What is the limit of rocket reuse? The vertical recovery "posture" must be correct!

Reuse is an inevitable trend in the development of rockets

Speaking of "reuse", people often have a feeling of both familiarity and unfamiliarity. Speaking of "familiarity", the means of transportation used in people's lives, cars, ships, airplanes, which one is not reusable, which one is used once and then thrown away? Speaking of "unfamiliarity", in the past, it was really rare to hear about the reuse of launch vehicles, which were all disposable, and no one knew where they fell after launch. Why is the launch vehicle like this? We have to start with the origin of the launch vehicle, the famous rocket formula.

Falcon 9 rocket achieves soft landing of first stage

At the beginning of the last century, with the advancement of aviation technology, people gradually discovered that the atmosphere has boundaries and even estimated its thickness. Traditional aircraft, whether balloons relying on air buoyancy or airplanes relying on aerodynamic lift, cannot go beyond the atmosphere.

So is there a tool that allows humans to escape the atmosphere? Since the earth is a sphere in the universe, through the deduction of basic mechanics, we know that if an object's speed is fast enough, the centrifugal force generated by its movement around the earth can balance the gravity generated by the earth, and then it can escape the atmosphere and enter the orbit around the earth. This is the first cosmic speed of 7.8 kilometers per second that we are familiar with.

The key is how to make an object reach such a high speed. This is when the great father of rockets, Tsiolkovsky, came into play. He derived the rocket formula and proposed a feasible path to use a launch vehicle to send an object into orbit around the earth.

The core of the rocket formula is to convert fuel into the speed increment of an object. However, due to the characteristics of chemical propellants themselves, the vast majority of the rocket's takeoff mass must be the propellant mass. Currently, the fuel of a rocket usually accounts for 80% to 90% of its own weight. The rocket body we see can even be said to be a propellant tank. When the technical level was relatively low, even a disposable launch vehicle could not use such a light aircraft mass and load so much propellant to take off. After all, the mechanical environment experienced by a rocket flight is much worse than that of an ordinary aircraft.

The greatness of Tsiolkovsky lies in that he proposed the concept of multi-stage rockets only through mathematical deduction, laying the foundation for people to achieve space flight with relatively low technical levels. Because the use of multi-stage rockets can gradually discard some of the rocket sub-stages that have used up the propellant, thereby ensuring that the final payload can obtain sufficient speed increments.

The rocket concept diagram drawn by Tsiolkovsky

Liquid rockets generally only need two stages to enter low-Earth orbit, while solid rockets generally need three or even four stages. This is because the specific impulse of solid rocket propellants is lower. In order to achieve geosynchronous transfer orbit launches, the current mainstream rockets in the world generally use three or two-and-a-half-stage configurations. For example, the United States' Delta and Atlas, Russia's Angara and Proton, the European Space Agency's Ariane, and China's Long March 3A series of launch vehicles.

The requirement for reuse often means that more structural mass needs to be paid, such as the need to install landing recovery equipment, thicken and strengthen the structure to cope with repeated flight loads, etc. Under this situation, early launch vehicles must be multi-stage disposable launch vehicles.

However, with the advancement of technology, the structural mass of the rocket can become lighter and lighter, while the strength is getting higher and higher, so the evolution trend of the launch vehicle is determined, that is, from multi-stage to single-stage, from disposable to reusable, and from partially reusable to fully reusable.

At present, the Falcon 9 can be said to be the most advanced launch vehicle. There are two reasons for this: on the one hand, it is a rare two-stage tandem rocket on the market that can adapt to the mainstream geosynchronous transfer orbit (GTO) payload and does not use hydrogen and oxygen power. This shows that the Falcon 9 has reached a high level in terms of structural lightweight and engine comprehensive performance. On the other hand, it is because the Falcon 9 has achieved partial reuse, and the reuse rate has reached 80%.

Vertical recycling increases reuse capabilities

Since the technology of disposable launch vehicles matured, humans have begun to move towards the goal of reusable launch vehicles. To achieve this goal, humans have made many attempts and proposed various technical paths.

The Saturn I rocket was the earliest attempt by the United States to use the concept of reusable launch vehicles. In the 1960s, NASA tried to use paraglider wings to recover the first stage of the Saturn I rocket in an effort to achieve autonomous landing. However, due to the immature landing technology at the time, the plan only remained in the scaled-down test stage. In the 1950s, the Soviet Union also tried to recover a small meteorological rocket by parachuting, and succeeded. After a simple repair, the small rocket achieved a second flight.

In 1977, the U.S. space shuttle took off vertically and landed horizontally for the first time, and the two solid rocket boosters used by the space shuttle were decelerated by parachutes and splashed down on the sea surface, thus being recovered and reused. In the 21st century, SpaceX's Falcon series rockets and Blue Origin's New Shepard rocket both achieved rocket recovery and reuse through vertical launch and vertical recovery.

