There are many types of fuel you can load onto your rockets in SimpleRockets 2, each has different properties and characteristics that make them unique in their usefulness. Understanding the different types of rocket fuels and how they work will help you to build better rockets by choosing the right fuel for the job!
What is liquid rocket fuel?
Pretty much any rocket that you could name off the top of your head uses liquid propellant. The propellant is made up of two separate liquids, the fuel, and the oxidizer. These two chemicals are required for a combustion chemical reaction. In a combustion reaction, a fuel and oxidizer combine and form water, carbon dioxide, and energy. This energy is used to propel the rocket. The oxidizer and fuel are stored in separate tanks as liquids, then are pumped to the combustion chamber where they are mixed and ignited.
Kerolox
Kerolox, or kerosene and liquid oxygen, is one of the most widely used fuels in the history of rockets. Also known as RP-1, is essentially a highly refined jet fuel, mixed with cryogenic liquid oxygen. It’s the least efficient of the liquid propellants, largely because the hydrocarbons that make up all petroleum based fuels, whether its diesel, gasoline, or RP-1, don’t burn perfectly clean, meaning there are unburnable particles that are being expelled with the rest of the fuel. Its these carbon atoms that give kerolox its yellow exhaust color, from the black body radiation emitted from the glowing particles of soot in the exhaust. These soot particles don't add to the energy of the exhaust, making it less efficient and slower. Essentially you want as high of an exhaust velocity as possible, which is going to give you a highly efficient engine. Kerolox on this stock Mage Engine with a bell nozzle has a thrust of 265 kilonewtons, an ISP of 253 seconds, an exhaust velocity of 2321 meters per second, and on this particular rocket gives a total delta V of 3,937 meters per second.
Methalox
Methalox, which is what Starship’s Raptor engines run on, is a combination of liquid methane and liquid oxygen. Methane and oxygen are both gasses at room temperature so in order to keep them in their liquid state while inside the fuel tanks, they must be cryogenically cooled. This makes handling methalox a lot more complicated in the real world. Obviously in a game like simplerockets 2 you don’t have to deal with the engineering challenges of cooling and storing cryogenic fuels though, which is very nice. Methalox is a lighter, less dense fuel than kerolox, which means it takes a larger fuel tank to get the same delta V between rockets. Methalox has a blue hue when burned in simplerockets 2, in real life the blue color is far less noticeable and the exhaust is almost clear. Methalox burns significantly more cleanly than kerolox, because the chemicals in methalox are only methane (assuming no impurities) while kerolox has hundreds of different types and sizes of hydrocarbons. Because of this uniformity, it burns clear, with the slight blue tint coming mostly from the hydrogen atom’s emission spectrum. Loading up our mage engine with methalox fuel gives us the following performance: a engine thrust of 266 kilonewtons, 1 kilonewton higher than kerolox. An ISP 0f 262 seconds, significantly higher than kerolox, an exhaust velocity of 2400 meters per second, and a total delta v for this rocket of 3571 meters per second. Now you may be wondering, how does a fuel with a higher thrust, efficiency, and exhaust velocity have a lower delta v than kerolox? Well like I said earlier because its less dense it requires a larger fuel tank to get the same amount of delta v, and for this test we are keeping all of our dimensions for the rocket constant. If I had the same mass of methalox as I did of kerolox then you would see the higher efficiency lead to more delta v, for the same mass rocket.
Hydrolox
Rounding out our liquid propellants we have Hydrolox.. You can probably guess what hydrolox is at this point, if kerolox is kerosene and oxygen and methalox is methane and oxygen, hydrolox is… you guessed it, hydrogen and oxygen. Hydrolox is the most efficient of liquid propeelants by far, its chemical reaction is simpler than the other two, 2 liquid hydrogen molecules combine with 1 liquid oxygen molecule to form 2 water molecules. No waste, no other byproducts, just hydrogen and oxygen into water. This reaction with hydrogen and oxygen is extremely energetic. Far more energetic than burning hydrocarbons or methane. Hydrolox in simplerockets appears white, but in reality is almost completely clear, with a very subtle blue hue to it from the hydrogen atoms emissions. Hydrolox is also extremely light, since hydrogen atoms are the absolute smallest element on the periodic table there aren’t any fuels lighter than hydrogen. Looks at the performance analyzer, hydrolox engines have a thrust of 266kn, the same as the methalox engine, but they have an ISP of 334 seconds, this is way higher than all the other liquid rocket propellants. It has an exhaust velocity of 3058 meters second. On this rocket, the hydrolox fuel gives us a delta V of 2613 meters per second. but has almost ⅓ the propellant mass of methalox and almost ¼ the propellant mass of kerolox.
Here are all three of the l=different liquid propellants compared side by side.
