False KSP Lessons

Moving from Kerbin to Earth

Growing up on Kerbin, we often learn facts about the universe that just don't apply when we move to Earth.

1. Myth: Rocket engines all throttle down to 0.1%.

   Fact: In fact, very few rocket engines throttle; engines designed for landing (like the LMDE) do--that's called deep throttling, and the LMDE got down to about 10% max thrust--and some modern first-stage engines do, to decrease G loads on the crew (called shallow throttling, i.e. down to 70% or so). The RS-25 Space Shuttle Main Engine (SSME) is an example of the latter.

2. Myth: A "gravity turn" means flying up to 10km and pitching over 45 degrees.

   Fact: In fact, a gravity turn is exactly what its name implies: a turn caused by gravity. To perform one, kick over to about 85 degrees pitch when you reach 100m/s (or slower, if you have higher than 1.5 liftoff TWR). Then don't touch the controls and, as the name implies, let gravity turn you. This is because every instant of time your engine is adding horizontal and vertical velocity, but gravity is only subtracting vertical velocity; this will tend to pull your velocity vector "down" over time, and the air will keep your rocket aligned with the velocity vector.

3. Myth: All fuels are created equal.

   Fact: In fact, rocket fuels are the subjects of books and hundreds of thousands of hours of research. Each type of fuel is carefully selected for it's strengths and compatibility with the mission profile. Kerosene-liquid oxygen (Kerolox), liquid hydrogen-liquid oxygen (Hydrolox), and hypergolics are the three most common fuel types for chemical rocket engines, and each have their advantages and disadvantages. Nuclear-Thermal Rockets (NTR) in most reference designs 'burn' liquid hydrogen, though are capable of using other fuels such as ammonia, methane, and water. Ignition! by John D. Clark is an entertaining informal history of the subject of rocket fuels.

4. Myth: Rocket engines are infinitely restartable

   Fact: In fact, restarting an engine is a tricky prospect and requires just the right conditions, and most engines only have a limited number of restarts. Issues such as zero-gravity causing fuel to float away from the engine further complicate the matter.

5. Myth: Gyroscopes are magical all-powerful devices.

   Fact:* In fact, attitude on spacecraft is often handled through the use of gimbaled thrust and reaction control thrusters. gyroscopes have limited ability to modify the attitude of a spacecraft, especially under thrust.