Space Shuttle Atlantis

In addition, people have also explored fully reusable single-stage orbital vehicles. In 1986, the United States proposed to develop the National Space Plane (NASP) powered by an air-breathing engine, but due to the high cost and technical difficulty of the project, it was forced to stop in 1994. In 1996, the United States decided to develop a scaled-down test vehicle X-33 of the rocket-powered single-stage orbital vehicle "Venturestar", but due to the high technical difficulty and the plan overdue, it was abandoned in 2001.

Reusable launch systems can be divided into three types according to the differences in take-off and landing methods.

The first method is horizontal takeoff/horizontal recovery.

The power form of the launch system of horizontal takeoff/horizontal recovery is generally air-breathing combined power. Air-breathing combined power can enable the optimal working mode under different flight altitudes and Mach numbers to achieve the best acceleration and cruising requirements. It can make full use of the oxygen in the atmosphere to reduce its own takeoff weight, becoming the most promising power system in the future. After the single-stage orbital space plane program of various countries with air-breathing power was aborted due to the high technical difficulty in the 1990s, all countries adopted a more pragmatic approach, first developing a more mature rocket power, and then developing a more technically difficult air-breathing combined power.

The second method is vertical takeoff/horizontal recovery. For rocket-powered two-stage orbital reusable rockets, vertical launch is adopted. During takeoff and flight, the main load is axial load. The structural design is simple. At the same time, vertical takeoff can quickly pass through the atmosphere with small aerodynamic drag loss. The aerodynamic shape of the wing-body combination can adopt a horizontal landing mode, using atmospheric drag to slow down before landing. However, the aerodynamic drag and aerodynamic heating during flight are greater than vertical landing, and heat protection measures need to be taken on the wings and fuselage. In addition, horizontal landing requires a longer runway for taxiing and deceleration. The technical difficulties in aerodynamics, control, thermal protection, etc. are relatively large, and the construction period of infrastructure such as landing runways is relatively long. The winged reusable carrier with horizontal return has excellent hypersonic flight capability and rapid response capability.

The typical representative of vertical takeoff/horizontal recovery is the US space shuttle. It is a spacecraft that transports people and payloads between the ground and low-Earth orbit. It has the functions of a manned spacecraft and a carrier, and lands like an airplane. Because the space shuttle is too complicated, it can carry both people and cargo, and its operation efficiency is low. It flies less than 10 times a year, and instead becomes an expensive aircraft. Practice has proved that it has not met the expected goals in terms of economy, safety and reliability. In July 2011, the US space shuttle officially retired after completing its 135th mission. Statistics show that the space shuttle program cost a total of $196 billion, of which each space shuttle cost about $12 billion, and the cost of a single launch was about $450 million (nearly 10 times over budget).

The third method is vertical take-off/vertical recovery.

The rocket adopts a spiral aerodynamic shape and a simple structural design. It generally adopts a vertical landing method. The additional weight of the structure used for landing is relatively small. Compared with the horizontal landing method, the aerodynamic, control, and thermal protection technologies are less difficult. However, the engine is required to have a wide range of thrust adjustment capabilities, and propellant deceleration needs to be reserved, resulting in a partial loss of carrying capacity.

Typical representatives of vertical takeoff/vertical recovery are SpaceX's Falcon series rockets and Blue Origin's New Shepard rocket. Both have achieved rocket recovery and reuse many times, fully verifying the maturity of the technology. At the same time, the cost of using rockets has also dropped significantly. According to Elon Musk, the recovery and reuse of the first-stage rocket can reduce the cost by another 70% based on the existing cost.

Blue Origin New Shepherd

German Orbital Vehicle

To sum up, horizontal take-off/horizontal landing technology is relatively difficult and it is difficult to have engineering application capabilities in the short term; although vertical take-off/horizontal landing has the conditions for engineering practice, the cost of use and maintenance is high, which operators usually cannot afford; vertical take-off/vertical landing does not require changing the rocket configuration, and the technical difficulty is relatively small, which truly reduces the cost of rocket launches and has become the most popular reusable technology route.

How to measure the life of a reusable aircraft

Before discussing the maximum number of times a reusable launch vehicle can be reused, that is, the life limit, we first need to understand the factors that affect the life of a reusable launch vehicle. According to different influencing factors, the life of a reusable launch vehicle can be divided into three types: design life, economic life, and technical life. The economic life is much longer than the design life, and the technical life and economic life depend on their respective costs, and usually the higher-cost side gives way to the lower-cost side.

On February 7, 2018, the Falcon Heavy rocket successfully launched a cherry-colored Tesla sports car into deep space orbit, and two boosters were successfully landed and recovered.

Design life refers to the time from when a reusable launch vehicle is used from the time it is brand new until major equipment failure occurs, rendering the launch vehicle unusable. For example, impact failure of the landing bracket, aging and cracking of the rocket body, excessive coking of the engine cooling pipes, fatigue cracking of the turbo pump, etc., will render the launch vehicle unusable.

