Working out how many and type of engine to use.

[SiroVai]

Hi,

So I want to build a inter-planetary vessel.

How do I work out the best type and number of engines to use?

I know I would have to take into account the mass of the rocket, thrust of the engine and the fuel efficiency. So for example, I could use a mainsail with a high thrust but low efficiency. Or I could replace it with a number of atomic motors. How would I work out how many atmoics I would use and if that's more efficient than a single larger engine?

Thanks

[Zerro]

Hi,

So I want to build a inter-planetary vessel.

How do I work out the best type and number of engines to use?

I know I would have to take into account the mass of the rocket, thrust of the engine and the fuel efficiency. So for example, I could use a mainsail with a high thrust but low efficiency. Or I could replace it with a number of atomic motors. How would I work out how many atmoics I would use and if that's more efficient than a single larger engine?

Thanks

I'm not an expert but it seems like everyone uses the nuclear rockets for max efficiency

[capi3101]

Nukes (also called "LV-N" in the game; NERVA on the boards occasionally) are definitely how you want to go; they have over twice the Isp of any other engine in space (aside from ion thrusters, which, as Scott Manley puts it, have about as much thrust "as a mouse fart"). As far as how many, that's another matter altogether. It's going to be a matter of how massive your payload is. Nukes only put out 60 kN thrust at full power, so unless you're going with something light, you're probably going to want more than one. I've had success with a combination of ten X200-32 tanks and five nukes; the TWR is lousy, but efficiency counts more than thrust once you're in orbit.

To define success, look at my salad bar. I don't reset it with every update...too much work.

I do have a design with 25 nukes (LV-N efficiency, Mainsail-equivalent thrust); I haven't really tested it yet. The idea is to send that one to Moho, which is a world folks say is extraordinarily difficult to intercept (on account of the burn time for capture). It currently has the same fuel loadout as the five nuke design - and about 5,000 m/s less delta-V. So mass definitely counts. Five's a good balance - that ten -32/five nuke design? 12,000 m/s delta-V without a payload...

[Platofan2]

While Launching Use The Bigger Engines

And In Space Use The Nuclear One Because It Is More Efficinet

[Zephram Kerman]

For interplanetary stuff, the high ISP of the LV-N makes it the popular favorite. The quantity doesn't matter much, except for how patient you feel.

Tavert has made a nifty chart of optimal engine types vs dV, payload, and TWR:

http://forum.kerbalspaceprogram.com/showthread.php/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR?p=579323&viewfull=1#post579323

It basically says LV-N for large payloads in vacuum, unless you need TWR higher than 0.9. Interestingly, small payloads on shorter trips should use Rockomax 48-7S.

[SiroVai]

Ok great, thank you for the reply. So really in a vacuum, the mass doesn't matter as much. With a smaller motor it will take longer, but still eventually get up to speed. Compared with a larger motor, makes sense.

Now I've seen the delta-v maps out there. Is there a 0.21.1 mod that people commonly use to workout their rockets potential delta-v?

I haven't tried landing on a planet with an atmosphere yet, but I see that aero breaking is pretty much essential as to stop the use of fuel to slow down. It seems pretty straight forward, just aim my trajectory for an altitude that's within the planets atmosphere and hope for the best? Are there any tips for this?

Say a planet has an altitude of 70k. Where would I aim my initial trajectory to slow down sufficiently, or to a good amount to enable capture or minimal decelleration fuel consumption?

Thanks!

[annallia]

General rule of thumb, go with the LV-N. As to how many... depends really on how big a fan of long burn times you are. I find that 1 engine for every 22 tons (25 tons when you count the engine/mount) works well. Keeps most of my burn times under 4 minutes though some of the trips do take longer like Kerbin to Jool.

Edit- and that 22 tons is including the weight of the fuel for the engine as well.

[capi3101]

I use Kerbal Engineer Redux; folks also use MechJeb. Before KER, I did stuff by hand, and still do when I'm bored at work.

Aerobraking tips: quicksave when you enter your target world's SOI. You will probably screw it up the first time in.

Aerobraking is largely a guessing game; too high and you fly out into interplanetary space, too low and you're liable to go lithobraking. I don't have a comprehensive chart in front of me at the moment for good aerobraking altitudes......IIRC (and this is off of memory, mind you), Eve's about 72,000 for aerobraking and Duna is just 15k. Basic principle is the same though; just get your periapsis down to that height and let the atmosphere do the rest. If you have to burn at all to get captured, you've done it wrong...whole reason why my first two Duna missions failed.

[Tiron]

I use Kerbal Engineer Redux; folks also use MechJeb. Before KER, I did stuff by hand, and still do when I'm bored at work.

Aerobraking tips: quicksave when you enter your target world's SOI. You will probably screw it up the first time in.

Aerobraking is largely a guessing game; too high and you fly out into interplanetary space, too low and you're liable to go lithobraking. I don't have a comprehensive chart in front of me at the moment for good aerobraking altitudes......IIRC (and this is off of memory, mind you), Eve's about 72,000 for aerobraking and Duna is just 15k. Basic principle is the same though; just get your periapsis down to that height and let the atmosphere do the rest. If you have to burn at all to get captured, you've done it wrong...whole reason why my first two Duna missions failed.

Mechjeb's landing module also features realtime, accurate aerobraking prediction. If you hit the 'show landing prediction' button, that activates the re-entry simulation feature. If the simulation indicates you're going to re-enter and land, it indicates where (or times out if it's a really shallow trajectory). If you're going to rise up out of the atmosphere afterwards, it instead shows the predicted orbit that will result after one pass (and can even set up a maneuver node for the aerobrake!)

I also saw that someone did some kind of external aerobraking calculator, but so far as I know only Mechjeb's landing module can do it ingame, in real time.

[Specialist290]

The aerobraking calculator mentioned above is here, incidentally.

The guy responsible for that has also put together some nifty aerobraking charts for Jool and Kerbin as well, which I'd highly recommend you take a look at if you're interested.