While answering a question about if N2O was a practical monopropellant, I found that hydrazine was considerably more efficient. So, why do people want to replace it with N2O, if hydrazine is more efficient. According to a Harvard sub-page, they are. So, why? There must be some current problem with hydrazine that people want to switch. What is that problem?

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    $\begingroup$ Because it is a toxic propellant? IIRC, see articles like this newscientist.com/article/mg15621033-800-what-goes-up $\endgroup$ Commented Feb 11, 2022 at 12:26
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    $\begingroup$ @CamilleGoudeseune Gaseous nitrogen, N2, is used as a monopropellant. It is the favored gas for cold gas thrusters. $\endgroup$ Commented Feb 12, 2022 at 3:25
  • $\begingroup$ The book Ignition! (Clark) - also available from other sources - discusses hydrazine as of the late '40s and the '50s. Clark brings up two issues: pure hydrazine has an extremely high freezing point and so needs to be mixed with a "freezing point depressant" to get it usuable - not of which seemed to work - or to be changed to something else - "a derivative" - and they came up with UMDH. But that was not a monopropellant. 1/2 $\endgroup$
    – davidbak
    Commented Feb 13, 2022 at 18:18
  • $\begingroup$ For monopropellant use the problem (discussed on pages 122-123) was that the hydrazine was sent over a catalyst bed, "but restarts, if the catalyst bed has cooled, are just about impossible." But, again, that was in the late 40's early 50's, and maybe things have been discovered since then ... 2/2 $\endgroup$
    – davidbak
    Commented Feb 13, 2022 at 18:20
  • $\begingroup$ @davidbak hydrazine was used as a mono(fuel, not really prop) in the shuttle Auxiliary Power Units and caused a lot of trouble including almost blowing up the orbiter on STS-9. As you say, heaters had to be supplied to keep it from freezing, and if they failed on, that was Real Bad too. $\endgroup$ Commented Feb 15, 2022 at 22:15

2 Answers 2


There are a few reasons why a hydrazine replacement is being looked at:

  • Hydrazine is highly toxic, and carcinogenic so it's dangerous to handle. A leak can put astronauts or technicians at serious risk
  • Hydrazine is corrosive and can damage unprotected surfaces, think metal, skin, lungs, etc.
  • Its breakdown products are also toxic and corrosive, so it's still nasty after it is used
  • As hydrazine is toxic, corrosive, etc. it requires special equipment and procedures to transport and handle, which increases complexity of the overall system
  • High costs: it's expensive to produce to start, then there are additional costs to safely transport and handle it

Note that's not the complete list of why hydrazine is ugly stuff, it has the tendency to spontaneously explode on contact with certain materials, but that's why it's used in rocketry.

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    $\begingroup$ For small satellites that are piggybacked during launch with a larger spacecraft, the owner of that larger spacecraft along with the launch provider may both take a dim view of hydrazine on the piggybacked satellites. $\endgroup$ Commented Feb 11, 2022 at 15:19
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    $\begingroup$ Another bullet item: It and its decomposition products are also quite corrosive nj.gov/health/eoh/rtkweb/documents/fs/1006.pdf $\endgroup$ Commented Feb 11, 2022 at 15:21
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    $\begingroup$ Thanks @OrganicMarble, I've edited with some of those details. $\endgroup$
    – GdD
    Commented Feb 11, 2022 at 16:53
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    $\begingroup$ and another bullet item, rust, i.e. oxidised steel, is just another oxidiser as far as hydrazine is concerned, thereafter once decomposition has started the generated heat can make it self-sustaining. One can''t assume that stainless steel (i.e. CRES) will be rust free and so the propellant handling equipment has to be in pristine condition. $\endgroup$
    – Puffin
    Commented Feb 11, 2022 at 17:23
  • $\begingroup$ @Puffin: I smell a liquisolid engine. $\endgroup$
    – Joshua
    Commented Feb 12, 2022 at 5:20

Hydrazine, as for small monopropellant thrusters, has been in use for decades on hundreds of satellites. One model the "1N Monopropellant Hydrazine Thruster" is claimed to be in use in 165 units. Other models have similar deployment records.

Reference: space-propulsion.com.

Many studies of monopropellants were done during the 60s and 70s. A book by: Belal, Hatem -- 2009/05/26SP Modeling of Hydrazine Decomposition for Monopropellant Thrusters presents a summary performance table of a few monopropellants, including hydrazine and $N_2O$. I am attempting to paste in a summary table. Here it goes:

enter image description here

Hydrazine has higher Isp but $N_2O$ has higher combustion temp likely needing more exotic materials.

Overall, hydrazine has such a built-in infrastructure, long-term installed performance base that it unlikely that it would be displaced.

The toxicity, carcinogenic properties, corrosiveness, etc. of hydrazine are greatly overstated from my experience. I was working in the 60s in a small combustion research company. In one work area studies of hypergolic ignition parameters of $N_2O_4$ and hydrazine, UDMH, Aerozine 50, etc. were done. High speed movies and temperature and gas composition measurements were done to support the testing. I lived in clouds of hydrazine and $N_2O_4$ fumes. $N_2O_4$ was treated with respect. Hydrazine was not treated with much concern. Hydrazine has an ammonia odor and it was attempted to keep that as low as possible but there were no real safety limits. Also, there were no easy measurement instruments for ambient concentrations. No one in our group has become ill from anything the could have been related to hydrazine and $N_2O_4$. Likely this work could not be done today especially in a suburban industrial park.

In its document EPA estimates that, if an individual were to continuously breathe air containing hydrazine at an average of $0.0002 \hspace{1mm} {µg}/{m^3}$ ($2.0 * 10^{-7} \hspace{1mm} mg/m^3$) over his or her entire lifetime, that person would theoretically have no more than a one-in-a-million increased chance of developing cancer as a direct result of breathing air containing this chemical.

I think hydrazine will be used for a long time.

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    $\begingroup$ Fascinating! Hope you do not suffer any consequences. $\endgroup$ Commented Feb 12, 2022 at 2:55

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