Shulou(Shulou.com)11/24 Report--

On September 7, NASA's giant rocket space launch system (SLS) uses a mixture of liquid hydrogen and liquid oxygen as fuel. This kind of fuel has compact volume and high combustion efficiency, but its inherent properties also bring a series of problems that can not be solved. Industry insiders said that in order to please the US Congress, NASA had to continue to use the liquid hydrogen and liquid oxygen propulsion system used on the retired space shuttle on the SLS rocket.

On Saturday, SLS's second launch attempt was cancelled after engineers failed to resolve a hydrogen leak from the quick disconnect device connected to the rocket. The problem could delay the earliest launch of the SLS rocket until October, which means that the Artemis 1 mission carrying the Orion spacecraft to and from the moon has been postponed again.

Last Monday, the NASA ground team fixed the hydrogen leak during the first launch attempt, but the launch was cancelled after a faulty sensor mistakenly showed that the engine did not reach the required ultra-low temperature. The leak that occurred on Saturday proved much more difficult to control, and engineers tried unsuccessfully to fix it three times. "this is not a controllable leak," Mike Mike Sarafin, head of the Artemis project, told reporters after the meeting.

NASA is still evaluating the next step, but the rocket must return to the spacecraft assembly building to carry out mandatory security checks related to the flight termination system. Engineers believe that an inadvertent order made during Saturday's attempted launch caused a brief increase in pressure in the system, accidental overpressure affected the tightness of the pipeline, and the rocket may need hardware repair. they are currently assessing the possibility of this happening.

The long-existing hydrogen leak is nothing new to NASA. The erratic launch of the space shuttle is mostly caused by hydrogen leakage. The most infamous event was the hydrogen Summer, when ground teams spent more than six months trying to find the source of the hydrogen leak, which led to the grounding of the space shuttle throughout 1990. SLS rockets largely mimic the space shuttle's propulsion system, including the use of liquid hydrogen propellant, so there must be hydrogen-related problems. But NASA had no choice.

Jordan Bimm, a space historian at the University of Chicago, says NASA's insistence on using liquid hydrogen as a propellant on SLS rockets is not for technical reasons.

"since the establishment of NASA in 1958, it has been using contractors across the United States to maintain congressional policy support and financial support for space exploration," Beem said. "the first system to use liquid hydrogen was the Centauri rocket developed in the 1950s and 1960s. In 2010, Congress explicitly required NASA to use existing space shuttle technology in a new generation of launch systems in the authorization bill to fund NASA." "the purpose of this decision is to maintain the jobs of NASA contractors in key constituencies so that they can benefit from congressional funding and support for NASA," he added.

But the decision also means that although the space shuttle is about to retire, the RS-25 engine and the liquid hydrogen and liquid oxygen fuel will be transferred to the SLS rocket. In total, NASA collected 16 engines from decommissioned space shuttles, four of which were installed on SLS rockets still standing on the launch pad of Kennedy Space Center in Florida.

Bim revealed that NASA "must always give priority to how to win the support of the U.S. Congress in order to maintain the space exploration program." He said that "the continued use of RS-25 engines" is a typical example of how "things like fuel selection can be politicized in the United States, while the most direct and ideal solutions are often not feasible."

In the end, instead of choosing propellants such as methane or kerosene for the SLS rocket, NASA chose to use a mixture of liquid hydrogen and oxygen to power such a heavy rocket. By contrast, the starship to be launched by SpaceX uses liquid methane as the fuel and liquid oxygen as the oxidant. "because their goal is to land on Mars, SpaceX chose liquid methane as fuel in the hope of extracting this element from Mars and creating a cost-effective form of resource utilization," Beem explained. NASA followed a different set of principles when designing SLS rockets because it was always cash-strapped and had to please Congress.