Solid rockets
Solid Rocket fuel works the same as liquid rocket fuel on a chemical level. Fuel and oxidizer combines to form carbon dioxide, water, and energy, as well as other byproducts. The difference is that while liquid rockets use liquid fuel, solid rockets use… you guessed it, solid fuel. The oxidizer and fuel are premixed and held together with a binding agent. The propellant ends up being a somewhat rubbery material, but once ignited the combustion reaction can’t be stopped or throttled. Solid rocket fuel in simplerockets 2 has a bright white/yellow color and generates a ton of smoke. The smoke comes from the unburned particles of fuel and binding agent. Rocket fuel is far less efficient than liquid propellant, and the fuel itself is more expensive than liquid fuels, but comes with the trade off of producing a lot more thrust and uses a cheaper, simpler engine. The stock solid rocket engine in simplerockets 2 has a thrust of 2305 meters per second, 10 times greater than the liquid rockets. But it has a dismal 208 second ISP and a relatively slow exhaust velocity of 1850 meter per second. The solid rocket gives this rocket a delta v of 3538 meters per second, but is significantly heavier than even kerolox. Heres how our solid rocket compares against the liquid rockets.
Nuclear Rocket Fuels
Liquid Hydrogen
Nuclear rockets operate differently from the solid and liquid chemical rockets we’ve discussed so far. Nuclear rockets don’t rely on a combustion reaction to generate the energy used to accelerate the ship, but instead use the energy created by a nuclear reactor in the engine. The fuel doubles as reactor coolant and is passed over the reactor core, taking the heat energy from the reactor and accelerating out the nozzle. This system is way more efficient than regular chemical combustion, but comes at the cost of a heavy and expensive nuclear reactor core, which creates its own operational limitations in flight. You can’t just flip a switch and get to full throttle right away, that would cause a reactor meltdown and destroy your engine. Instead the nuclear rocket engine has to be throttled up gradually to avoid prompt supercriticality, and it has to be throttled down slowly to remove decay heat from the core until its cool enough to remove propellant flow all together. There are two options for fuel for nuclear rockets, pure liquid hydrogen and water. Liquid hydrogen is far lighter and because its so light it can be accelerated to much greater speeds when leaving the engine. Here’s how liquid hydrogen performs on our rocket, it gives us a thrust of 123 kilonewtons, which is about half that of the kerolox engine. But it has an unbelievably high ISP of 908 seconds, and an equally unbelievable exhaust velocity of 8412 meters per second. The same ship with a liquid hydrogen nuclear engine has a total delta v of 961 meters per second, which is way lower than every other fuel type we’ve talked about sofar, but since the fuel is pure liquid hydrogen its incredibly light, lighter than even hydrolox, which gets most of its weight from the liquid oxygen. The same tank that held several thousand kilograms of liquid propellants can only hold 613 kilograms of liquid hydrogen. So to have enough fuel to make a liquid hydrogen nuclear rocket usable you'd need a very large fuel tank.
Water
The other fuel type that can be used in nuclear rockets is also one of the most abundant molecules on earth, good ol H2O. Water has great thermal properties and is the most common coolant for nuclear reactors in general, so it makes sense that it could also be a good option for the fuel of a nuclear rocket. A water fueled nuclear rocket has the same thrust as a liquid hydrogen fueled one, of 123 kilonewtons. But because of the larger mass of water molecules compared to hydrogen molecules, the ISP is only 415 seconds, still significantly higher than a chemical rocket could achieve, but less than half that of a liquid hydrogen rocket. On the other hand, this increase in density means you don’t need as massive a tank to get higher delta v. On our reference rocket the water based nuclear rocket gives us 3897 meters per second of delta v, more than most of the rockets so far.
Here is how our two nuclear rocket fuels compare against the other fuels we’ve looked at so far.
Ion Engines: Xenon
There is one other rocket engine in simplerockets 2 we have yet to talk about, the ion engine. Ion engines use a radically different mode of propulsion than any rocket engine we’ve talked about so far. If comparing liquid and nuclear rockets is like comparing apples and oranges, then comparing any of those rockets to an ion engine is like comparing fruit to a crescent wrench. Ion engines operate on a whole different field of science, electrodynamics. Ion engines work by stripping atoms of their electrons, ionizing them. These positively charged atoms are drawn to the negatively charged plate at the end of the engine, accelerating to speeds of up to 50000 meters per second. This insanely high exhaust velocity gives ion thrusters hands down the best efficiency of any rocket engine technology invented yet, but it comes at the downsize power. Ion engines emit streams of atoms, at a rate of about half a gram per second. Compare that to the mass flow rates of our liquid rockets which burn fuel at a rate of around 100 kilograms per second and you see the issue. While its incredibly efficient, ion engines are incredibly slow. If we just powered our rocket with a single stock ion engine it would have a thrust of about 20 newtons. This comes with an ISP in the range of tens of millions of seconds. Which is so far off the charts that the game doesn’t even display it in the performance analyzer, this engine gives our ship a total delta v of 110,000 meters per second. But it would take over a year, roughly 11,000 hours of continuous burning to achieve that velocity. This severely limits their usefulness as a main propulsion system, but makes them ideal for small correction burns for space stations.
Hopefully these explanations of the different types of rocket fuels in game have helped you in your future missions!
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