Musk's "Starship" launch failed, but the engine "Raptor" is still epoch-making

On April 20, 2023, the "Starship" super-heavy rocket developed by Musk's Space Exploration Technologies Corporation (SpaceX) disintegrated unexpectedly three minutes after launch and exploded at high altitude. Musk said that the company would sum up its experience and try to launch again in a few months. Although the "Starship" launch failed, the engine it used, the "Raptor", still received widespread attention and was hailed as a groundbreaking product.

The Starship system launched this time consists of two parts, the super-heavy booster below and the Starship spacecraft above. There are 33 engines on the super-heavy booster and 6 on the Starship spacecraft above. These 39 engines are all Raptors, the only difference is that the three engines on the spacecraft have slightly larger nozzles to make them more suitable for working in a vacuum environment.

The Raptor engine is a liquid oxygen-methane engine, while the mainstream engines on the market are liquid hydrogen-liquid oxygen and liquid oxygen-kerosene. For example, the core stage of the Long March 5 rocket that we use to launch the space station uses liquid hydrogen-liquid oxygen engines, while the booster uses liquid oxygen-kerosene. The Merlin engine used in Musk's highly successful Falcon 9 rocket is also liquid oxygen-kerosene. Since the Falcon 9 is so successful, why does Musk want to open a new technology tree and use methane as a fuel?

We can compare these fuels. Liquid hydrogen and liquid oxygen are the most efficient rocket propellants, but the design is also relatively difficult, mainly because hydrogen molecules are very small, and it is easy to leak in structures such as pumps. They are also extremely flammable and explosive, and have a high chance of exploding on the spot. Liquid hydrogen has a very low temperature and is difficult to handle. Its boiling point is minus 252 degrees, which is not far from absolute zero and lower than the freezing point of liquid oxygen. In other words, if the liquid hydrogen tank and the liquid oxygen tank are placed very close together, the liquid oxygen will be frozen. In addition, the density of hydrogen is very low, which means that I need a very large tank to hold it, so the rocket has to be thick and large.

The efficiency of liquid oxygen-kerosene is not as good as that of liquid hydrogen-liquid oxygen, but the design difficulty is lower and the price of fuel is much lower. However, kerosene is easy to coke, which is similar to the carbon deposits when we drive. If it is used once, it is not a big problem, but if it is to be recycled, it is more troublesome. If a more complex and efficient closed cycle is adopted, it will be more difficult to clean it after recycling. Therefore, the Merlin engine on the SpaceX Falcon 9 still uses the most primitive gas generator cycle, that is, the open cycle.

We can see that there is a small tube next to its nozzle, which is where the gas generator exhausts. The performance of methane is between kerosene and liquid hydrogen, and each performance requires a golden mean. Although liquid methane also needs to be stored at low temperatures, it is not as convenient as kerosene, but its storage temperature is similar to that of liquid oxygen. The two tanks can be placed next to each other and separated by a wall. This is called a common bottom tank, and the structural weight of the rocket can be reduced (liquid oxygen and kerosene rockets now also use a common bottom tank, with thermal insulation foam in the middle).

For Musk, the most attractive thing is that methane does not coke, which is particularly suitable for reuse. It can be ignited again soon without cleaning after burning. Musk said that if you want to make rockets cheap, complete and rapid recovery is very important. The so-called complete recovery means that nothing is thrown away except fuel when launching a rocket, and everything can be reused, which will definitely save money. The so-called rapid recovery is like our current civil airliners, which will fly away again immediately after landing for one or two hours, because rockets, like civil airliners, make money when flying in the sky, and spend money when they are parked on the ground for maintenance. The shorter the time they stay on the ground and the longer they fly in the sky, the lower their operating costs will be. Therefore, engines like liquid oxygen-methane engines that are easy to maintain and can be refueled after flying once are very consistent with Ma's expectations. Of course, he has a more unrealistic reason, that is, he wants to colonize Mars in the future. There is no kerosene on Mars, but there is water and carbon dioxide, which can be used to make methane, so he may have to open a methane plant on Mars in the future.

The picture comes from Tuchong.com

Of course, although the use of methane itself is relatively new, it is not unique to the scorpion. What is truly unique to the Raptor engine is the engine cycle mode. What is the engine cycle mode? Simply put, it is how to supply fuel to yourself. You know, the fuel flow required by the rocket engine is extremely large. Take the RS25 engine on the space shuttle and SLS as an example. Its fuel pump can drain a standard swimming pool in just over three minutes. Who will drive such a large flow pump? The answer is its own fuel. The simplest gas generator cycle, that is, the start of the cycle, is to guide a part of the fuel and oxidizer out separately, light them, let them drive the turbine to rotate, and then drive the fuel pump to pump the fuel into the main combustion chamber. The part that burns separately in front is the gas generator. The main purpose of the fuel and oxidizer here is to drive the turbine. After pushing, it is discharged. We can observe our Chang Er F engine, the engine of the American Saturn V, and the engine of the Falcon 9. There is a small pipe next to the rocket nozzle, which discharges the gas in the gas generator. These discharged gases cannot generate thrust efficiently, so there is some waste.

In order to make the engine more efficient, a staged combustion cycle, also called a high-pressure regenerative combustion cycle, is introduced. This is equivalent to sending the gas that has been burned once and pushed through the turbine into the combustion chamber to burn again. It is called staged combustion because it burns twice, and it wastes less energy. The Long March 5 uses an oxygen-rich staged combustion cycle, which means that there is a lot of oxygen in the pre-combustion chamber, and then these gases carrying a lot of oxygen push the turbine pump and then enter the main combustion chamber to mix with the kerosene sprayed in and burn again. The one used on the space shuttle and SLS is called a regenerative staged combustion cycle, which means that there is more fuel in its pre-combustion chamber, and then these gases carrying a lot of fuel are sent to the main combustion chamber to mix with oxygen and burn again.

You may be confused when you hear this. Either oxygen-rich combustion or fuel-rich combustion, there must be more of one component. Why can't it be just right? Because the gas is used to drive the turbine. If its combustion ratio is just right, the combustion efficiency will be too high, and the temperature will rise along with the combustion efficiency, and the turbine will melt. Therefore, the gas that drives the pump must have a lot of oxidant or a lot of fuel.

Let's go back to the Raptor. It also has a staged combustion cycle, but it has three more words in front of it: full flow. What does full flow staged combustion cycle mean? It combines the oxygen-rich and fuel-rich that I mentioned earlier. The pre-combustion chamber that pushes the liquid oxygen is oxygen-rich, and the pre-combustion chamber that pushes the fuel is fuel-rich. In this way, the main combustion chamber burns a mixture of oxygen-rich gas and fuel-rich gas. Gas plus gas, the best mixing, more complete combustion, and because the fuel-rich gas pushes the fuel pump and the oxygen-rich gas pushes the liquid oxygen pump, even if there is a leak, the fuel-rich gas leaking into the fuel will not further burn and explode. You can imagine that it already has too much fuel. Similarly, the oxygen-rich gas leaking into the oxygen will not burn and explode. Therefore, the sealing requirements of this pump are not as abnormal as those of the RS25 of the space shuttle. It is also convenient for maintenance.

In history, there have only been three types of engines that used this cycle. The first two failed and never went into space. If Starship is successful, the Raptor will be the world's first engine that successfully sends a rocket into space using a full-flow staged combustion cycle. This is why I said at the beginning that Starship's Raptor is an epoch-making engine.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Gou Sheng (Lecturer at Xi'an Aviation University)

Reviewer: Zhou Binghong (Researcher at the National Space Science Center, Chinese Academy of Sciences)

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.