Friday is here, and the 'A Taste of Science for the Weekend' column is back — number 101.
This time: rocket fuel, and what keeps causing spacecraft to explode again and again on the launch pad.
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The spectacular, apocalyptic fireball in the video shows the premature demise of New Glenn, Blue Origin's massive launch vehicle.
The launch site was completely destroyed, and the next launch won't be possible for roughly two years. The U.S. government and NASA, which had planned to use similar launch vehicles, will be forced to reschedule.
The one person genuinely enjoying all of this is Elon Musk. Starlink is meanwhile celebrating a monopoly in the low-Earth-orbit communications satellite market, and SpaceX just went public at a truly staggering valuation of $1.77 trillion.
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Conventional jet fuel relies on drawing oxygen from the atmosphere to enable combustion in a mixture with the fuel. It also contains a certain amount of sulfur and other hydrocarbons, which reduce friction with engine components and prevent wear.
Rockets launched into space need to carry an oxidizer inside the vehicle's body to allow the fuel to burn, since there is no oxygen in space to draw from. Typically this means oxygen, hydrogen, or methane cooled to near absolute zero in order to remain in a liquid state.
Using such extremely cold liquids offers an additional benefit: their feed lines can be routed along the body of the rocket so that they double as a cooling system.
The problem is that the rocket's high temperatures cause the hydrocarbons in the fuel to form solid deposits that clog the lines and can trigger an explosion. The problem is compounded in reusable rockets, because these deposits must be cleaned from the tank before every relaunch.
The most common propellant combination for space launch vehicles today is liquid methane and liquid oxygen. Methane is a clean-burning gas that produces almost no byproducts, and its operating temperature is close to that of oxygen, eliminating the need for heavy insulation between the two.
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The reason rockets like these explode on the launch pad varies from case to case, but the range of possible causes is so vast that every time one of them makes it to space, it is an engineering miracle.
Methane and hydrogen are cryogenic liquids, becoming liquid only at temperatures near absolute zero. Earth's atmosphere is warm, and a small amount of heat always manages to penetrate the propellant tank somehow. This heat rapidly converts the liquid to gas, and special valves continuously vent that gas to prevent the pressure buildup that would lead to an explosion.
A valve failure, or excessive heat leaking through the plumbing, can instantaneously convert a large quantity of liquid to gas, and the rapid pressure surge causes an explosion.
Heat leaking into the tank can also create bubbles. The bubbles rise quickly, and the waves they generate inside the tank can destroy the valves. Small bubbles can also accelerate erosion of the pump impeller blades and cause them to fail.
The rocket's proximity to the ground and its rigid mounting in the launch structure generate shockwaves and vibrations when the engines ignite, which can crush pipes and other critical components.
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The New Glenn explosion is still under investigation.
Before it, in 2016, SpaceX's Falcon 9 exploded — with Israel's Amos-6 satellite on board.
The investigation revealed that SpaceX's own engineering ingenuity was responsible for the failure. Inside the oxygen tank, liquid helium vessels insulated with carbon fiber were installed. Tiny gaps in the insulation allowed liquid oxygen to seep in, absorb the cold from the helium, and solidify. The carbon fibers then rubbed against the solid oxygen particles until an explosion occurred.
In the final analysis, SpaceX's more freewheeling approach — relying on inexpensive materials and repeated attempts — is proving itself superior to Amazon's strategy of trying to get everything perfect on the very first try.
Shabbat Shalom 😊
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