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Transitioning space propulsion to a nitrous-based industry standard

The future success of the space industry’s supply chain depends on standardizing all aspects of a space mission, from human safety to the oxidizer used with fuel. This collective piece dives into how nitrous oxide is the optimal choice featuring input from: 

  • Eric Anderson of And One Technologies
  • Max Haot of Vast
  • Stefan Powell of Dawn Aerospace
  • Tom Mueller of Impulse Space
  • Umair Siddiqui of Phase Four

Tactically responsive space 

A few weeks ago, for the first time, the US Space Force requested $60M over the next two years for its tactically responsive space program. 

As tactically responsive space continues to gain momentum as a funding priority, propulsion technology will have to catch up. Satellites will have to be ready to launch within 24 hours notice, meaning satellite propellant loading must be seamless.

The US government is getting serious after years of lobbying from small satellite launch companies, but successful execution of the total launch program will require more than a budget line—it will require the most efficient satellite propellant process possible, including: 

  • A resilient domestic supply chain
  • Easy storage on-site for long periods of time
  • Competitive on-orbit performance in the form of a high Isp 
  • Expedient and safe loading operations with minimal PPE required (no SCAPE suits)

Enter nitrous oxide

Yes, just like in Fast & Furious. Nitrous-based systems are the fastest-growing category of green propulsion in the commercial space market. Suitable for all mission types, from LEO to deep-space, nitrous-based systems are scalable across many thrust classes. Government organizations like the European Commission, ESA, DLR, and the New Zealand Space Agency are already investing in nitrous projects. 

“I’ve always felt that for future spacecraft to be truly low-cost, the industry would need to move away from toxic propellants like hydrazine and instead optimize around bipropellants that prioritize safety but still have good performance,” said Tom Mueller, the chief executive officer of Impulse Space. “That’s why when I started Impulse, I knew we would use nitrous as the oxidizer for our engine developments.”

A resilient domestic supply chain: The U.S. propulsion supply chain is unsecured. The United States is heavily reliant on high-purity xenon and krypton, which are primarily manufactured in Russia, China, and Ukraine, as highlighted in a recent article by Phase Four CTO, Umair Siddiqui. There is also no domestic production of hydrazine, the U.S. default propellant of choice, with the majority of production happening in China. 

“For satellites with meaningful payloads that have significant propulsion requirements measured by large delta-v, xenon meets a need, but prices are high and lead times are long. Costs that reach millions of dollars and uncertain availability can break budgets and schedules,” said Eric Anderson, president of And One Technologies. “With nitrous oxide costs measured in thousands of dollars per satellite, mission owners can concentrate funds and effort on areas other than propellant.” 

Supported by other large industries like the medical industry, industrial-grade nitrous oxide is in high supply, domestically produced in the United States, and available for dollars per liter. There are at least seven N2O facilities in the US, producing 50 to 100 million lbs per year, according to ChemView. Fun fact: The production facilities of Air Liquide in Washington, Pa, are located at 1 Nitrous Lane. 

Propellant storage: N2O is a pure chemical compound. It does not demix, decompose, come out of solution, or crystalize. It does not degrade in any manner and is, therefore, theoretically infinitely storable. Tanks are transported directly to the launch site or integration facilities by trucks of the industrial gas supplier. It’s stored in industrial-grade gas cylinders within a ventilated area. Anyone that can handle a welding torch can manage these kinds of cylinders.

“We started building nitrous-based systems as it’s a high performance, yet safe propellant we could get our hands on,” Stefan Powell, the Dawn Aerospace CEO, said. “Turns out that commercial satellite builders love it for all the same reasons.”

Expedient loading operations: Propellant loading of a nitrous-based OTV that flew on a Transporter mission was done in the SpaceX Payload Processing Facility east bay in the general assembly clean room.  The ground operator that loaded each propellant into the orbital transfer vehicle (OTV) took roughly two hours each, so four hours total. PPE required through the process was safety glasses and hearing protection — no SCAPE suits were required. The only restriction at the time was that propellant loading needed to happen at night when fewer people were around.

French Guiana range safety personnel for the Vega rideshare took the same approach.  Propellant loading was allowed to happen within daytime hours. Safety screens were installed; however, other payload integrators could continue their business as usual within the same facilities.

State of the Nitrous Industrial Base

The commercial market loves it. Nitrous-based systems are the fastest-growing category of green propulsion. There are 50+ thrusters on orbit powering 12 satellites using nitrous bi-propellant, and hundreds more on order from commercial companies and space agencies. 

