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: 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 (assuming your rocket is aerodynamically stable) 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.

2. 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. 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. Other than that, engines do not throttle, and g-loads are managed by shutting down some engines early.

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 its 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, requires just the right conditions, and even then 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: Rocket engines and fuel tanks are heavy.

   Fact: In fact, rocket engines have very, very high thrust to weight ratios (maxing out over 150:1). As for fuel tanks, perhaps the rocket stage with the highest fuel fraction was the Atlas D sustainer: loaded mass 113 tonnes, dry mass 2.347 tons. That includes not just the tanks' dry masses, but the engine, guidance, pressurant, and everything else a rocket stage needs. This only applies to liquid engines and tanks in KSP, however: KSP's solid rockets have more or less reasonable dry masses (a bit high, but well within the range) and its nuclear engine has a spot-on TWR; it's really only the conventional liquid engines and their tanks that underperform so horribly (too heavy by a factor of 3 to 8).

6. Myth: Reaction wheels 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. Reaction wheels 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.

7. Myth: Orbital rendezvous is easy and can be done from almost any starting orbit.

   Fact: In fact, orbital rendezvous is quite difficult to get correct, especially when starting from a non-equatorial launch site. While a rocket can indeed carry enough propellant to make KSP-style rendezvous from wildly different (more than 1*!) orbital inclinations and planes, this requires not putting as much mass to orbit as possible with the given launch vehicle. This is not done because each launch costs many millions of Dollars/Rubles/Euros, so launch providers and their clients want to use payload mass as efficiently as possible.
   To do so, you must first wait until the target orbit is over the launch site and launch into the same plane as the target. In reality, a small amount of reserve delta-vee (dV) can be used near the beginning of launch to turn a 'dogleg' to move the ascent path into the correct plane. The Space Shuttle had enough excess capacity to launch in a ten-minute window and still ascend to the target orbit. This is done early in the ascent when steering losses are low due to the low downrange velocity.
   Secondly, the ascending payload is usually launched into a slightly lower orbit slightly behind the target spacecraft. This is called a 'chaser orbit.' This is again, done for efficiency reasons, as it takes less dV than ascending to a higher 'leader orbit.' The chaser vehicle then takes a number of orbits to catch up to the target, slowly adjusting its orbit to get a low-velocity approach, again for efficiency reasons. Low velocity approaches are also safer for both vehicles in the event of an abort. This is both easier and safer than a direct 'launch to rendezvous.'