Author Topic: Propellant Toxicity (Read 7886 times)

Skye · « **on:** 07/03/2025 01:43 pm »

Hydrazine is known for being extremely toxic, but it’s pretty damn tempting, basically the perfect fuel, right? N2H4 + O2 reacts to form 2H2O & N2. The exhaust of a hydrolox engine, but with some Nitrogens, & they’re basically free energy (weak N-N bond strength —> forming strong N三N bond, gives out loads of energy), and all room temp storable! My question is - it it worth it? Yes it’s extremely toxic, but damn that exhaust species is amazing.

Same goes for N2O4 - extremely toxic but again, Ns for loads of energy, plus four Os. ~25% higher density than LOX, & free Ns!

Thoughts?

Jim · « **Reply #1 on:** 07/03/2025 01:51 pm »

Quote from: Skye on 07/03/2025 01:43 pm

Hydrazine is known for being extremely toxic, but it’s pretty damn tempting, basically the perfect fuel, right? N2H4 + O2 reacts to form 2H2O & N2.

No, hydrazine is mostly used as a monopropellant. Usually MMH is part with N2O4 mixtures.

Skye · « **Reply #2 on:** 07/03/2025 01:52 pm »

Yes, but is there any reason why it can’t be used in main prop? It’s higher density & Isp than MMH.

Jim · « **Reply #3 on:** 07/03/2025 01:52 pm »

spacecraft people don't mind using them.

Skye · « **Reply #4 on:** 07/03/2025 01:54 pm »

Yup. Why shouldn’t LV people? It’s basically superior to MMH in every way (Isp, Density, No Coking, etc.) and is less toxic, plus has easier stoichiometry & more direct calculations, so why not?

VSECOTSPE · « **Reply #5 on:** 07/03/2025 02:14 pm »

N2O4 was the oxidizer in many earlier launch vehicle generations around the world, like Titan II and Proton. The exhaust is toxic and carcinogenic and can wind up in places you don’t want in the event of launch accidents. (Same is true for the physically dangerous unburnt propellant in the event of an accident.) Everyone has moved away from N2O4 for launch except for some legacy launch vehicles in China, which are on their way out.. More here:

https://www.twz.com/why-chinese-rockets-spew-toxic-bright-red-gas-clouds-on-launch

Skye · « **Reply #6 on:** 07/03/2025 02:18 pm »

The exhaust is toxic? What fuel was it burned with? If N2O4 were burned with a hydrocarbon or N2H4, surely the exhaust would be:

Hydrocarbon: CO2 + H2O + N2

N2H4: 2H2O + N2

Is N2O4 (aside from toxicity) superior to O2? It has higher density, similar Isp, and nitrogens. Just don’t burn it with something that produces carcinogenic exhaust.

Jim · « **Reply #7 on:** 07/03/2025 02:49 pm »

Quote from: Skye on 07/03/2025 02:18 pm

The exhaust is toxic? What fuel was it burned with? If N2O4 were burned with a hydrocarbon or N2H4, surely the exhaust would be:

Hydrocarbon: CO2 + H2O + N2

N2H4: 2H2O + N2

Is N2O4 (aside from toxicity) superior to O2? It has higher density, similar Isp, and nitrogens. Just don’t burn it with something that produces carcinogenic exhaust.

Propellants are never burned at stoichiometric ratios. There always unburnt residuals of one of the propellants in the exhaust.

Titan II burned N2O4 and Aerozine 50 (50% Hydrazine and 50% UDMH) at 1.91 mass ratio.
I will let you do the math,

Apollo22 · « **Reply #8 on:** 07/03/2025 06:01 pm »

Quote from: VSECOTSPE on 07/03/2025 02:14 pm

N2O4 was the oxidizer in many earlier launch vehicle generations around the world, like Titan II and Proton. The exhaust is toxic and carcinogenic and can wind up in places you don’t want in the event of launch accidents. (Same is true for the physically dangerous unburnt propellant in the event of an accident.) Everyone has moved away from N2O4 for launch except for some legacy launch vehicles in China, which are on their way out.. More here:

https://www.twz.com/why-chinese-rockets-spew-toxic-bright-red-gas-clouds-on-launch

