Why Chinese Rockets Spew Toxic Bright Red Gas Clouds On Launch

By OLiver Parken,
Published by The War Zone, 22 March 2023

The rusty red-colored exhaust plume that comes with many Chinese rocket launches looks dangerous because it is.

Footage of China’s most recent Long March 3B Y90 rocket launch appeared last week. Aside from the high terrain of the launch site, which really looks like a scene out of You Only Live Twice, what is impossible to miss is the sinister-looking billowing bright reddish plume that engulfs the rocket as it lifts off. It certainly doesn’t look like something you would want to breathe.

As it turns out, that is all too true. 

In fact, this is the case to such a degree that when asking rocket aficionados and experts alike about the scarlet cloud, the answer we repeatedly received as to the nature of it was “if you ever see it, run.”

Lifting off from Xichang Satellite Launch Center, which is located in Sichuan Province, southwest China, the rocket sent a Gaofen-13 (02) orbital satellite into geosynchronous transfer orbit on March 17, according to China’s Aerospace Science and Technology Corp (CASC). Seeing the footage showing the red plume, seemingly a staple of Chinese rocket launches but not ones in the West, The War Zone’s Tyler Rogoway took to Twitter in order to figure out what the exhaust is and why the Long March 3B Y90 produces it. As the Twitter replies were so detailed, we decided to write an explainer on the science behind the emissions.

As several commenters noted, the clouds seen in the video above are the result of a chemical known as dinitrogen tetroxide (N2O4) being used as part of the rocket’s hypergolic propellant. Hypergolic propellants allow rocket engineers to do away with ignition systems that could potentially fail.

‘Oxidizers’ such as dinitrogen tetroxide spontaneously ignite when they come into contact with a fuel source (typically from within the hydrazine family of chemicals) in the rocket’s engine. The reaction between the dinitrogen tetroxide and the fuel is exothermic, and releases a tremendous amount of heat energy and toxic gas.

Astrophysicist and video creator Scott Manley has produced an informative YouTube video on the science behind hypergolic rocket propellants – a particularly good watch for those new or returning to the topic – which explains why the dinitrogen tetroxide burns a rusted red color.

As Manley explains, dinitrogen tetroxide molecules are made up of two nitro groups, each consisting of a nitrogen atom and a pair of oxygen atoms. The two groups are joined by a weak bond between the nitrogens. Dinitrogen tetroxide is an extremely hazardous substance that can cause severe skin burns and eye damage. It can also be fatal if inhaled.

Dinitrogen tetroxide’s chemical formula. Kemikungen via Wikimedia Commons

At low temperatures of around −11.2 °C (11.8 °F), dinitrogen tetroxide’s freezing point, nitrogen dioxide (NO2) converts to the clearer dinitrogen tetroxide. As dinitrogen tetroxide is heated, it breaks down to nitrogen dioxide, becoming darker in color. What we see in the video, therefore, are the gasses that are produced as the dinitrogen tetroxide breaks down to nitrogen dioxide, due to the heat of the reaction between the oxidizer and the fuel. The gas produced is highly toxic and very likely carcinogenic.

An overlay of the same 99.9% pure NO2/N2O4 sealed in an ampoule. From left to right -196 °C, 0 °C, 23 °C, 35 °C, 50 °C. Efram Goldberg, Florida Institute of Technology, Dr. Knight Research Group via Wikimedia Commons

Avoiding these toxic gasses isn’t as easy as staying clear of the rocket launch site, sadly. As has been the case in the past, Chinese rocket boosters commonly land in or near populated areas when dropped, spewing toxic hypergolic fuels in the process. Concerns have also been raised over local populations being recruited to clear away rocket booster debris, owing to the health risks involved.

A booster seen still burning on the ground after a launch. Chinese Internet

It should be noted that dinitrogen tetroxide is rarely used on its own as an oxidizer. Frequently, it’s mixed with nitric oxide (NO) as well as nitrogen dioxide (NO2) to make the agent less corrosive. ‘Mixed oxides of nitrogen’ (MON), as this is commonly referred to, also has a much lower freezing point than pure dinitrogen tetroxide. MON3, the number refers to 3% of added nitric oxide, has a freezing point of around −15 °C (5 °F). MON25, 25% added nitric oxide, has a freezing point of around −55 °C (−67 °F).

