The right engine for the right job?

[Masked Turk]

Hi all,

Currently in my sixth iteration of the PO-03 KESTRAL Duna orbital probe & launch vehicle, with the aim of testing the waters for a ground probe and then Kerbal'd landing.

KESTRAL project event list

• 
  KESTRAL I
  - Crashed into the sea, bad LKO flight course.
  KESTRAL II
  - Managed to get into a Duna transfer orbit, but ran out of fuel at the end to create a parking orbit, ended up flying straight past and into a Kerbol orbit...
  KESTRAL III
  - Gravity assist around the Mun x2 to save fuel... didn't help. Same result as II.
  KESTRAL IV
  - Switched to using nuclear engine for Orbital/Transfer stage. Failed to make LKO due to low thrust...
  KESTRAL V
  - Added 4x LV-T45 (small, liquid fuel) engines to Orbital/Transfer stage. Same result as IV.
  

So...

After much guesswork, wiki'ing and googling, I'm still slightly miffed as to which engines I should be using for which stages of a mission. As far as I know, I need high thrust for quickly creating/correcting orbits, while I can use more efficient and low-thrust engines for transfer/etc maneuvers (I could do with a brush up on the lingo still, too, I guess...).

However, whenever I try to apply logic to my choices, it fails... so, yeah. Can someone just lay down the basic logic paths of why rocket X is good for stage 0A, and the likes?

Cheers in advance,

JB

Edited July 31, 2013 by Masked Turk

[dlmarti]

However, whenever I try to apply logic to my choices, it fails... so, yeah. Can someone just lay down the basic logic paths of why rocket X is good for stage 0A, and the likes?

Its not your logic that is failing, but your experience in the game.

Which btw, you are getting the right way. Test, test, play, test, play, test, etc.

If anyone here tries to give you the skinny on how they do it, it will most certainly be wrong for you.

Take a look at this craft:

https://www.dropbox.com/s/uovsk0a8jazgcqt/Fueler%20Mk6.craft

That is my absolutely most favorite craft, I probably launch 2-3 each day, and a dozen on a good day.

But I bet it doesn't fit the way you play, cause we all do it differently.

Edited July 31, 2013 by dlmarti

[FrostOmega]

Its not your logic that is failing, but your experience in the game.

Which btw, you are getting the right way. Test, test, play, test, play, test, etc.

This.

And a quick tip: NERVAs are better for use in space (Isp on vacuum: 800) but they're kind of weak!

[numerobis]

You need high thrust to get off the planet, but "high" only means a thrust to weight ratio of about 2 -- on Kerbin, that means you need a bit under 20 kN of thrust for every tonne of mass. At every stage of your rocket, add up how much mass your rocket has, and add up how much thrust. If you have more than 20 kN per tonne, you can do with fewer engines.

[Dawun]

I haven't done too many rockets - which sounds like what your probe uses to be launched - but I have had the same problem as you. For interplanetary travel you pretty much need nuclear engines, they are over twice as fuel-efficient as any other engines (except ions). I've found that the best way is to have some big engines, depending on how heavy you are at that point, can be the LVT series or skipper/mainsail, to get yourself into orbit. Bring along a pair or two of nuclear engines as well. This is a case where I have found that there isn't one engine for the job and that its better to have multiple.

Here's an example of my ship's engines:

http://postimg.org/image/dqpovaz0r/

It reaches Kerbin orbit with more than enough fuel to make it to Duna orbit, despite how heavy it is. You get fairly long burns (~12min) but if you divide it like you did with your 2x lunar assist then it works out well.

I can't answer much about which engine is good for each stage because I pretty much have done only planes :/. But I use jets to get to around 30km, then turn on the big Skippers and the nuclears to get to Kerbin orbit. Once I'm in orbit I use the nuclears for everything.

I do recommend you use the moon of Duna to slow yourself down when you enter Duna's orbit, its the same maneuver as with the Mun but backwards

[Ahmed]

Quick tips for You about engines I prefer to use at all stages of my mission to Duna. Of course quantity and types of engines and tons of fuel depends on mass You want to orbit and deliver to Duna orbit or its surface:

1.Going to orbit: Rockomax "Mainsail" for sure... or LV-T45 for smaller crafts. If more thrust needed give some more mainsails every with 3 or 4 Rockomax X200-32 Fuel tanks

2.Inserting parking orbit: Rockomax "Skipper" or Rockomax "Poodle" or LV-T45 for smaller crafts, You need some thrust to insert orbit fast.

3.Interplanetary transfer, orbit changes, nodes correction : there You can use more efficiency engines like LV-N Atomic Rocket Engine, add 3 or more if You got heavy craft. Remember they are efficient but very heavy! You can also use Rockomax "Poodle" or LV-909.

4.Braking to establish orbit on Duna: master aerobraking but if developers end their project it will be harder because of heating. Else, use the same engines like in point 4.

5.Landing/Descent: I preffer radial engines:Rockomax Mark 55 or Rockomax 24-77 or new one: LV-1R Liquid Fuel Engine. Other small engines are also good. Dont use LV-N Atomic to land on Duna because it is heavy and loose some efficient at surface.

6.Ascent: It isnt hard to get in orbit on Duna. Remember to take some fuel with you.

But like dlmartin wrote You need to practice orbital/landing techniques. It is more important than engines used in any phase of mission. Gravitational assist is also useful.

Have a nice day with creating new duna mission!

[Masked Turk]

Cheers all, gents, very useful posts (and craft files)!

I'll crack on and try to factor in your suggestions/design ideas/concepts, and hope that by KESTRAL XXXIV I'll be in Duna orbit

[zerotwo]

Masked Turk:

Look at the engine ISP and guess where you will be in flight when that engine turns on.

Mainsails, LV30, LV45 for going to orbit, Aerospikes, Poodles, skippers, and 30/45 for LKO stage, then 909 for short range (Kerbin influence) or 1 LV-N for any kind of long range. (General rules here, can be broken)

TWR in space is meaningless, just Dv counts.

If you can keep your fuel ratio high and your engines efficient, you can get anywhere. (Fuel ratio is amount of weight of craft that is fuel, multiple LVNs ruin this)

Hope that helps

[Hour Hero Yes]

Remember -- you can attach the bi-tri-quad adapters to the bottom of big fuel tanks and attach multiple smaller engines to it. I see a lot of players using large fuel tanks, but attaching 2 nuclear engines to the bottom. Still highly efficient, but faster burns!