Technical life refers to the time from when a reusable launch vehicle is put into use to when it is eliminated due to technological backwardness. Technological progress will shorten the service life of existing rocket models and lead to early retirement. For example, new rockets have higher single-shot carrying capacity due to certain technological innovations and can obtain higher market returns, so the life of old rocket models will naturally be shortened.

Take the Falcon 9 launch vehicle for example. It can be divided into three generations according to technology iteration, codenamed v1.0, v1.1 and v1.2. v1.2 can also be divided into three versions, Block3~5. Starting from v1.1, SpaceX conducted a first-stage vertical recovery test, and achieved the first successful land and sea recovery of F21 and F23 of v1.2 Block3 respectively. The F55 is the first flight of the v1.2 Block5 configuration, and the first stage starts with the number B1046. Even though the versions before Block5 have achieved reusability, they have never been used more than twice. They have been naturally eliminated due to backward technology and ended their technical life.

Economic life refers to the most economical service life of a reusable launch vehicle, that is, its service life is determined based on the evaluation criteria of the lowest cost of use or the highest economic benefit. As the number of times a reusable launch vehicle is used increases, the performance of its main components will gradually deteriorate. At the end of its life, the maintenance costs caused by frequent failures will increase sharply. After a reusable launch vehicle is put into use, the longer it is used, the less the construction cost is allocated each year, but the more maintenance and operation costs each time. The average launch cost of the rocket is the lowest within the most appropriate number of uses, which is the economic life.

If the reusable technology is only broken through, but it is not economically viable, it will be difficult to achieve long-term application. For example, the space shuttle, although technologically advanced, the cost of maintenance and launch after each recovery far exceeded expectations. The initial estimate was only more than 30 million US dollars, but the actual cost reached 400 million to 500 million US dollars, which became an important reason why the space shuttle was difficult to maintain.

The current limit on the number of times the Falcon 9 launch vehicle can be reused is mainly determined by its economic life.

Reusable rockets: exploring the limits of lifespan

In order to accurately and reliably determine the economic life of a transportation tool, it is necessary to conduct statistics on a large sample size and establish an accurate prediction model. In this way, manufacturers can determine how many times a vehicle needs maintenance, how many times a vehicle needs overhaul, and how many times a vehicle should be scrapped, just like the cars we are familiar with. In fact, the same is true for airplanes. For example, an Airbus 320 needs an A check after 600 hours of flight, and a C check after 18 months of flight. Basically, the vehicle needs to be completely disassembled for inspection, and the repair time may be as long as 20 to 60 days.

At present, the Falcon 9 launch vehicle has not yet reached such a sample level. SpaceX is still exploring its economic life limit. Although Musk once claimed that the Falcon 9 rocket can be refilled with propellant without maintenance within 10 times of recovery, just like its first successfully recovered booster. But from the actual time interval of the same booster reuse, the shortest interval is 38 days, the longest is 619 days, and the average is 167 days. There has not been a case of reuse within 1 month, which shows that SpaceX is using this time to conduct in-depth inspections and tests on the recovered boosters. Only by understanding the performance degradation of each component as detailed as possible can we establish accurate life models and operating specifications more quickly. An economical reusable rocket must not be disassembled into eight pieces every time it flies back, and each part must be carefully inspected, and it will not dare to be used again if there are some defects.

Falcon 9 rocket recovery failed, the first stage fell into the sea

In early February 2020, an engineer from SpaceX revealed that they had set up a rocket body refurbishment team similar to the maintenance of civil airliners. During the refurbishment process, it is necessary to check the connection parts and welds, and ensure that all avionics equipment is working properly. The current maintenance work requires very detailed work and takes 1 month. The team is still exploring in practice. This shows that rapid detection and reuse in the true sense are still under research and testing.

B1051.7 booster undergoing overhaul inspection in the workshop

On March 18, 2020, the sixth batch of Starlink satellites was launched, using a five-hand first stage for the first time. However, at the end of the first stage flight, an engine on the outer side failed, and the first stage recovery also had problems and failed to land successfully. On the eve of the launch, President and COO Shotwell said that the company would no longer make design improvements to the first stage of the Falcon 9 rocket, and did not plan to reuse the first stage more than 10 times.

It seems that if SpaceX does not upgrade the technology of the first stage of the Falcon 9 rocket, its reuse limit may be limited to 10 times.

We know that the reason why the Falcon 9 rocket was able to achieve a breakthrough in reuse is largely because SpaceX needs to use this killer feature to significantly reduce launch costs and gain an absolute advantage in the commercial satellite launch market. It can be expected that if new competitors emerge, under strong competition, the reuse of rocket technology will continue to be promoted and its life limit will continue to increase.