Public announcements of companies using nitrous-based systems include:

  • Impulse Space—Mira OTVs
  • Impulse Space & Relativity—Mars lander
  • Launcher (now Vast)—Orbiter OTVs

Satellites with Dawn Aerospace-built propulsion systems (non-exhaustive):

  • ALE—space entertainment
  • AstroForge & OrbAstro—deep-space asteroid mining
  • Blue Canyon Technologies—tailored propulsion systems as needed for their customers
  • BRIN, the Indonesian Space Agency—early tsunami-warning system constellation
  • D-Orbit—ION Satellite Carrier OTVs
  • Lynk—satellite-direct-to-standard-phone telecoms constellation
  • Hiber—Hiber 3 & 4 IoT CubeSats
  • Pixxel—hyperspectral imaging constellation
  • Sidus Space—LizzieSat line of satellites
  • UARX—Ossie OTV
  • Zentrum für Telematic—volcanic activity measurement constellation

Rideshare friendly: Nitrous-based propulsion systems have flown on every SpaceX Transporter mission, Starlink rideshare, and Arianespace’s Vega rideshare. They are planned to fly on Terran R. If there is a leak, propellants with a high-vapor pressure like nitrous naturally vaporize and disperse, so it is unlikely to harm any satellite or person around it as long as the bay or capsule is ventilated, which it usually is. This is the opposite to low-vapor pressure propellants like HTP and ASCENT.

High production and lead time friendly: These systems are designed for high-production rates with lower supply chain risk. 3D printing enables complex designs and regenerative cooling for thrusters, enabling the use of common metals like Inconel. Exotic metals like Niobium are no longer required to manage high temperatures, which in turn leads to higher production rates, more suppliers, lower costs, and lower risk. Nitrous-based systems bypass the catalyst beds used on traditional thrusters, like the Shell 405 catalyst, which can take more than 12 months to procure. Dawn is already supplying N2O-based systems to commercial customers within four to six months from the first order.

“Our plan is to ramp up to build 200 satellites per month and hit 5,000 satellites within two years,” said Charles Miller, the CEO of Lynk.

Storage pressure -> 3D printing: Storage pressures are relatively low compared to other propellant types, meaning unique designs can be made to comfortably comply with launcher pressure vessel requirements. This makes them well suited to rapid manufacturing techniques like additive manufacturing, dramatically reducing lead times to satisfy custom requirements. 3D printing enables custom tank designs, including cylindrical, spherical, and toroidal designs. 

But isn’t it dangerous?: Nitrous received scrutiny in 2007 due to a Virgin Galactic and Scaled Composites accident. Although the exact cause was never reported, the improvement plan in Cal OSHA’s Accident Investigation teams report included: 

  1. Conducting increased compatibility testing between N2O and any materials that contact it in the tank and eliminating incompatible materials in the flow path;
  2. Revising cleaning procedures to further minimize the risk of contaminants in the system;
  3. Replacing the composite liner in the N2O tank with a metal tank liner;

Nitrous oxide is a propellant, and all propellants must be treated with respect. That means proper training and knowing how to work with it in a safe, clean, and well ventilated environment and only using materials that are scientifically proven to be compatible—not epoxies that could decompose into the oxidiser to create a premixed solution. There is a long history of companies and institutes working with and using N2O correctly, which demonstrates its safety.

A future-proofed propellant for enduring advantages

Extreme scalability: Nitrous-based thrusters already range from 0.25-lbf (1N) to 240-lbf (1068 N) in thrust class. It is a highly flexible propellant that can be used for a range of programs, including CubeSats, large interplanetary transfer stages, Mars landers, and space stations.

Rendezvous and docking: Removing catalyst beds and moving to spark-based ignition systems leads to thousands of cold-start steady-state and tiny pulse operations. Thrusters can operate in both bi-propellant and cold-gas modes, independently of each other or in unison, unique for rendezvous and close proximity operations.

Simple on-orbit refueling and propellant gauging: Nitrous oxide-based propulsion systems use liquified gasses stored at saturation conditions in liquid-vapor equilibrium at typical operational temperatures of 14-95°F. Propellant transfer from a servicing depot to a client spacecraft can be achieved through thermal control, heating the tanks of the servicer to create a pressure differential, which transfers propellant from one tank to another. Once the transfer is complete, propellant within both the client and servicer tanks can be independently heated to the point that each propellant reaches a supercritical state. Pressure can then be measured, which will correlate to the expected amount of propellant mass in a tank. 

Don’t try to beat the cold; use it to your advantage: Colder deep-space environments are better for long-term storage of N2O, which has a freezing point of -130°F (-90°C). It doesn’t require constant heater power, so systems can be turned off and it will survive, unlike hydrazine, water, and HTP, which all have high freezing points. 

A dual-purpose propellant for orbiting human habitations: “We haven’t released in public details of our space station yet, but it’s no secret that we have a large nitrous based-team within Vast via the acquisition of Launcher,” said Max Haot, president at Vast. “As we work to expand humanity across the solar system, it would be smart to have a dual-purpose propellant that can also be used for surgery and dentistry, or distilled into breathable air.”

You can make it on Mars: Although it’s harder to make than liquid oxygen and methane, you can store it for many years. In fact, Impulse Space will use nitrous oxide as the propellant for the first commercial mission to land on Mars, in partnership with Relativity. 

Long-life reliability: Collectively, many tens of thousands of thruster restarts have been conducted with no measured degradation in start-up performance. Unlike catalyst-based thrusters, there is likely almost no limit on total throughput. Coupled with on-orbit refueling, this is the future of space transportation that will allow us to transcend the tyranny of Tsiolkovsky’s rocket equation.

To learn more about the companies leading the nitrous effort, read on: 

Dawn Aerospace: www.dawnaerospace.com/green-propulsion 

Impulse Space: www.impulsespace.com 

Vast: www.vastspace.com

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