Moreover, similar-looking rusty red clouds are produced in other oxidizer combinations. ‘Red fuming nitric acid,’ for example, consists of a mixture of nitric acid (HNO3), dinitrogen tetroxide, and water. Again, the distinctive red exhaust created is a product of the dinitrogen tetroxide breaking down to nitrogen dioxide.

In the case of the Long March 3B Y90, dinitrogen tetroxide – the oxidizer – is combined with unsymmetrical dimethylhydrazine, also referred to as UDMH – the fuel source. In terms of its chemical composition, UDMH has one nitrogen atom that has two methyl atoms, while another nitrogen atom has two hydrogen atoms. Compared to the symmetric version’s −9 °C (16 °F) melting point, UDMH has a much lower melting point of around −57 °C (−71 °F). UDMH can be stored in liquid form for long periods of time, although it remains highly flammable, explosive, toxic, corrosive and requires careful handling with proper safety equipment.

UDMH’s chemical composition. NEUROtiker via Wikimedia Commons

“Old-style N2O4/UDMH toxic storable propellants with their distinctive bright orange-red exhaust… were derived from Chinese rocket pioneer Qian Xuesen (Tsien Hsue-Shen),” Dr. Jonathan McDowell, astrophysicist at the Harvard-Smithsonian Center for Astrophysics, told The War Zone. 

“[Qian Xuesen] was part of [Dr. Theodore] von Karman’s Aerojet/JPL [Jet Propulsion Labratory] team until … his deportation to China in 1955. The U.S. Titan missile (same propellants [N2O4/UDMH]) came out of the same team and the engines look remarkably similar to the Chinese ones.”

During the Cold War, many rocket designs used hypergolic propellants, including the Titan II ICBM, as well as the wider family of Titan rockets, which used N2O4/Aerozine 50 (a 50:50 mix of hydrazine and UDMH). N2O4/UDMH has also been used by Russia for its Proton family of rockets, the first of which was launched in 1965. India’s PSLV medium-lift launch vehicle, the first flight of which took place in the early 1990s, also uses N2O4/UDMH.

A Titan II being launched from its silo. CSIS.org

While fuels such as hydrazine can be used on-orbit or for backup power – the Space Shuttle Orbiter and the X-37B uses it, as well as some aircraft, like the F-16 – many launch vehicles now favor cleaner fuel alternatives to N2O4/UDMH for actual rocket fuel used in the launch stages. 

As Janet K. Tinoco et al. indicate in their 2020 book, An Introduction to the Spaceport Industry: Runways to Space, most modern U.S. launch vehicles (LVs) use solid propellant with liquid oxygen (LOX) and kerosene. Modern Russian rockets, they indicate, use both N2O4/UDMH and LOX/kerosene, while China still favors N2O4/UDMH, followed by solid propellant.

A SpaceX Falcon 9 rocket carrying the company’s Crew Dragon spacecraft launches on the Demo-2 mission to the International Space Station with NASA astronauts Robert Behnken and Douglas Hurley onboard at Launch Complex 39A on May 30, 2020, at the Kennedy Space Center, Florida. SpaceX rockets use a combination of liquid oxygen and kerosene as propellants. Photo by SpaceX via Getty Images

China’s rocket production was born out of the country’s ICBM development. As Manley explains in another YouTube video dedicated to Chinese rocket production, China had direct assistance from the Soviet Union in the 1950s on this score until the Sino-Soviet split of 1960. Development on Long March 1, also known as the Changzheng-1 (CZ-1), began in 1965 and was launched in 1970. Long March 2 (CZ-2) and 3 (CZ-3) were launched in 1974 and 1984 respectively. Long March 3B first took flight in 1996, while an enhanced Long March 3B/E version first took flight in 2007.

As McDowell highlights, the current main stable of space rockets used by China – which includes the CZ-2C and CZ-3B from the China Academy of Launch Vehicle Technology in Beijing and the CZ-2D, CZ-4B, CZ-4C from the Shanghai Academy of Spaceflight Technology – feature old-style N2O4/UDMH toxic propellants. In the 1990s, China undertook the development of a new generation of rockets with cleaner LOX/kerosene and LOX/liquid hydrogen (LH2) propellant, but this initiative ran into many problems and delays. Only now in the early 2020s are cleaner Chinese rockets beginning regular flights. Although the old CZ2/3/4 series will be phased out in the coming years, in 2022 there were 39 launches of that old series and only 10 of the new series, he indicates.

While efforts are being made by China to move away from hypergolic propellants, it’s likely that the toxic red clouds seen engulfing its rockets aren’t going away any time soon.

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