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Engine Usage and Stage Sizing

Théo Verhelst edited this page Mar 15, 2019 · 1 revision

Engine selection and Stage Sizing in Realism Overhaul can be somewhat daunting if you're not familiar with how rocket engines work in the real world, hopefully this guide will help with that.

Fuel

Oddly enough, fuel choice is among the most important aspects of choosing an engine. If you're a little familiar with how engines work this will make a lot of sense. If you're not interested in the technical aspects, here are the basics:

  • Low density fuels are very efficient, but low thrust. (Example: Liquid Hydrogen & Liquid Oxygen)
  • High density fuels are not as efficient, but provide much more thrust (Example: Kerosene & Liquid Oxygen)
  • Anything with 'Lqd' in the name is a cryogenic fuel, and will boil off.
  • Hypergolic propellants are very useful for long missions because of the lack of need for ignition sources. (For more information, check out the Wikipedia Page)
  • Hypergolic propellants tend to be higher density, lowering their efficiency.

Details on why this is the case go here.

Thrust / Time to Orbit

The second consideration you'll want to use in selecting engines is the Thrust it provides. In general, there are two useful metrics here:

Thrust to Weight Ratio

Mechjeb provides some excellent information here, showing TWR as well as SLT (Sea Level TWR) per stage. Sea Level Thrust is very important since more often than not you'll be launching from inside the atmosphere. Kerbal Engineer can give the same information, though it's not quite as accessible. General guidelines for sizing stages:

  • Your first stage should have a starting sea level TWR of between 1.2 and 1.7 (Up to 2.0 if you have SRBs that burn out fairly quickly)
  • Below 1.2 you'll accelerate too slowly, and waste fuel.
  • Above 1.7 you're not making effective use of your launch vehicle's thrust, and can potentially damage your launch vehicle with aerodynamic stresses.
  • Mid-atmospheric stages should be sized to have a starting TWR of about 1.(Expect to be in near-vacuum when this stage fires)
  • Upper stage insertion stages can have even lower TWRs, which often means achieving orbit after apoapsis.
    Example:
  • Saturn V stage 1 (S-1C) SLT = 1.18
  • Saturn V stage 2 (S-II) TWR = 0.86
  • Saturn V stage 3 (S-IVB) TWR = 0.88

Additionally, for Crewed vehicles it's often wise to keep the acceleration experienced by the crew under 4 Gs (TWR of 4) to minimize discomfort during launch, and maximize the ability for your Launch Escape System to extract the crew from a rapid unplanned disassembly.
Saturn V Example: Saturn V Launch Acceleration

To maintain this acceleration, the Saturn V would switch off its central engine.

Time to Orbit

The other metric which will help you determine if you have sufficient thrust between all of your stages is the total burn time to orbit. You can often skimp on thrust if you can still finish your full launch burn within about 10 minute time frame. You can extend this time by injecting into a higher orbit, but this requires more Delta V. Example:

  • Saturn V time to orbit: 11.7 minutes

Delta-V

The final thing you'll want to consider in sizing your rocket is Delta-V. An efficient LEO insertion can be done with about 9.3km/s Delta V, though it's wise to budget some margin for inefficient flying. A decent number is about 9.6km/s.

For the rest of your rocket, a Delta-V chart like this one can help immensely: Real Solar System Delta-V Map

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