Proton, Baikonur, April 2, 1969
Titan 34D-9, Vandenberg, April 18, 1986

Those were two very nasty low altitude failures involving toxic storable hypergolic props.
Kourou got a very scary close brush with similar disaster with Ariane Flight 36, as told here.
https://www.thespacereview.com/article/4085/1

Downrange Baikonur (Tuva republic) and downrange Long March 1-4 launch complexes have important storable propellants pollution - because of spent stages.

Proponent · « **Reply #9 on:** 07/03/2025 06:17 pm »

According to Clark's Ignition!, in the late 1940s and early 1950s engineers liked hydrazine as a fuel (and it features prominently in von Braun's plans of the period), but the armed services rejected it because of its high freezing point, -1.5 ^oC. All sorts of additives were tried before MMH and UDMH were discovered. They freeze below -50 ^oC, have similar performance and density as hydrazine and are neither dangerously unstable nor, in this context, extremely toxic. Regarding toxicity, consider that hydrogen cyanide was one additive considered to depress hydrazine's melting point.

EDIT: "Clarke" -> "Clark", per Donosauro's observation, below.

Skye · « **Reply #10 on:** 07/04/2025 07:57 am »

Quote from: Jim on 07/03/2025 02:49 pm

Quote from: Skye on 07/03/2025 02:18 pm
The exhaust is toxic? What fuel was it burned with? If N2O4 were burned with a hydrocarbon or N2H4, surely the exhaust would be:

Hydrocarbon: CO2 + H2O + N2

N2H4: 2H2O + N2

Is N2O4 (aside from toxicity) superior to O2? It has higher density, similar Isp, and nitrogens. Just don’t burn it with something that produces carcinogenic exhaust.

Propellants are never burned at stoichiometric ratios. There always unburnt residuals of one of the propellants in the exhaust.

Titan II burned N2O4 and Aerozine 50 (50% Hydrazine and 50% UDMH) at 1.91 mass ratio.
I will let you do the math,

Riiiight, sorry

My dad, a chemist, told me that N2O4 is also extremely corrosive, and gave various other reasons that hydrazines & NTO should be kept far away from.

Quote from: Proponent on 07/03/2025 06:17 pm

According to Clarke's Ignition!, in the late 1940s and early 1950s engineers liked hydrazine as a fuel (and it features prominently in von Braun's plans of the period), but the armed services rejected it because of its high freezing point, -1.5 ^oC. All sorts of additives were tried before MMH and UDMH were discovered. They freeze below -50 ^oC, have similar performance and density as hydrazine and are neither dangerously unstable nor, in this context, extremely toxic. Regarding toxicity, consider that hydrogen cyanide was one additive considered to depress hydrazine's melting point.

Yes, freezing point is a huge issue, another thing my dad reminded me. I will say, however, that MMH has 20% less density & UDMH has 30% less.

Proponent · « **Reply #11 on:** 07/04/2025 02:26 pm »

Less dense than straight hydrazine but still denser than common fuels: RP-1, methane, hydrogen. And what really matters is the bulk density. UDMH-NTO really shines on this score compared to other common propellant combinations. I would think that the O/F ratio for UDMH-NTO would be higher than for hydrazine-NTO, possibly resulting in a higher bulk density for the former, because NTO is very dense.

ugordan · « **Reply #12 on:** 07/06/2025 02:29 pm »

Quote from: Skye on 07/04/2025 07:57 am

My dad, a chemist, told me that N2O4 is also extremely corrosive, and gave various other reasons that hydrazines & NTO should be kept far away from.

I might be wrong on this, but rocket engines typically run fuel rich (primarily to keep the combustion temp down to prevent melting everything) so hydrazine would be the excess compound that's released to the atmosphere, not the N2O4 oxidiser.

Nasty stuff either way.