How do you set up the purpose of each stage of your rocket ship?

[Moonfrog]

I've been thinking about this, if I can make my ship better. I used to design my ships pretty much like this. Taking into account how much each stage needs in D/V.

1. Lander

2.Planetoid Transfer module.

3.Takeoff launch booster.

after noticing that my take off launch boosters always had some fuel left over, I search over forums to try and find out why. I did some research found out my launch booster could use another stage as it was using non space efficient engines while in orbit. I use kerbal engineer mod to help calculate DV. Now trying to design ship like this adding a new stage.

1. Lander

2. Planetoid Transfer module

3. Orbiter

4. Launch take off booster.

Idea being that ISP changes when u reach orbit, so DV calculations are different. Also I try to make the orbiter have a very high TWR so it orbits quickly.

I was wondering do you other rocket makers, have different stages for your Landers or transfer modules? I don't mean asparagus or onion fuel type staging, but purpose behind the stage your making. I've seen some landers that I think it has an ascent module they called it. Is that better to have an ascent module?

Mostly interested if u have other types of stages u use.

[Nuew]

I think it really depends on you - you might need an ascent stage for the dV, or you might not. Depends on how you built your lander.

[blizzy78]

Also I try to make the orbiter have a very high TWR so it orbits quickly.

As long as your orbital insertion stage can actually establish the orbit in a reasonable timeframe (seconds to about a minute instead of hours), the TWR can be ignored. Which can also save weight because of engine choice.

I've seen some landers that I think it has an ascent module they called it. Is that better to have an ascent module?

Leaving the descent stage behind on a world and only taking the ascent stage with you is better because of reduced mass, thus you won't need as much fuel to bring the entire craft into orbit.

[allmhuran]

- Ascent stage - big and powerful. Mainsails.

- Orbit injection - small, light, weak, but efficient. Engine choice depends on craft size, LVT30's are a common go-to for me here.

- Interplanetary. Kerbin to wherever and back. Nukes.

- Lander. Engine choice highly dependent on target object and craft size.

- Optional: Dedicated lander descent and/or ascent. Only really needed for certain bodies - bigger ones with no atmosphere to assist with descent. Eg, Tylo.

Obviously your actual choice of engines will depend on the size of your payload.

[Spyritdragon]

Generally, a big launch stage with reasonably inefficient engines with high thrust, used to get my craft mostly into orbit. For rockomax, i tend to rely on mainsails or skippers at least untill i hit a 80-by-80 LKO. Then a 'transfer stage' to put stuff either in the orbit it needs to be, bring it to a station i want to dock it with, or put it in the sphere of influence where i need it. This stage always comes with a probe core to deorbit it if in any way possible, and is propelled by NERVA's, 909's, possibly poodles. Sometimes the last part of LKO injection and this stage fall together, usually for station parts, which will usually use a skipper. Then finally, my payload :-).

Keep in mind when designing, that ISP is a gradual line - it gradually changes going from sea level to in space, rather than switching the moment you exit the atmosphere.

[blizzy78]

Keep in mind when designing, that ISP is a gradual line - it gradually changes going from sea level to in space, rather than switching the moment you exit the atmosphere.

Not necessarily, as I've been witnessing with the latest KOSMOS mod update. Engines have what's called an "atmosphere curve". This curve can be designed by the mod maker to be non-linear.

[Spyritdragon]

Okay, my bad. Its not a line, but its still a curve :-)

[M5000]

Not necessarily, as I've been witnessing with the latest KOSMOS mod update. Engines have what's called an "atmosphere curve". This curve can be designed by the mod maker to be non-linear.

Okay, my bad. Its not a line, but its still a curve :-)

Indeed, the density of the atmosphere is not a straight line if it were to be graphed against altitude, rather, it would follow an exponential curve that I believe is also logarithmic. So after a certain point, it does get MUCH easier. That's why there's a big difference between 0-10Km, but not too much of a difference between 50-60Km.

[capi3101]

Once I have my mission defined (I say, "Okay, I want this hunk of junk to do <x>"), the first place I go is to the delta-V map on the wiki and figure out how much delta-V is going to be needed for each phase of the mission. I know I'm not all that great of a pilot, so as a rule I add a 25% reserve to those figures. I then work backwards; say I'm going to do a manned round-trip back to Kerbin mission. I know I'm going to need a command capsule, a chute or two and some batts; that's the stuff I'll need to get my guys back. A service module comes next; this has the delta-V to get back from where I'm going. I may or may not combine this stage with the lander; having done things the Apollo way for a while, I can say that a dedicated lander module allows you to land heavier payloads as a rule. Lander has enough delta-V to land and launch on its own, with no other payload. A transfer stage comes next; this has the delta-V to get the CM, SM and lander to the target and get it in orbit. The booster comes last and as a rule it's the largest piece.

CM - chutes, capsules, batts. Docking ports if necessary,

SM - RCS, return fuel and engine, solar panels. Nukes are recommended though I have also used Poodles with success on interplanetary missions.

Lander - engine depends on the target and the mass of the lander. Usually an LV-909, though I have used large numbers of 24-77s to good effect (6 of those weigh as much as a -909 and have 2.5 times as much thrust). Poodles and even nukes on occasion. On low gravity worlds like Pol or Gilly, RCS is sufficient for landing; no, seriously.

Xfer Stage - Same as the SM: nukes are best for interplanetary trips but I've used Poodles and even LV-909s successfully.

Booster - Depends on how you set things up. Mainsails/Skippers usually, but I've had luck with all engine types except the LV-1s and Ion engines (nukes even work if you pack enough of them).

Really, a mission is more dependent on its delta-V budget than available thrust in most cases. Figure out the mass of your payload, add any staging equipment and engines you have selected for a stage, then work Tsiolkovsky backwards (adding your payload mass to both mass portions of the equation - i.e. (M + Payload / Md + Payload) - and bearing in mind that for most fuel tanks their full mass is nine times that of their dry mass - i.e. M = 9 md - so you can substitute in that relationship). When you know how much fuel you need, add tanks that match that amount, then work the equation forward to see how well you did. If its unacceptable, try again. If it is acceptable, set the resultant total mass as the payload mass for the next stage and start the process over with that next stage, and keep working until everything's done. You can check TWR if you'd like at any time, though it's generally necessary only for launch and landing phases. Add more engines if what you have isn't sufficient and recalculate.