Choosing the right rocket engine for greatest efficiency in 1.0 and up

[Halsfury]

Ok so I decided to write this tutorial because of my own experience with upper stage engines in 1.0 KSP

I've been playing this game for a few years and as such I got very used to using nuclear rockets for all my deep space needs. At the time there was nothing wrong with this.

Now all the upper stage engines have their own distinct advantages and as you play the campaign you will notice that even though the Terrier is less efficient than the Poodle, that in some cases using the poodle might be wasteful, or using the terrier engine might be wasteful.

So this tutorial will be basically a list compiled from ÃŽâ€v -> calculations involving a theoretical rocket made of one 2 meter tank with a 2 meter command pod on top of it I hope to show the actual efficiency of each engine by compiling this data on efficiency.

48-7S "Spark" Liquid Fuel Engine

This engine, has a low vacuum Isp because of it's small size, as such it has it's limits in efficiency but it has it's uses.

with a Rockomax X200-8 fuel tank and the Spark the ÃŽâ€v -> for our simple rocket with a total mass of 8.7 tons (full) would be 1811.56 m/s

with a X200-16 it does even better with more fuel and a better ratio between dry mass and fuel, now it has a higher ÃŽâ€v -> of 2740.64 m/s

Now lets compare it with the LV-909 "Terrier"

with the X200-16 the Terrier performs much better than the spark. for an increase in total mass of just 0.4 t we get 3002 m/s of ÃŽâ€v ->

but with the smaller fuel tank the Terrier gives us less for our money with the X200-8 tank the ÃŽâ€v-> is only 1959 m/s

so with the larger tank the Terrier gave us + 262 m/s of ÃŽâ€v-> while with the smaller rocket the Terrier only gave us 148 m/s extra.

The Rockomax "Poodle" engine is even less forgiving because of it's increased mass, with the X200-8 tank (the smallest 2 meter tank) the Poodle only gives 1666 m/s so if you decided to go for the poodle's extra 5 seconds of Isp with a smallish upper stage you actually lost - 293 m/s of ability to go places (maybe more or less depending on payload)

the Rockomax is also pretty much just more pounds on the larger rocket with the X200-16 tank since it only gives 2642m/s of ÃŽâ€v-> which is - 360m/s less than the "Terrier" and 98m/s less than the "Spark".

and which one is cheapest? the spark. the terrier is only 120 bucks extra though so with a small rocket the terrier is probably your choice for shifting medium payloads and even really small rockets benefit from the terrier.

Note: It's only with very small payloads that the Spark really excels on all fronts (things less than 3 tons or so) and at that point the "Ant" engine is probably more efficient with 315 secs of Isp. the Spark is really a landing engine for small landers on atmosphere-less bodies. Good places to use the Spark for landing (on a probe for instance) have low gravity like the Mun but not so little as to make the high thrust redundant like on Gilly, Pol or Bop

The Moral of the Story is; do your Delta V calculations, the game gives you all the information you need to make the right engine choice for each part of the mission. You don't have to be exacting about it but in deep space, with no reason for great amounts of thrust, I would suggest going to this website http://www.strout.net/info/science/delta-v/ and at least make sure that you are making the most economically sensible decision with your rocket.

Here are some rules of thumb:

-In order for an LV-N "Nerv" rocket to make any sense you need something the size of a big orange tank.

-A poodle is rarely more efficient (need a very big tanker) than the slightly less efficient-on-paper Terrier, and the only reason to really use it for anything is as a landing stage for big heavy landers, and as a final stage on smaller rockets where the thrust is still necessary to get to orbit.

-your rocket must be positively tiny for the Spark to be more efficient than a Terrier

I hope this tutorial helps someone, just sitting down to write it was enlightening for me though

Good Luck, and Fly efficiently!

Edited May 4, 2015 by Halsfury