After decades of development in green propulsion of various kinds, we are seeing the first substantial industry-wide uptake of nitrous-based propulsion as a replacement for hydrazine.

The uptake spans thrust classes ranging from 1N – kN, all major space markets (USA, Europe, Asia), and a broad spectrum of users – constellation owner/operators, integrators and space transportation service providers. As of writing, there are 50+ thrusters on orbit on 12 satellites using nitrous bi-propellant, and hundreds more on order from 15+ companies and space agencies.     

Dawn Aerospace has led the development of nitrous-based thrusters since 2018, with the first bipropellant thruster demonstrated on orbit in January 2021. Since then, we’ve seen several other providers start work on nitrous-based systems, including Impulse Space, led by ex-SpaceX head of propulsion Tom Mueller and Launcher, a USA-based space transportation company.

Europe has been an early adopter and looks set to continue using the technology in both government and commercially-backed missions.

At Dawn Aerospace, we consider this traction to be an early sign of an industry-wide transition to nitrous-based technology for in-space propulsion.

By Stefan Powell, CEO & Co-founder

As we enter 2023, I wanted to update you on a trend we’re witnessing in the satellite propulsion market – the proliferation of nitrous-based propellants.

Dawn Aerospace is a Dutch, Kiwi, and now US-based space transportation company - we’re in the business of nitrous-based propulsion, and it’s hardly surprising we think it’s great. That said, we’ve seen a significant increase in queries – from prospective customers, regulators, and trade press - about the potential of nitrous to become an industry standard.

 While we see ourselves as a leader in this technology, I’ll say upfront that several other credible companies are delivering nitrous-based systems. This article aims to collate these trends and give some context as to why the industry is moving in this direction. We hope this will provide you with the information you need to let you decide for yourself if this technology is something you want to know more about.

Who is developing nitrous-based propulsion and spacecraft?

Dawn Aerospace
In 2017, we selected nitrous oxide and propylene (aka propene) as our “future in-space transportation propellants”. Although we also experimented with ethane as the fuel, which we also see as a viable substitute for propylene, the outcome of our investigation determined nitrous was the best and only real choice for the oxidizer.

In January 2021, D-Orbit’s ION SCV002 ‘space tug’ satellite was launched, which, later that year, demonstrated the first firing of a nitrous bi-propellant thruster in space. It has since demonstrated altitude and significant inclination changes, totaling hundreds of burns. D-Orbit has remained our most prolific customer, with nine spacecraft in orbit successfully using Dawn propulsion.

In the five years since its founding, Dawn Aerospace has seen 51 of our thrusters reach space, and over 200 are with customers destined for space or under contract to be delivered this year. Our development has centered around 1N and 20N class thrusters, with a 200N class thruster in development. The CubeDrive product line is for CubeSats and bolt-on collision avoidance kits, while our SatDrive products are for larger satellites. They use standardized interfaces and flexible form factors to suit Microsats, ESPA Grande, and larger.

Other nitrous propulsion developers
There are at least seven other companies or institutes developing components such as rocket engines, including the Deutsches Zentrum für Luft- und Raumfahrt (DLR), who have been working on the technology since at least 2015. Their studies, led out of their Lampoldshausen facilities, have focused on premixed combinations and demonstrated good performance across various conditions. TNO in the Netherlands, and Moog in the USA, have also been involved in similar nitrous-premixed experiments.

Gate Space in Austria, and Valiant Space in Australia, have active development projects for nitrous-based thrusters for the commercial market.

Two (known) US companies are actively working on nitrous-powered spacecraft, including in-house developed propulsion systems. Impulse Space, headed by Tom Mueller, ex-VP of Propulsion at SpaceX, plans to launch its first spacecraft in late 2023. Launcher, a California-based company, has already deployed a space tug as of January 2023 and is undergoing testing and commissioning. Both systems are in the kN range and use ethane as fuel in combination with nitrous oxide.

What missions are using nitrous?

Here are some (publicly known) nitrous propulsion users grouped by application. Many of these are Dawn customers - please note we have a number of customers we are yet to name publicly, and this list is not exhaustive.

