Delft, The Netherlands - 18 July 2024

Electric propulsion systems (EP) have gained popularity in the low Earth orbit (LEO) propulsion market over the last decade, largely due to their lower launch costs. This is driven by their relatively high fuel efficiency (specific impulse - Isp) which reduces propellant mass and therefore launch costs. These savings have come at the expense of other factors —largely that electric propulsion is slow, power-hungry, and less responsive. 

Recent reductions in launch costs, now approximately 1/20th of what they were during the space shuttle era, are expected to continue decreasing even further. This trend is making launch costs a relatively minor factor in mission budgeting. Given these changes, does it still make sense to prioritize trade-offs involving reduced thrust and responsiveness? 

We look at some key decision factors for satellite operators when choosing between chemical and electric propulsion: 

1. Fuel Efficiency:

Specific impulse (Isp) measures the amount of thrust produced per unit of propellant. As mentioned above, EP is the clear winner here. It’s incredibly fuel-efficient with Hall Effect thrusters having Isp values typically around 1,600 seconds or even higher. 

By comparison, Dawn’s B20 thrusters have an Isp of 277 seconds. This is more than enough to sustain most missions in LEO, including orbit raising, station-keeping, and debris avoidance maneuvers. Dawn’s thrusters have gained significant heritage in LEO with 76 thrusters in orbit.  

However, for high delta V missions, such as interplanetary or deep space exploration, chemical propulsion systems would require very large amounts of propellant, making electric propulsion the most suitable choice. 

2. Lost Mission Revenue:

Satellites with chemical propulsion typically reach their operational orbit quickly, from mere hours to 2-3 days, while electric propulsion is slow—it typically takes 90 days to reach orbit. For a satellite that is producing $20,000 of revenue per day, that’s a revenue cost of $1.76 million. In constellations, this figure can be many multiples higher. Where getting to operational revenue quickly is a key factor for a mission, then chemical propulsion remains the better choice. 

3. Responsiveness:

In an increasingly crowded LEO environment, modern satellites must not only be equipped with propulsion to avoid debris but be able to utilize those propulsion systems quickly. Additionally, “tactically responsive space” is language that has been commonly adopted by the Space Development Agency (SDA) in recent times—this means having the ability to move away from threats in space, respond to on-earth events for observation, and being able to replace any lost assets quickly.
Electric propulsion systems are generally unsuitable for rapid maneuvers due to their slow start-up and longer time to reach operational orbit. By comparison, chemical propulsion not only means satellites can get to where they need to go fast, Dawn’s propellant combination is cold-gas capable. The systems can bypass their usual ignition to produce instantaneous thrust in situations where urgency is required, making them ideally suited for rapid response situations. 

4. Thrust Requirements:

Chemical propulsion systems have a much higher thrust-to-weight ratio than electric propulsion. For maneuvers requiring high thrust, such as lunar orbit injections and LEO to GEO transfers, chemical propulsion is generally preferred. 

5. Deorbiting:

When choosing between chemical and electric propulsion for deorbiting, the ability of a satellite to fully burn up upon re-entry (demisability) is not dependent on the propulsion type but rather on the satellite's design and materials. However, the time spent deorbiting is critical: chemical propulsion allows for rapid re-entry, minimizing time off-mission and maximizing revenue generation 

6. Upfront Cost:

EP systems typically cost more up-front. They contain more complex and expensive components such as solar arrays and power management systems than typical chemical systems. By using self-pressurizing gases, Dawn has reduced the complexity even further by eliminating the need for complex pressurization systems.  

7. Propellant Availability & Storage:

EP systems typically use noble gases such as Xenon, Krypton, and Argon, with Xenon being the most popular due to its higher Isp. Xenon is very rare and found only in trace amounts, so prices are known to fluctuate widely, and availability is severely constrained, limiting the ability to scale.  NASA famously had to purchase and stockpile Xenon years in advance for its Dawn mission, the first deep space probe to orbit extraterrestrial bodies. 
 
Dawn’s propellants, nitrous oxide (N2O) and propene (C3H6), are widely used, easily domestically available, non-toxic, and very easy to store. But not all chemical propellants are created equal. For example, Hydrazine has a long shelf-life but is incredibly toxic to handle, which comes with significant handling costs. 

8. Fine Pointing Maneuvers:

EP is usually favored over chemical propulsion to perform very fine pointing maneuvers because of its very low thrust levels, something not possible for traditional high thrust chemical systems like hydrazine. However, not all chemical propulsion has this limitation. Dawn has developed a 1N thruster that, with its propellants, nitrous oxide, and propene, can operate in cold-gas mode and perform the same fine pointing tasks at low thrust levels. It is common in Dawn’s propulsion systems to combine B1’s and B20 (20N) thrusters in the same system to enable both large and small maneuvers. 

Summary 

Ultimately, the choice between electric and chemical propulsion depends on the specific mission requirements. But with launch costs becoming a smaller factor in mission planning, other factors take on more importance such as destination, duration, power availability, revenue opportunity, and budget constraints, to name a few. 

Electric propulsion is well suited for deep space and long-duration missions because it can generate power for a long time through solar arrays, from a very small amount of propellant. 

But for missions where revenue, responsibility and responsiveness take high priority, chemical propulsion systems provide quick orbit insertion, responsible re-entry, and instantaneous thrust and is often the better choice. 

Although traditional chemical systems have some disadvantages in areas such as handling and toxicity, and an inability to perform low-thrust maneuvers, innovations such as Dawn’s cold-gas capable thrusters and non-toxic propellants present viable, low-cost alternatives.