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Firstly, I understand the need to use hypergolics over cryogenics as part of Dragon 2's abort and landing sequence - they provide "instant on" thrust and are throttlable with very wide margins (SuperDraco itself can throttle down to 20%).

But why use Nitrogen Tetroxide & Monomethyl Hydrazine specifically? Newer chemicals such as AF-M315E (which will be tested as part of the Green Propellant Infusion Mission) offer some pretty cool advantages. Consider the numerous downsides Hydrazine possesses:

  • Highly toxic. Hazmat suits are a requirement when handling Hydrazine. It's toxic in very low concentrations and also carcinogenic. This slows loading & unloading times and increases cost and complexity significantly - something that is diametrically opposite to SpaceX's goals.

  • Costly. In 1990, NASA was paying $17/kg for Hydrazine. I highly doubt it has become cheaper since then. Stricter environmental & handling regulations will only have increased the cost, and when Dragon 2 holds a few hundred kilograms of it, that cost stacks up.

  • Low specific impulse. SuperDraco has an Isp of 235 seconds, which is terrible. AF-M315E has "50% increased performance". This could translate into a number of options for SpaceX. They could reduce the weight of Dragon 2 significantly, which could make recovery of Falcon 9 easier, alternatively, it could allow them to keep the same weight and pack more scientific instrumentation or cargo aboard. Another option is keeping the same amount of propellant and allowing for a longer, less aggressive landing burn or providing more maneuvering margin during abort situations.

  • Fiddly. Hydrazine has to be kept warm too - which requires heaters which requires power.

Now, I realize...

that AF-M315E is still relatively cutting edge, but so is Dragon 2. NASA is requiring that landings take place under parachutes for the time being, which leaves ample R&D time (something SpaceX are fond of) for Dragon integration with this newer chemical.

Can anyone explain this decision to prefer Hydrazine? Is a greener, more performant propellant in the upgrade line?

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  • $\begingroup$ Regarding the I$_\text{sp}$ of 235 seconds, that number comes from a document prepared for the FAA document by a consultant. Who knows the provenance of that number? From that FAA document, that value apparently is sea level Isp. Even then, it looks low for N2O4/MMH. That's the Isp of monopropellant hydrazine. $\endgroup$
    – David Hammen
    Commented Mar 9, 2015 at 9:58
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    $\begingroup$ @David Hammen - Theoretical Isp for N2O4/MMH id 288 s at SL. For deeply throttled, radiatively cooled engine 235 s looks very realistic. Cited "50% increased performance" is referring volume Isp of "hydrasine monopropellant sysems" winch theoretical Isp is ~200 s. In best case "green propellants" may approach but not exceed N2O4/MMH performances. $\endgroup$
    – Val
    Commented Jun 22, 2015 at 15:06
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    $\begingroup$ "that cost stacks up" From your own numbers it's unlikely to be more than a few thousand dollars a trip. Even assuming the alternative was free, that's not a huge saving - especially in terms of engineer-hours. $\endgroup$
    – NPSF3000
    Commented Jun 23, 2015 at 9:40
  • $\begingroup$ Reliability. Sometimes useful. $\endgroup$
    – Starship
    Commented Mar 21, 2023 at 0:30

2 Answers 2

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TL;DR:

The low technology readiness, the very, very low thrust, and the need for a catalyst bed means this was and still is the wrong technology for the intended purpose of a launch abort system and maneuvering in low Earth orbit.


Low technology readiness

SpaceX began working on Dragon V2 over five years ago. At that point, AF-M315E had a rather low technology readiness level. The technology readiness had been raised to 5 by 2013, but that isn't that great and that was far too late.

When designing a new spacecraft, one needs to very carefully pick and choose which low TRL items truly need to be used in the spacecraft design. The spacecraft itself is, by definition, at a rather low TRL. Every low TRL item used in the spacecraft needs to be looked at with a rather skeptical point of view. In the case of Dragon V2, SpaceX had engineers and operations people who were already familiar with hazardous propellants.

Very, very low thrust

The Green Propellant Infusion Mission referenced in the question is aiming at bringing 22 newton AF-M315E up to TRL 9. That's irrelevant with regard to the kinds of thrusters needed for a launch abort system for a human-rated spacecraft. Each Super Draco thruster provides 73,000 newtons of thrust. To provide the same level of thrust, the vehicle would need 26,500 of those 22 newton thrusters -- or it would need to increase the thrust by a factor of 3300. SpaceX took a huge risk in increasing the thrust of its Draco thrusters by a factor of 200. Proposing to increase thrust by a factor of 3300 is beyond risky. This is TRL 1 territory.

Need for a catalyst bed

The Draco and Super Draco thrusters use nitrogen tetroxide and mono methyl hydrazine. Igniting this mixture is easy: Mix the two and they ignite. They are hypergolic. In contrast, the monopropellant proposed in the question needs a catalyst bed to trigger ignition. There's a big problem here: Catalysts beds don't scale up to the size of thruster needed by Dragon V2. Large engines have an annoying tendency of ingesting catalyst beds. Catalyst beds are by their very nature a bit flimsy. They have to be; gas needs to flow through them. They don't scale up that well, particularly so in rocket engines.

