kspalculator Documentation

kspalculator is a tool which determines the best rocket propulsion designs for one stage of a rocket, given a set of constraints and preferences.

Constraints are properties of the spacecraft which have to be fulfilled. These are the possible payload and the Delta-v as well as the minimum acceleration which is reached in an environment with given air pressure. Preferences are further properties a propulsion design might fulfill in order to be preferred. Examples for preferences are the thrust vectoring angle, the radial size, whether the engine is able to generate electric power, etc.

Which is the best design depends heavily on the specific application. A design might be better than another one, if it is cheaper or has a lower mass, but it might also be considered better if it is buildable using less technology or if it better fulfills some of the given preferences. Obviously, it is impossible to sort all propulsion designs by their “goodness”, so there might be more than one which is the best at least by some criteria. This tool presents exactly all best designs.

Check out the official web frontend of kspalculator at: https://kspalculator.appspot.com/


kspalculator evaluates all possible designs, checks whether they fulfill the user’s requirements, and then checks whether it is the best design using the relation “A is better than B iff A is better than B by any of the user’s criteria”. Only the best designs are then presented to the user. This way, the user has maximum flexibility to use the type of propulsion which serves best his needs, still without being spammed by non-optimal solutions.

The stage might have different requirements for minimum acceleration for different *flight phases* through different air pressures and different Delta-v requirements. For example you might require the vessel accelerating by 1000 m/s with an acceleration of 3 m/s², and later 500 m/s with an acceleration of 7 m/s².

Besides considering the classic liquid fuel engines as well as solid fuel boosters, kspalculator also considers using the LV-N Nerv Atomic Rocket Motor, the IX-6315 Dawn Electric Propulsion and the O-10 Puff MonoPropellant Engine.

Considered criteria to decide whether a design is better than another one are

  • Mass,
  • Cost,
  • Whether it is buildable with easier available technology,
  • Whether gimbal (thrust vectoring) is available, or Thrust Vectoring Range,
  • Whether it uses MonoPropellant as fuel, which is also used by Reaction Control System (RCS) thrusters,
  • Whether its engine generates electric power,
  • The length of the engine, as might be meaningful when building landers,
  • Whether it meets user’s radial size preference.

Even though calculating this sounds highly sophisticated, the best designs are presented to the user usually within less than a second. The information shown about each design includes a detailed listing of the performance characteristics, i.e. the actually reachable Delta-v (which might be slightly more than required, because of rounding to tank sizes), the acceleration at full thrust as well as the mass at beginning and end of each flight phase.