From failure to take-off, how did the Rockets be reborn?

On August 29, 2020, when the United Launch Alliance's Delta 4 Heavy rocket was carrying out the NROL-44 launch mission, the rocket ignited and staged a thrilling "flame on the stage, but the rocket remained motionless" drama. Rockets are essentially controlled "explosions" of large amounts of dangerous high-energy propellants. Once this "explosion" is out of control - it really becomes an explosion, which is what we often call a launch failure or anomaly. However, as a complex means of transportation with multiple systems coupled, modern launch vehicles have common problems and failures are inevitable. When faced with anomalies or failures at different stages, how do different rockets do things differently?

After the Delta 4H rocket ignites, the rocket remains motionless

What should I do if the engine suddenly shuts down after ignition?

On August 29, 2020, the three RS-68 hydrogen-oxygen engines of the Delta 4 Heavy rocket's core stage and booster suddenly malfunctioned 3 seconds before takeoff. The hydrogen discharged from the engine rose and ignited, but because the rocket failed to leave the frame, the hydrogen fire quickly surrounded the rocket, and the rocket control system automatically performed an emergency shutdown, and the launch was forced to abort. Fortunately, the payload in the rocket fairing was normal. This payload was the latest model of the reconnaissance satellite code-named "Mentor". This series of satellites is as famous as the famous "Keyhole" and is known for its mystery and high cost. It is said that the cost of the satellite is more expensive than the same weight of gold. Therefore, the launcher has always been cautious with this type of payload, treading on thin ice, and dare not make any mistakes.

Coincidentally, in January 2019, the same rocket of the United Launch Alliance had the same "emergency shutdown" drama when launching a new Keyhole satellite for the same customer at the Vandenberg Launch Center. Fortunately, the payload was also safe at the time. A few days later, the fault was found to be caused by the sensor. After the fault was eliminated, the rocket was launched normally.

In fact, emergency shutdown failures before launch are not uncommon, and are mostly triggered automatically by the system when an abnormal signal is detected during the engine ignition process. For example, when the Falcon 9 rocket attempted to launch the SES-9 satellite for the fourth time in February 2016, the engine ignition system detected an abnormal decrease in thrust of one engine, and the rocket immediately and automatically aborted the launch. This was due to the restraint release device, which firmly "held" the rocket and did not leave the launch pad. However, as a liquid oxygen-kerosene fuel rocket, the Falcon 9 rocket lacks the "flaming burning" effect of hydrogen-oxygen fuel rockets, and it just shut down after the self-igniting ignition agent emitted a burst of green light. Subsequent investigations stated that the failure was caused by the long-term standing of the supercooled propellant, which caused the temperature to rise, and the pressurized helium in the tank was mixed into the propellant and sucked into the turbopump, causing abnormal thrust.

Similar situations are not uncommon in the history of space exploration, but as long as the rocket has not left the launch pad, there is usually a chance to remedy the situation.

How to save yourself after taking off?

What if a problem occurs after the rocket is launched? Since the rocket is separated from the ground support, it can only rely on "self-rescue". There are two common ways to remedy the problem.

The first is that if the rocket itself is designed with power redundancy, once an engine fails, the rocket can automatically shut down the faulty engine, and the other engines in parallel can take over the work of the abnormal engine. Through the re-iterative planning of the rocket control and navigation system, the working time of the remaining engines can be appropriately extended to save the launch. Historically, the Saturn V, the Space Shuttle, and the Falcon 9 have actually used power redundancy to save launches, so this design indirectly improves the reliability of the rocket.

The second remedy is also to extend the working time of the engine, but it is the next stage of the faulty engine that has to work hard. For example, in March 2016, during the launch of the Cygnus OA-6 mission, the Russian-made RD-180 engine of the first stage of the Atlas V rocket suddenly shut down 5 seconds in advance, and then the RL-10 engine of the second stage of the rocket used the reserved propellant in the tank to extend the burning time by up to 1 minute, and finally sent the spacecraft into orbit.

The Cygnus spacecraft, which took off safely, is approaching the International Space Station

This early shutdown may seem trivial, but it is actually very dangerous, because if the RD-180 is shut down one second earlier, the launch will fail, and even if the second stage of the rocket uses up all the reserved propellant, it will be "unrecoverable." For launch vehicles that do not have power redundancy, a failure of the power system during flight is fatal and can easily lead to irreversible launch failure.

So, once a launch vehicle fails, it will naturally learn from its mistakes and start over. For example, the Vega rocket, which just resumed its launch on September 2, sent 53 satellites from 21 customers in 13 countries into orbit at once, successfully completing its "rebirth journey" and breaking the European record of launching multiple satellites in one launch.

Vega rocket launches resume

The rocket had previously encountered an accident during its launch in July 2019, after which it fell silent. The accident investigation believed that the high-temperature gas generated by the combustion of the rocket's solid charge impacted its second-stage structure, causing the second-stage carbon fiber structure to burn through, eventually causing the rocket to disintegrate in flight. The root cause was that when the rocket was manufactured, the thickness of its second-stage thermal protection layer was about 1 mm different, and it escaped quality control inspection, which ultimately led to the accident.

How to make a comeback after a launch failure?

After a launch failure, R&D personnel will generally conduct a systematic review and analysis of the subsystems of the entire rocket. While solving the problem, they will sometimes dig deep into the pain points and improve the rocket. Therefore, a launch failure is usually an excellent opportunity to update, upgrade, and tap the potential of the launch vehicle.

For example, the Electron rocket, which is a major player in the field of microsatellite launches along with the Vega rocket, also completed a recovery launch after a failed launch. Previously, the rocket failed during a launch due to a failure of an electrical connection, which caused the electric pump to lose power and the engine to shut down prematurely, resulting in the launch failure.

Electron rocket resumes launch

By learning from the experience, Rocket Lab also designed a weight-reducing battery pack for the rocket's engine electric pump, thereby increasing the new rocket's low-Earth orbit capacity.

Coincidentally, SpaceX also used the opportunity of "returning to zero" to improve its rockets. In July 2015, during the CRS-7 cargo mission to the International Space Station, the Falcon 9 rocket disintegrated in mid-air due to the failure of the COPV support rods in the second-stage tank of the rocket, destroying both the rocket and the rocket. Then in December of that year, SpaceX launched a new full-thrust version of the Falcon 9 as soon as it resumed launches.

This upgrade can be described as "drastic". The company not only used supercooled propellant, but also lengthened the rocket's second-stage tank, replaced it with a longer and stronger carbon fiber interstage, added hydraulic push rods for separating the first and second stages, improved the engine compartment, upgraded the recovery landing legs, and finally increased the thrust of the Merlin 1D engine, ultimately increasing the rocket's low-Earth orbit capacity by up to 33%.

However, when it comes to a typical example of a major modification after a failure, the Antares rocket may be the one. Compared with the upgrade and modification of the Falcon 9, this type of rocket directly chooses to replace the engine.

In October 2014, when the Antares rocket was carrying out the Cygnus launch mission, the AJ-26 engine of the first stage of the rocket suddenly exploded only 15 seconds after it was launched. The accident analysis speculated that debris or manufacturing defects caused damage to the engine pump bearing, which in turn triggered a chain reaction and caused the explosion. The two AJ-26 engines of the first stage of the rocket are actually NK-33 engines that were stored in the Soviet Union more than 40 years ago. They were purchased from the United States in the 1990s and refurbished, so the price is extremely low. So the company had to reluctantly abandon this type of engine and use the RD-181 engine instead, and finally resumed launches in October 2016.

Antares rocket explosion scene