Look here, this rocket is not ordinary!

Produced by: Science Popularization China

Author: Hanmu Diaomeng (popular science creator)

Producer: China Science Expo

For a space power like my country, many rockets are successfully launched every year, which has become "common practice" in my country's space sector.

However, the successful launch of the Suzaku-2 Yao-2 rocket on July 12, 2023, has caused heated discussions. Why is this?

The Suzaku-2 Yao-2 carrier rocket was launched from the Jiuquan Satellite Launch Center in my country

(Photo source: Xinhua News Agency)

Liquid oxygen and methane rockets: a new attempt by various countries in space exploration

It turns out that this is the world's first liquid oxygen-methane rocket that successfully sends a payload into its intended orbit.

China is not the only country developing liquid oxygen-methane rockets. Other major countries are also developing them intensively. For example, in the first half of this year, two companies in the United States launched two liquid oxygen-methane rockets, namely the Terran-1 rocket and the Starship rocket.

Terran-1 liquid oxygen-methane launch vehicle

(Photo source: Guangming Online)

However, the launches of both rockets by the United States were unsuccessful.

At this point, you may have this question: Since we already have liquid oxygen-kerosene and liquid oxygen-liquid hydrogen rocket engines, why are both China and the United States now rushing to develop liquid oxygen-methane rocket engines?

If the LOXM engine is so good, why wasn't it developed in the first place? Or decades ago, before developing the LOX/LH rocket engine, why wasn't the LOXM engine developed first?

What are the advantages of liquid oxygen-kerosene over liquid oxygen and liquid hydrogen?

During World War II, Germany's V-2 rocket used liquid oxygen ethanol as propellant. Ethanol is what we commonly call alcohol.

As a fuel, alcohol containing water obviously does not perform well. After World War II, various countries began to develop new liquid propellants.

Thus, the two routes of liquid oxygen and kerosene and liquid oxygen and liquid hydrogen were born, and both routes have been successful so far.

A refined kerosene "RP-1", rocket fuel

(Image source: Maxhaot)

The reason why kerosene was chosen instead of methane at the beginning was that kerosene was already very common and used in large quantities at that time . The large-scale use of natural gas around the world has only occurred in recent decades.

In terms of cost , kerosene is the inevitable choice over methane. Even if history were to start over again, scientists would still choose kerosene over liquid oxygen and methane, which require higher technology and are more expensive to obtain.

Therefore, since World War II, both the liquid oxygen-kerosene and liquid oxygen-liquid hydrogen routes have been fully developed. After exploring two suitable routes, people no longer need to look for other routes.

Liquid oxygen and methane: making rocket recovery easier

Since liquid oxygen-kerosene has so many advantages over liquid oxygen and liquid hydrogen, why are people still pursuing the liquid oxygen-methane route?

Because human requirements for rocket engines have increased significantly.

Before, scientists only had to ensure that a rocket engine could be used once. After a successful use, it didn’t matter what happened to the rocket engine; it would just fall to the ground or into the sea anyway.

But now, humans hope that rocket engines can be reused more than 10 times, or even 100 times. At this time, liquid oxygen-kerosene engines are a little unsuitable, because when kerosene burns, it will cause carbon deposition inside the rocket engine , which will bring great difficulties to reuse.

Currently, liquid oxygen-kerosene rockets must have their engines thoroughly cleaned after recovery before they can continue to be used.

Use a car engine to demonstrate carbon deposits. Of course, the carbon deposits in a rocket engine cannot be that serious.

(Photo credit: KINGKAR)

Liquid oxygen and methane are different. They are highly volatile fuels . After use, the residue of this fuel in the rocket engine is extremely rare , which greatly reduces the logistical maintenance workload for reusable rockets.

When a rocket engine is working, the maximum gas temperature can exceed 3300 degrees Celsius, which exceeds the melting point of most metal materials.

In order to avoid damage to the engine due to high temperature, the rocket engine needs to be cooled. There are many cooling methods, and the current mainstream method is "regenerative cooling" , which is to use fuel cooling.

The temperature of liquid oxygen is around minus 183 degrees Celsius, which seems to be ultra-low temperature and is most suitable as a coolant. However, due to the high activity of oxygen, it easily reacts with metals or other materials that make up rocket engines .

Therefore, people generally do not use liquid oxygen as a coolant, but use fuel as a coolant. Specifically, part of the fuel first goes back and forth around the engine nozzle, which will inevitably take away a huge amount of heat, thus playing a role in cooling the rocket engine.

"Tian Que" 80-ton liquid oxygen-methane engine

(Photo source: People's Daily Online)

Compared with room temperature kerosene and liquid methane, the latter is definitely better because its temperature is around minus 162 degrees Celsius, and the cooling effect is immediately increased by more than 3 times.

That is to say, in order to make the rocket reusable, it needs to be cooled with the highest efficiency, and in terms of cooling, liquid oxygen and methane are much better than liquid oxygen and kerosene.

Liquid oxygen and liquid hydrogen: Why not choose me?

Through the above analysis, we know that liquid oxygen and kerosene are prone to carbon deposition and coking, while liquid oxygen and methane are not. But the question is, liquid oxygen and liquid hydrogen will not deposit carbon and coke either, so why not use liquid oxygen and liquid hydrogen? Moreover, during regenerative cooling, the temperature of liquid hydrogen is lower, so wouldn’t the cooling effect be better than that of liquid methane?

Yes, the above statements are all reasonable, but liquid hydrogen actually has many difficult to overcome problems.

One problem is that compared with liquid oxygen and methane, the density of liquid hydrogen is too low, resulting in a large volume . Another problem is that the temperature of liquid hydrogen is too low, minus 252.8 degrees Celsius, and it is too difficult to handle such a low temperature, which in turn is very expensive.

The big brown box under the space shuttle is called the external tank. Most of the volume inside is liquid hydrogen.

(Image credit: NASA)

Moreover, the temperature of liquid oxygen is only about minus 183 degrees Celsius, while that of liquid hydrogen is minus 252.8 degrees Celsius, a temperature difference of about 70 degrees Celsius. Therefore, the two boxes storing liquid oxygen and liquid hydrogen on the rocket need to be well isolated, otherwise the liquid hydrogen may freeze the liquid oxygen into a solid .

If liquid oxygen and kerosene are used, isolation is also required, otherwise the liquid oxygen will freeze the kerosene into a solid.

At this point, the superiority of liquid oxygen and methane can be reflected, because its temperature is similar to that of liquid oxygen, so there is no need to worry about one freezing the other into a solid.

So, originally there were two separate boxes, one for liquid oxygen and one for kerosene. But now it can be made into one big box, with an extra partition in the middle of the big box, one side for liquid oxygen and the other for liquid methane. This structure is called a "common bottom storage tank."

There is no doubt that the common bottom tank can effectively reduce the weight of the tank and shorten the length of the tank, thereby reducing the weight of the rocket and enhancing the carrying capacity.

At this point, the mystery is all solved. Compared with liquid oxygen-kerosene and liquid oxygen-liquid hydrogen, liquid oxygen-methane has many advantages. As the requirements of various countries for rockets are increasing, the development of liquid oxygen-methane engines has gradually become a major focus in the field of aerospace research.