Space tugs / Orbital Transfer Vehicles (OTVs)
Good Isp and system performance enable high delta-v space tugs. High precision due to low impulse bits enables the possibility of rendezvous and docking in the future.

• D-Orbit SCV001 to 009

• Launcher — Orbiter (in-house build propulsion)

• UARX

• Impulse space (in-house build propulsion)

Earth observation constellations and integrators
Low lead times let these constellations prototype quickly now. Low costs at high production volume enable them to scale in the future.

• Pixxel – Indian/USA Hyperspectral imaging constellation

• Zentrum für Telematic - German constellation measuring volcanic activity

• BRIN (previously LAPAN) – Indonesian Tsunami early warning system

• Blue Canyon Technologies – BCT uses Dawn SatDrive systems as a highly modifiable offering, allowing them to tailor the propulsion system as needed for their customers

Other users with high delta-v requirements

• AstroForge & OrbAstro – asteroid mining

• ALE – previously launched on dedicated launch. High-performance propulsion enables the use of lower-cost rideshare launch

Licensing, regulation, and heritage

Nitrous systems, specifically those produced by Dawn, have flown on Vega, Soyuz, and Falcon 9, each instance through a different set of launch licensing and compliance standards. In all cases, the regulatory challenges, although significant due to the unique and unfamiliar nature of the propellant, were not insurmountable. In fact, once the propellants and associated risks were understood, it was found that regulators and launch site staff were willing to relax requirements normally placed on hydrazine systems. An example of this was while fueling a customer satellite at Cape Canaveral for a Falcon 9 launch -we were cleared to conduct propellant loading in the assembly hall, outside the dedicated fuelling facility, due to the non-toxic nature of the propellants. This not only saved cost but meant that we were not subject to the tight scheduling normally associated with the propellant loading facility.

Flight heritage

When accounting for Dawn and Launcher, there are 52 nitrous bi-propellant thrusters aboard 12 satellites currently in orbit. The satellites of D-Orbit, with thrusters built by Dawn, have performed (an estimated) hundreds of burns, over 20 kN.s of total impulse, with no reported misfires.    

All satellites to date have remained in Low Earth Orbit (LEO).

Why nitrous?

If you have read this far, I imagine you want to know why so many companies are looking at nitrous-based propulsion for their satellite.  There are very few options available among chemical propellants, particularly when it comes to oxidizers.

One of the main reasons to choose nitrous is because all alternative propellants (specifically oxidizers) have at least one major drawback. Skip to the ‘Nitrous oxide and propylene’ section for why we use nitrous. Read on for how we ruled out other propellants before we landed on nitrous.

Why green?

We have a love-hate relationship with the term ‘green’ propellant. Although an important factor, the reality is that no customer selects propellant based on toxicity and the associated environmental impact alone. For most customers, the choice to use green propellant is primarily driven by a solid business case.

‘Green,’ in this context, does not say anything about the environmental impact or carbon footprint of the propellant. Rather, it imprecisely refers to the idea that you can open a bottle without having to first put on something akin to a spacesuit (a SCAPE suit). For example, nitrous oxide, hydrogen peroxide, and ammonium dinitramide (I) are all considered ‘green,’ but I would not recommend consuming them or fertilizing your garden with them. They are also not doing the environment any favors. However, unlike hydrazine, they can be handled and stored with comparatively little fuss.

Traditional propellants – hydrazine being the prime example – come with enormous complexity and large fixed costs of associated compliance and handling, usually summing to $100–500k USD per satellite before any actual hardware is considered. This fixed cost is prohibitive for missions where the total cost per satellite should be in the order of $2-10M. Green propellants hold the promise of nearly eliminating these compliance and handling costs.

Isn’t HTP the obvious green propellant?

Hydrogen peroxide (aka high test peroxide or HTP) has always been a prime candidate as a low-toxicity in-space propellant. At Dawn, we use HTP in our spaceplane demonstrator, and are generally comfortable with it and advocate for its use. A year-long study to find the best future fuel for in-space propulsion brought us to HTP and nitrous oxide as the two most viable candidates to replace hydrazine. However, we could not reconcile HTP’s failure modes associated with leakage and the literal “flow down” effects on the satellites around them. It’s also not storable, so for satellite missions that need to last more than twelve months, it’s not feasible. Thus, nitrous won out. But let’s explain our choice against our friend HTP, and why we see nitrous as the “rideshare friendly” fuel for satellites.

