than it loses for the higher altitude.  Thus, there's something of a weight limit to this option unless you augment with extra thrust, like a small SRB underneath, or a couple on hardpoints.  

-This did not appear to be cost-competitive as a radial booster on large rockets. The Kickback is just too cheap if all you want is big dumb thrust.  And in those cases you really want the thrust help at the beginning, not when your rocket is picking up speed and TWR anyway.  

 

Has anyone else played with jet-powered boosters as a semi-serious career mode option?  Any other notable findings?  I have ranted a few times on here about the lack of good small-vehicle launch engines right now.  So this seems like a fun avenue to explore, for a change of pace if nothing else.  

 

[samstarman5]

Using jet engines as a launch platform is handy for getting low orbit satellites up in orbit for a cheap price.

[Shpaget]

I used them, but just as a novelty thing. Never did any research into economies of it, since all my rockets are binary. They either achieve what I want them, or explode. 

I don't spend time optimizing.

[Streetwind]

It used to work even better in older versions of KSP, where jet engines (given enough intakes) could go as high as 40-50 km.

But eventually Squad made "airhogging" (intake-spamming) stop working when the atmosphere model was improved.

Edited November 8, 2016 by Streetwind

[Rath]

Airhogging was funny.  I made a plane where the wings flexed from number of intakes.

[swjr-swis]

3 hours ago, Aegolius13 said:

Has anyone else played with jet-powered boosters as a semi-serious career mode option?  Any other notable findings?

This challenge from a while back may be relevant:

From page #2 on you will find entries using a jet engine as booster, quickly bringing the weight down of the launchers.