After completing the 9-meter high jump test, what is the origin of this "gas tank"?

Recently, the US Stock Company successfully conducted a jump test on the reusable rocket upper stage. During the 15-second flight test, the rocket's second-stage prototype Hopper2 rose to a height of about 9 meters and then landed 4.5 meters from the launch pad. So, what is a jump test? What novel designs does this rocket use? What are its future prospects?

Jump test should not be underestimated

The hop test is a vertical take-off and landing test, the purpose of which is to demonstrate the ability to control the rocket during ascent and descent and to make a soft landing. This is a very difficult guidance, navigation and control problem, especially for the new distributed thruster system of the rocket's second-stage prototype developed by Stocker.

The moment the rocket's second-stage prototype takes off

The significance of the jump test is significant. The hundred-meter and kilometer-level vertical recovery flight tests can verify that the engine's state is different from that during the ground test, and assess the stability of the engine in a real flight environment. The jump test is more authentic and has practical assessment significance in engineering development.

At the same time, the jump test can also verify the lateral guidance algorithm: add the lateral guidance algorithm during the test, pre-set the target point coordinates, calculate and plan through the onboard computer, issue lateral guidance instructions, guide the rocket to move horizontally to the target coordinates, and carry out the rocket's off-site return recovery technology verification. This can provide early technical verification for the off-site return recovery and reuse of future orbital rockets, and even intercontinental travel.

The jump test can also further verify the aerodynamic problems of the rocket body during the vertical recovery of the rocket, as well as key core technologies such as long-term adaptability to thermal and vibration environments, long-term and wide-range variable thrust working capabilities of the engine, reusable rocket body structure and landing cushioning device, lateral guidance and attitude control capabilities, laying a solid foundation for conducting subsequent research.

The vertical take-off and landing technology route has now become the absolute mainstream. There are 28 types of reusable space launch systems under research, in service, and in the conceptual design stage in the world, of which 19 are vertical take-off and landing types, accounting for 68% of the total. At present, Stock is the fastest company from raising funds to demonstrating jump test technology. It has become the second company in the world to test a fully reusable rocket upper stage prototype and the third company in the United States to test a liquid hydrogen and liquid oxygen rocket engine. This test marks an important step for Stock to develop a fully reusable launch vehicle.

Novel design creates fully reusable rocket

As we all know, SpaceX has used the vertical take-off and landing technology of the Falcon 9 rocket to demonstrate the launch and recovery of the first stage of the rocket, while Stocker has innovatively proposed to build a fully reusable rocket. For the second stage of the rocket, the company mainly proposed three novel designs.

The first is a brand-new thruster design. The second stage of a traditional rocket generally adopts a single-engine structure, and the engine has a large bell-shaped nozzle. The purpose of the structure is to optimize the flow of engine exhaust in a vacuum. However, the disadvantage of this structural design is that the extended nozzle is often quite fragile, making it very difficult to protect the engine nozzle when the rocket re-enters. Stock uses a ring composed of 30 smaller thrusters as a propulsion system. In a vacuum, the plumes from these nozzles merge into a whole to work. The use of partial thruster ignition during the rocket re-entry process makes it easier to protect the nozzle, and the structural design can better meet the needs of vertical landing.

The second is to try a regenerative cooling heat shield. During the reentry of the second stage of the rocket, the entire spacecraft needs to be protected from the overheated atmosphere. In response to this problem, Stock said that "trying a regenerative cooling heat shield is the most meaningful." During the reentry of the rocket, the ductile metal outer layer of the spacecraft will be lined with small cavities to keep the propellant cool through the material. The second stage of the rocket is like a space capsule using a regenerative cooling heat shield during the reentry process.

The third is a new first-stage rocket engine. Stock said that engineers have begun to research and develop full-flow, staged combustion rocket engines for the first stage of the rocket, seven of which will provide power for the booster. Currently, component testing of these engines is underway.

However, unlike the first-stage reuse and recovery, Stock's development of a fully reusable rocket still has a long way to go. At present, there are still a series of technical difficulties in the development work.

The first is how to ensure that the second-stage rocket engine can be used normally in both atmosphere and vacuum. The two stages of this rocket will use the same type of liquid hydrogen and liquid oxygen engine, but the second-stage rocket works in a vacuum during the launch phase and in the atmosphere during the return phase. When the nozzle works in these two different environments, the area ratio of the rocket engine (the ratio of the nozzle outlet to the throat area) is different, so it is necessary to ensure that the engine can be used normally in both environments.

The second issue is how to consume the huge kinetic energy when the rocket returns. After the second-stage rocket sends the payload into orbit, it will return to the earth at the first cosmic speed. How to consume such a large amount of kinetic energy to slow down and land the several tons of precision equipment safely is a technical problem. Stock plans to recover the second-stage rocket by ballistic reentry and powered landing, but this method requires carrying enough fuel and will increase the size and mass of the second-stage rocket, thus affecting the rocket's carrying capacity, so the feasibility needs to be demonstrated.

The third is how to ensure the sealing of the fairing under repeated use conditions. When the second-stage rocket and the fairing re-enter the atmosphere, severe aerodynamic heating will occur. If there is a problem with the sealing of the second stage of the rocket, the high temperature during re-entry will be extremely destructive to the rocket, so a good thermal protection layer is needed to protect the rocket. How to ensure the sealing of the seams at the fairing and achieve multiple repeated uses is a problem that Stoke needs to solve further.

Watch the rocket's second-stage prototype take off from another angle

A strong competitor in commercial spaceflight

Stocker aims to design and build a 100% reusable rocket with a 24-hour turnaround time that can deliver up to seven tons of cargo to low Earth orbit. The rocket is scheduled to fly for the first time in 2025.

The rocket developed by Stocker is divided into two stages. The recovery of the first-stage rocket is similar to that of the Falcon 9 rocket. The second-stage rocket is connected to the fairing. After entering orbit, the fairing unfolds like a petal to release the load. After closing, it lands with the second-stage rocket through rocket reverse thrust, making the entire rocket reusable.

A reusable system is bound to be more complex, and compared to a one-time launch, it needs to carry more fuel to meet the need for re-entry deceleration, which has an impact on the carrying capacity. However, if this technology matures, Stocker will be able to reuse the first and second stages of the rocket and the fairing, and increase the launch frequency to once a day, providing reusable rocket delivery services in low-Earth orbit or even farther, reducing launch costs and shortening the launch turnaround cycle.

In the future, commercial rockets will need to increase their launch frequency and reduce launch costs. There is a foreseeable huge demand for commercial reusable rockets around the world. As the only successful, mature, and large-scale reusable rocket currently verified in actual engineering, SpaceX's Falcon 9 rocket has been successfully recovered more than 200 times. Judging from its launch data, reusable rockets can bring huge economic benefits.

SpaceX is now able to recycle and reuse the first-stage booster and fairing of the rocket. According to Musk himself, the reuse rate of the Falcon 9 rocket has reached 80%. This means that during launch, only the cost of first-stage maintenance, second-stage manufacturing and fuel is required, which directly reduces the cost by 70%.

For the fully reusable rocket that Stocker plans to build, the cost will only include regular maintenance and fuel consumption. This rocket can further reduce the cost of putting payloads into orbit, becoming a powerful promoter and competitor of the space transportation industry. (Author: Zhang Tianliang, Wang Zhaolei, Image source: Stocker, gatekeeper expert: Jiang Fan, deputy director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)