We know rideshare on large rockets is the most cost-effective way to launch the projected 20,000 satellites by 2030. These launch, often 50 or 100 at a time, all stacked on top of each other. Each satellite has a propulsion system on board with dozens of leak paths. Designers at Dawn (and other companies, I am sure) go to great lengths to reduce leakage as much as possible. We use welded connections wherever possible, qualify systems with the most advanced and strenuous leak-checking methods, such as vacuum helium leak checks, and do these tests on every component and then again at the system level before it leaves our building. Still, one cannot be entirely sure a system is leak tight. There is always a risk of leakage.

If each propulsion system has a 1/1000 chance of leaking at the launch site during or shortly after propellant loading, the chance of any one satellite leaking approaches 10%.

In the case of a propellant with a high vapor pressure that naturally vaporizes and disperses, such as xenon or nitrous, a small bit of leakage may be a manageable risk. It will surely reduce the performance for that one satellite, perhaps even to zero, if a leak is large enough, but it is unlikely to harm any satellite around it provided the bay/capsule the satellites are in is ventilated, as it normally is.

In the case of HTP, propellant leakage can be disastrous. Its low vapor pressure means it will not readily vaporize and thus collects and drips like water. Anything below the leak could have corrosive propellant drip onto them, damaging hardware. 

The above would be manageable if it were not for the final Achilles heel of HTP – its reactivity. HTP will decompose into water and oxygen when in contact with almost any impurities. This leads to a dramatic volume increase, and when in a confined space, pressure increases as well.  This means a pressure relief valve must vent all pipework to ensure it does not over-pressurize during storage. But this leads to intentional leakage, the very issue we previously discussed. Of course, leakage is preferable to an uncontrolled disassembly, but in the case of rideshare, the fact that leakage is a required mitigation for a more serious failure mode becomes a major and unresolvable risk. 

What about the other green propellants?

The development of new propellants such as ADN (LMP-103S) and ASCENT (AF-M315E or HAN) have been fraught with issues, especially when scaled. They have had several high-profile, on-orbit failures, highlighting the difficulty of the technology despite over a decade of development. Development of higher-thrust classes, anything above 1N, remains slow.

The supply chain for these specialist chemicals is also very limited, and some of the more popular versions are ITAR controlled. This not only significantly constrains customers trying to buy fuel for their satellite, but it inhibits students, universities, and startups worldwide from developing systems that use these fuels. Thus, the supply of ADN/ASCENT systems comes exclusively from established industry, and is not likely to be challenged any time soon. As a result, prices for these systems remain high, and in many cases, higher than the hydrazine systems they were supposed to replace! This would leave the customer wondering why they chose ‘green’ fuels in the first place.

The “other” green option: electric

It’s worth briefly stating why our customers chose chemical propulsion over electric. There are various advantages, but the key reason is its significantly higher thrust when compared to electric propulsion systems, allowing satellites to be on station faster (in many cases 6+ months faster), providing higher satellite utilization and potential revenue. An extra six months of operations can substantially impact profitability, especially for LEO satellites with a total useful lifespan of less than five years.

For GEO satellites, much of their radiation tolerance budget is taken by significant time spent in the Van Allen belts during slow orbit-raising maneuvres. A chemical propellant transfer from GTO to GEO can significantly reduce this requirement, which we’re developing with our 200N-class thruster.

Decreasing launch costs and the growth in rideshare launches has mitigated some of the historic drawbacks of chemical propulsion, such as high mass of propellants. We see this trend continuing for the foreseeable future.

Nitrous oxide and propylene

Dawn’s underlying technology is based on nitrous oxide and propylene, utilizing a self-pressurizing configuration. N2O as the oxidizer, and C3H6 as the fuel. Both fuels are non-toxic, REACH compliant, and commonly available. Nitrous and propylene were selected as our propellants of choice for in-space propulsion for several reasons. We will briefly go through each.

