Space is very different from what we experience daily, so a bit of introduction is probably needed. If you already studied aerospace, this post will contain stuff you already know. But it will allow everyone to catch up, and we’ll have a reference to link when talking about space transportation concepts on this blog.
Cars move by rotating their wheels: that creates friction against the ground, and that makes them go forward. Boats and planes with propellers work a bit similarly, except they don’t push against the ground but against the surrounding fluid (water, air). Planes with reaction engines and rocket engines work differently: they throw something backward (hot gas), and that makes them go forward. The more mass and the faster we eject reaction mass, the more the spacecraft accelerates.
To measure how fast mass is ejected we use the “specific impulse” (Isp) metric. It gives you how efficient an engine is, like “liters per 100km” for cars. The bigger the specific impulse, the more you can accelerate with a given amount of fuel. Without going into equations, keep in mind that a higher Isp is often better, because we want to minimize the mass of the propellant (because it also has a mass we have to move with us).
There are two kind of distances in space. Sort of… At least, that’s how I like to think of it.
The first kind distance is obvious: it is how much physical space separates two things. For instance, the Moon is at 384,000km from the Earth. This physical distance is important because it gives you a clue about how much time it will take to reach something. Space is big so even if we go really fast, the distances are so large that it can take months (years!) to reach a desired target.
But there is another kind of “distance”, and it is the most relevant when transporting things in space. The ISS’s orbit is 400km above our heads, yet it is much more difficult to go there than to go from Toulouse to Paris (a 600km trip). This is because the ISS is moving very fast: more than 7500m/s (22 times the speed of sound)! The difference of velocity (Delta-V, DV) measures how much one needs to accelerate to go as fast as the target. Once you go as fast as your target (and in the same direction), your relative motion is 0, so you see your target fixed, and you can interact with it. Like when you are chatting with your friend during a jog session: you are both moving, but in the same direction, so you can talk even though you are moving.
There is a chart I love that maps the Delta-V between bodies of the solar system:
For our purposes on this blog, that will be mostly about the Earth, Moon, Mars, and Near-Earth Asteroids, this chart provides more details:
There are more exotic ways to move around, but we’ll see that in due time!
Conceptually, a rocket is made of 3 parts:
- The payload: that’s the useful thing you want to transport (satellites, humans in a pressurized module, …).
- The propellant: that is the thing you want to throw backwards in order to accelerate.
- The dry mass
What we call dry mass is the rocket’s parts that are not useful and that we can’t throw backwards. Dry mass is bad but it is needed: the rocket structure, the propellant tanks, the engine and nozzle, the on-board computers, …
Certain fuel combinations have a high specific impulse, like LOX/LH2 (liquid oxygen & hydrogen), but require big tanks and insulation because they need to be stored very cold (hydrogen evaporates at -253°C). There is no magic formula for designing a rocket: some have better Isp but higher dry mass, others have lower dry mass but need to carry more fuel because their fuel has a lower Isp, like LOX/CH4 (liquid oxygen & methane – methane evaporates at -161°C).
By the way, notice we must carry the oxygen to burn the fuel, because there is no oxygen in space, unlike planes that burn kerosene with ambient air. If you’re wondering how we can keep propellant so cold for so long, consider this: there are boats carrying liquid natural gas (that is essentially methane) sailing for multiple weeks!
Useful mass (payload)
Let’s say you have a rocket. You know where you want to go (the Delta-V), how efficient your engine is (the Isp), and the dry mass of your rocket. How much payload can you take with you? This is what the rocket equation is about. It relates the initial mass (payload + dry mass + propellant mass) to the final mass (payload + dry mass, no more propellant), using the Delta-V and Isp.
Imagine you have a rocket of 100 tons. The rocket equation is exponential, so if you want to accelerate 1000m/s, maybe you can carry 70 tons of payload. But if you want to accelerate 7000m/s, you can only carry 6 tons of payload.
The payload mass fraction when launching from Earth to LEO (Low Earth Orbit) is around 1/30th (it changes for every rocket). That means, to put 100 tons in LEO, you need a 3000t rocket on the ground.
This is why rockets are so big (it’s hard to realize when only watching videos). A French high-speed train locomotive weights around 400 tons. Can you imagine how much mass needs to be thrown backwards in order to move?
This notion of payload mass fraction is important to study the space economy: depending on where a resource is produced and where it is demanded, there can be big differences in price. Most of the cost of something is due to its transportation.
Such a sophisticated word, I love it.
Having 100% of payload mass fraction is theoretically impossible, but we can get close. It would mean that the payload is alone, that there is no rocket attached to it. Concepts like “catapults” have been proposed, where a rail or sling system launches the payload towards its destination, where it is catched. We’ll have to talk about this futuristic possibility in a dedicated blog post, because that would change a lot of things.
For trajectories arriving at Earth or Mars (or anywhere with an atmosphere – not the Moon), we can use aerobraking to lose some speed. That reduces the Delta-V performed with engines, so it improves the payload mass fraction (it goes up). The spacecraft hits the thin upper atmosphere, which creates friction, like a parachute, but less intense.
This process converts some kinetic energy (speed) into thermal energy (heat): the spacecraft slows down but heats up. The more extreme example of aerobraking is for Earth-return capsules: they are not equipped with rocket engines and are only based on thermal protection shielding and parachutes to slow down. It’s obviously a trick that can only work one-way. In practice it requires a bit of additional mass for structure & thermal handling. It also takes a lot of time, because we want only a bit of friction on every orbit to not stress the spacecraft too much.
As an example, ESA’s TGO mission aerobraking saved around 1km/s of Delta-V and took about a year.