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    $\begingroup$ Excellent answer, thank you. I suspected the catalyst issue may be a showstopper, and your answer seems to confirm that. Is there any possibility of the catalyst bed problem being solved and reaching a mature TRL in the next few decades? (Yes, sorry, I'm asking you to gaze into a crystal ball). If not, does AF-M315E remained confined to the realm of low thrust applications forever? $\endgroup$
    – marked-down
    Commented Mar 9, 2015 at 1:37
  • $\begingroup$ There have been some hydrogen peroxide engines (which need a catalyst) with fairly high thrust, so it doesn't seem impossible. $\endgroup$
    – Hobbes
    Commented Mar 9, 2015 at 9:50
  • $\begingroup$ @Hobbes - The largest monoprop hydrazine thruster I can find is Aerojet's 800 lbf (3560 N) MR-80B thruster. That's a long ways off from the 16,400 lbf (73,000 N) Super Draco thruster. $\endgroup$
    – David Hammen
    Commented Mar 9, 2015 at 13:06
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    $\begingroup$ @Hobbes - First off, that used a hydrogen peroxide+kerosene bi-propellant. No hydrazine! Secondly, the silver-plated mesh catalyst needed to start hydrogen peroxide decomposition (particularly when used as a bipropellant) is quite strong and durable compared to the pelletized iridium or ruthenium catalyst beds needed to start hydrazine (and apparently AF-M315E) decomposition. Iridium is hard and brittle, and doesn't deposit well on strong materials such as steel or nickel. Silver is ductile and malleable, and it's very easy to make silver-plated metals. $\endgroup$
    – David Hammen
    Commented Mar 10, 2015 at 16:31
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    $\begingroup$ The catalyst beds used in the shuttle hydrazine-fueled APUs were not metal frames, but porous cylinders filled with granular Shell 305 catalyst. I am not familiar with hydrazine monopropellant thrusters but I would imagine they use a similar design. $\endgroup$
    – Organic Marble
    Commented Sep 9, 2015 at 15:20
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  • The success of SpaceX is based mostly on using the best of well-proofed technologies, not on bold innovations. There are innovations in production process and in business model, but very few in rocket design.
  • Highly energetic monopropellants are very dangerous. Hydroxylammonium nitrate (AF-M315E) is rated as an explosive. It is probably unacceptable for human rated propulsion, and generally is not tested.
  • Monomethylhydrazine, dimethylhydrazine, and N2O4 are nasty chemicals but there is a solid experience with maintenance of these hazardous liquids. In fact, they work perfectly as propellants.
  • Development of new thruster on MMH/N2O4 is backed by enormous data base and experts' knowledge, as hundreds of such thrusters have been designed and are in use. Development of a new engine based on a new propellant may take long time, maybe decades. For example, methane based propellants have been under investigation for many years. There is no operational methane rocket, despite of benefits of the methane as rocket fuel.
  • There are other nontoxic propellant combinations as have be mentioned in comments. All they are bi-propellant.
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    $\begingroup$ I wouldn't call how SpaceX is using cross-feeding in the Falcon Heavy a "well-proofed" technology, let alone its attempts to land on a barge. SpaceX uses low TRL technologies when it makes sense. In the case of this question, it makes zero sense. $\endgroup$
    – David Hammen
    Commented Jun 24, 2015 at 16:29
  • $\begingroup$ The success of SpaceX not includes yet attempts to land on a barge nor cross-feeding. Highly successful Falcon 9 and Dragon spaceship are rather conservative than innovative. The future will show will the reusable VTVL rockets repeat this success. By the way, cross-feeding was theorized by Hermann Obertht - the mentor of Von Braun. $\endgroup$
    – Val
    Commented Jun 24, 2015 at 17:34
  • $\begingroup$ You missed my point, @Val. SpaceX is willing to take sensible risks. In addition to the two I mentioned, they are also looking into methane/LOX, once again a low TRL item. On the other hand, the "green" propellant referenced in the question is not a sensible risk. $\endgroup$
    – David Hammen
    Commented Jun 24, 2015 at 17:56
  • $\begingroup$ (1) Any new propellant is a sensible technological risk. Propulsion systems development shows slow progress historically. AF-M315E was discovered in 1970s. Very little progress since then. (2) I am not sure SpaceX willing to take sensible risks. Their costumers do not like risks. The public image of company as bold innovator is result of well-orchestrated propaganda. In some respect the question we try to answer may reformulated as: why my favorite company do not match the image I believe in?... I do not like to continue this debate because it became too personal. $\endgroup$
    – Val
    Commented Jun 25, 2015 at 0:51
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    $\begingroup$ Falcon Heavy crossfeed has been shelved. $\endgroup$
    – Vladimir
    Commented Jan 2, 2016 at 6:00

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