Some minor performance tips

C

ClockworkOwl

Guest
#1
I've seen several play videos and think I can provide some useful tips that could improve mission performances.

Rule 0: Go as light as possible.
A rocket changes its velocity by throwing something(propellant) behind it at fast speed. Heavier the rocket is, less velocity changed per propellant mass expended. And the best part? Your propellant has mass and it makes the rocket heavier. This means if you want to get something out of the earth, you need propellant to get it out of the earth, and propellant to get that propellant out of the earth, and propellant to... you get it. And no, you can't solve it by throwing more propellant at it because each batch of propellant comes with tanks to contain it, engines to use it and stuff. This results in diminishing return - you might have noticed when you strapped more rockets on your previous rocket(which worked well) and it can't really go as far as you expected - and took more time leaving the Earth atmosphere.
All of these means you want your dry mass(non-propellant mass) as small as possible. In real life, this includes using bigger tanks whenever possible(due to square-cube law, bigger tanks have more propellant per mass of tank wall), but all tanks in SFS has 9:1 ratio so it doesn't apply.
Except the payload - which is the whole point of the launch. Yes we do make them as light as possible IRL, but heavier ones provide more challenge here.

Detach the stages as soon as it runs out of fuel.
This one is fairly well-known and is practiced well - but I've seen some designs that has multiple engines per stage(that does not share their propellants) with differing lifetime, resulting in having to drag the shorter-lived ones all the way up untill the final rocket runs dry.

Detach the fairing as soon as you leave the atmosphere.
The sole purpose of the fairing is to make your payload aerodynamic during the launch stage. After you're in the space, it does nothing except being more dead weight.

Aerodynamic stuff stays in atmosphere.
Same as last one, you'll want aerodynamic caps on your side rockets since it helps the rocket punch through the atmosphere better. You won't want it, however, as soon as you leave the atmosphere because it no longer does anything but making your rocket heavier. Keep your side rockets small so that you can ditch them as soon as possible(preferrably before/shortly after leaving the Earth atmosphere). You can make it detachable but the mechanism happens to have same or more mass than the cap, so probably won't be worth it.

Rule 1. You have time. Go patiently and efficiently.
Space missions can take years. Well, most of them - we can't exactly do that with manned missions unless we can keep it supplied like ISS. In SFS, unless it is a part of a challenge, your mission time is practically unlimited. Your propellant, however, is not. This means you'll want to spend more time if it means spending less propellant.

Efficiency rules.
High-thrust engines tend to use propellant inefficiently. But without enough thrust, you can't get out of the Earths' gravity well to begin with(and if you strap several efficient engines together to match the thrust, it will weigh more, being actually more inefficient!). As explained in tooltip, use high-thrust engines for your first stage, which will be used to lift your rockets off the Earth. For the rest of the missions, avoid the word "inefficient" like plague. You can check each engines efficiency, called Specific Impulse(Isp) on the tooltip.

But is it really efficient?
Grasshopper L.F. engine under the Utility tab is a weird thing. It has very weak thrust, and also terrible efficiency. It's worse than RB-48(biggest non-DLC engine), and only slightly better than the Big Falcon engine(biggest DLC engine with worst efficiency)!
So is this useless? Let's run the numbers to actually check it. Efficiency is important, but not everything. The ultimate variable that determines how far a rocket can go is called Delta-V, change(delta) in velocity(v). If you can't change your velocity, you will be still moving in your orbit, but can't go anywhere else and is effectively stranded.
You can calculate delta-V with Tsiolkovsky Rocket Equation. I won't actually write it down here, but it says that the delta-v is proportional to specific impulse, and also proportional to natural log of (total mass divided by dry mass), and nothing else.
So, let's assume a simple lander that consists of a probe, a parachute, two landing legs, the smallest fuel tank, and an engine. Without engine, this payload weighs 4.1 tons including 2.25 tons of propellant. If we use Broadsword L.F(the most efficient non-DLC engine), that brings our total mass to 5.3 tons. ln(5.3/3.05) * 280(specific impulse) equals 154.71. If we multiply this by standard gravity(9.8 something) we get actual delta-V, but for purpose of comparison, this is unneeded. With Grasshopper, total mass becomes 4.45 tons, because it weighs mere 0.35 tons. Running the numbers again, we get... ln(4.45/2.5) * 244 = 171.88! It actually outperforms Broadsword in lighter crafts(and also weighs less) so there is reason to use this.

Know how to fly
Orbital maneuver is complex and sometimes unintuitive. For example, to catch up with something that is ahead of you in the same orbit, you actually want to slow down! As such, it is important to know how to manipulate your orbit in space in order to carry out your mission effectively.
We can't reliably pull off complex maneuver like gravitational slingshot since we have to do everything realtime and can't plan ahead, but something like Hohmann transfer is intuitive enough that you can use it without even learning about it. There's an entire post worth of stuff regarding orbital maneuvers. Something like powered flyby is likely not modeled, but it is worth taking a look at basic stuff if you're interested in it.