"based on current information and analysis, the proposed SLS rocket design represents the direction of developing a new generation of heavy launch vehicles with the lowest short-term cost, the fastest available time and the lowest overall risk," NASA wrote in a preliminary project report in 2011. "choosing this architecture means that there is no need to develop new rocket engines in the short term, thus shortening the first flight time and possibly minimizing the overall cost of SLS rockets."

Ironically, the SLS rocket was scheduled to launch in 2017, but it hasn't lifted off yet. Since the establishment of the project, the total cost of research and development, including the Orion spacecraft, has exceeded $50 billion, not including the $4.1 billion needed for each rocket launch. The rocket inherited the components of the space shuttle, and NASA inherited the hydrogen leak problem.

Useful but obnoxious hydrogen is very effective as a rocket fuel. This element is found all over the world, clean and light, and when combined with liquid oxygen, it is extremely efficient. According to NASA, "when combined with oxidants such as liquid oxygen, liquid hydrogen produces the highest specific impulse of any known rocket propellant." When hydrogen cools to minus 253 degrees Celsius, it shrinks sharply and can provide a large amount of fuel when fed into the rocket. "the advantage of hydrogen fuel is that it can effectively store the energy you want to release in the rocket, and it is very light, which has always been an important factor to consider in space flight," Beem said. "

NASA used liquid hydrogen in the second stage of the Apollo-era Saturn V rocket, as did the three main engines of the space shuttle. Liquid hydrogen is usually used as fuel for the second stage of a rocket or as a liquid fuel for spacecraft operating in orbit. Currently, rockets that use liquid hydrogen include Atlas's Centauri rockets and Boeing's Delta III and IV rockets. The BE-3 and BE-7 engines of blue origin also use liquid hydrogen as fuel.

"the disadvantage of hydrogen fuel is that it is difficult to transport and control, because the hydrogen molecule is very small, it is easy to leak, and it needs to be cooled to a very low temperature to maintain the liquid state," Beem said. " More importantly, hydrogen is extremely volatile when it is liquid, and there is a risk of large-scale combustion. As the lightest element known, hydrogen is also easy to leak. NASA explained many of the challenges of using liquid hydrogen as fuel: "to prevent evaporation or boiling, rockets fueled by liquid hydrogen must be carefully isolated from all heat sources, such as the plume of a rocket engine or air friction during flight through the atmosphere. Once a spacecraft goes into space, it must be protected from solar radiation. When liquid hydrogen absorbs heat, it expands rapidly, so exhaust is necessary to prevent the fuel tank from exploding under overpressure. Metals exposed to extremely cold liquid hydrogen become fragile, and liquid hydrogen leaks through tiny pores in the weld. "

Despite these problems, NASA chose liquid hydrogen as its fuel when designing the SLS rocket, and now it has paid a heavy price.

New rocket, old problem when fueling the SLS rocket, the sudden inflow of low-temperature hydrogen will lead to great changes in the physical structure of the rocket. According to NASA, the 40-meter-high liquid hydrogen storage tank will be 152mm shorter in length and 25.4mm in diameter when filled with ultra-cooled liquid. As a result, components connected to the tank, such as pipes, exhaust pipes, and brackets, must compensate for this sudden contraction. To solve this problem, NASA uses bellows, slotted joints, telescopic joints and ball joint hinges similar to accordion connectors.

But as the smallest molecule in the universe, hydrogen often leaks out through tiny pores. The problem of leaks in fuel pipes is particularly serious because they cannot be bolted directly to the rocket. As the name implies, the quick disconnect device is designed to be quickly detached from the rocket during launch, while preventing hydrogen from leaking under high pressure and ultra-low temperatures to ensure sealing effect. On Saturday, the concentration of hydrogen leaked near the quick disconnect device far exceeded the 4% limit. NASA had to cancel the launch because it was unable to solve the leak problem.

The most worrying thing was that NASA had not yet filled the first and second stages of the rocket with enough fuel, nor could it enter the countdown to launch. NASA has dealt with hydrogen leaks before, so engineers need to come up with solutions again to move the project forward.

In any case, the Artemis project has had a frustrating start.

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