Top four reasons we chose nitrous

1. Simplicity leads to industry-beating lead times and unmatched system performance.
   Nitrous and propylene are self-pressurizing and do not require pumps, pressurants or even propellant management devices. This dramatically reduces the number of components and simplifies those remaining, allowing us to quickly redesign key components such as tanks to meet a new customer’s need. Combined with additive manufacturing, the result is a relatively quick and low-cost customization process that is not limited to standard off-the-shelf components. 

   What’s more, because so many components are not required, the system-level performance is fantastic, despite the relatively low storage density and high tank mass usually associated with self-pressurizing propellants.  

2. Safe failure modes and being rideshare friendly.
   As per the section on HTP, we like nitrous (and propylene) because, should it leak, it disperses. Very few propellants have this fantastic property.

3. Extreme scalability – so that every satellite can benefit.
   Nitrous-based systems have been shown to work from 1N-25kN. It is a highly flexible propellant that can be used from CubeSats right through to large interplanetary transfer stages. No one will get left behind.

4. Ready for spaceflight when refueling becomes the norm.
   Electric ignition-based and highly repeatable, the system is cold-start capable and contains no catalysts, meaning they work for both steady-state and tiny pulse operations – key for rendezvous and docking.

   We have conducted tens of thousands of restarts with no measured degradation in start-up performance. We have yet to discover the actual limit, but it could conceivably be in the millions of starts and days of runtime. Unlike catalyst-based thrusters (Hydrazine, HTP, ASCENT, and ADN), there is likely almost no limit on total throughput. Tanks can be refilled, in principle, indefinitely, just like your car. This is the future of space transportation that will allow us to transcend the tyranny of Tsiolkovsky’s rocket equation.

Current drawbacks and the future roadmap

Nitrous is far from the perfect propellant, and our current generation of thrusters are equally imperfect. There is still much to improve and prove before we, as an industry, can say we have replaced hydrazine. After all, 70 years of heritage is not replaced overnight.

Nitrous systems are only available in limited sizes. Currently, only 1N and 20N systems are available. Although demonstrated at the 200N and kN range, no mature products are available on the market, making larger transfer stages still out of reach.

Nitrous thrusters burn very hot and have limited capabilities either as a film or regenerative coolant. This makes them particularly challenging to develop as a steady-state burning thruster. Consequently, many do not reach thermal equilibrium except at low-performance mixture ratios. This limits either the specific impulse[1] or the maximum impulse bit[2] available before the thruster must be allowed to cool down. 

Nitrous systems have yet to be proven in deep space. They rely more heavily on electronics for their high-voltage ignition systems. While reliable and repeatable on the ground and in LEO, they must be proven capable of surviving and performing in the high radiation environments of GEO, Lunar, and deep space.

Also related to deep space – Nitrous systems are, in theory, capable of survival and, in some cases, operation down to -40°C. This makes it uniquely suited to deep space operations where power for heating propellant systems may be limited. However, this has yet to be proven.

Conclusion

A lot has changed in the last two-and-a-half years since the first nitrous-based thruster was launched. The core technology is now proven on a dozen satellites, primarily from European companies. However, it is already scheduled to be flown on a wide variety of satellites, from 5-500kg, and from both commercial and government customers worldwide.

While currently only proven in LEO, some users are already planning to use it to visit asteroids or Mars, signaling their confidence in its performance and appropriateness for even the most challenging missions.  

The fundamental properties of nitrous make it well suited to the most important needs and trends of our industry – it’s low cost, highly adaptable to customer needs, highly scalable across satellite sizes, and appropriate for rideshare launch.  

We believe this substantial traction from multiple, credible, and independent developers constitutes the early stages of what could turn out to be an industry-wide trend.

If these trends in demand and widespread development continue, nitrous-based propulsion is set to become the prime candidate to replace hydrazine, industry-wide.  

If you’re interested in a friendly chat about whether Dawn can support your team, please contact sales@dawnaerospace.

[1] Rocket motor fuel efficiency

[2] Maximum amount of total ‘push’ you can get out of a rocket motor in one firing sequence before it must be shut down