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 propellants are created equal.

Fact: In fact, rocket propellants are the subjects of books and hundreds of thousands of hours of research. Each propellant mixture is carefully selected for it's strengths and compatibility with the mission profile. Kerosene-liquid oxygen (Kerolox), liquid hydrogen-liquid oxygen (Hydrolox), and storables are the three most common propellant mixtures for chemical rocket engines, and each have their advantages and disadvantages. Nuclear-Thermal Rockets (NTR) in most reference designs use liquid hydrogen as propellant, though are capable of using other propellants such as ammonia, methane, and water. Ignition! by John D. Clark is an entertaining informal history of the subject of rocket fuels. Just to put it in perspective, a modern kerolox mixture is about 1kg/liter and yields around 350s specific impulse in vacuum, whereas LH2 (as used by a NTR) is 1/14th as dense (0.07085kg/l) and may yield up to 1000s in a NTR. Hydrolox is only 1/2.84 as dense as kerolox, and provides up to around 460s specific impulse in vacuum. The advantage of hypergolic storables is that they do not 'boil off' since they are liquids at room temperature (storable) and ignite on contact with each other (hypergolic), as well as often being more dense (up to well over 2kg/l) but have much, much lower performance.

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 freefall causing propellants to float away from their feed lines complicate the matter. To solve this, LVs use small motors called "ullage motors" to settle the propellants before igniting their main engines. Spacecraft often use RCS for this purpose. Assuming propellants are settled, the engine must be able to ignite; most first stage engines have only one ignition (often provided externally), though some upper stage engines have multiple ignitions. Because of their simplicity--needing neither to ignite their propellants nor spin up a turbopump--pressure-fed hypergolic engines have effectively infinite ignitions. Indeed, that's all RCS is: sets of small hypergolic (or catalyzed monopropellant or cold-gas) pressure-fed engines.

5. Myth: Gyroscopes are magical all-powerful devices, you can turn a spacecraft on a dime with them.

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, and can only apply torque for so long since doing so spins them up. Eventually they have to be "spun down" with RCS. They are used for very fine, low-torque applications, like keeping ISS oriented correctly with respect to Earth or keeping a telescope oriented just so.

6. Myth: Rocket engines always produce the same thrust, but use varying amounts of propellant to do so.

Fact: In fact, rocket engine turbopumps don't magically spin faster at sea level. Isp is a measure of how much thrust one gets from a unit of propellant (think of it as "1 weight-unit of fuel will provide one weight-unit of thrust for this many seconds"); when air presses against the exhaust, the exhaust is less effective, and so a rocket engine produces less thrust for that unit of propellant. Thus a rocket engine will always expel propellant at the same rate, but the thrust (and thus the delta V) one gets from that load of propellant is dependent on Isp (which is dependent on ambient pressure). For this reason, one must be very careful to select an engine that will develop enough thrust at sea level; a 400kN engine with Isp 100/400 will only produce 100kN at sea level. If you are using MechJeb, open the Delta V stats window and click "All Stats" to show the SLT column; that is your Sea Level TWR. If you are using KER, switch to Atmosphere mode to see sea-level TWRs.