# Why is the Deep Space Industries Prospector 1 using water propellant instead of hydrazine?

A lot of satellites use hydrazine propellant but the Prospector 1 is using water. What are the benefits and drawbacks of that decision?

Unlike chemical rockets such as hydrazine (a monopropellant, which doesn't require separate oxidizer and fuel to burn), water is just the reaction mass — not the energy source. The Comet-1 thruster that the DSi Prospector series is using is electrothermal, so it requires a substantial electrical power supply in order to energize the water (presumably by turning it to steam). According to their data sheet,

Comet-1 thrusters are the ideal balance of cost and performance, occupying a place in the market between low-cost, low-performance cold gas and resistojets, and high-cost, high-performance monopropellant and electric systems.

In other words, the tradeoff is that in exchange for lower cost (and In-Situ Resource Utilization, ), $I_{SP}$ and probably specific power are lower than hydrazine, and $I_{SP}$ is much lower than ion engines. Since the Prospectors are intended to refuel as they go, and therefore they don't need to carry anywhere near all their fuel at launch, low $I_{SP}$ (150–175 s) isn't quite such a problem, but it's still a limiting factor.

Suppose the mission is to harvest two asteroids close enough that only 250 m/s $\Delta V$ is necessary to reach them from a suitable parking orbit. Each trip, the miner retrieves 10% of its dry mass in payload and leaves it in the same parking orbit. (These are made-up numbers, but they shouldn't be too far off, and things get more dramatic with higher $\Delta V$ or more payload.) With a 450 s $I_{SP}$ from a $LH_2 / LO_2$ chemical rocket (the best you're going to get), that requires a fuel fraction of about 21%, which is not too bad. But with the Comet-1, you can launch with only enough reaction mass to get to one of them and a tank large enough to contain a full round trip, then refuel at every asteroid harvested. The result is launching with only about a 14% remass fraction, which (even with the larger tank) is likely a cost improvement. And that stays the same no matter how many missions you go on, whereas with ten asteroids, $H_2 / LO_2$ requires a starting fuel fraction of about 69%, which is 2½ times the wet mass (starting mass) of the two-asteroid mission. (Doubling the asteroids to twenty increases the wet mass by almost another factor of 3, to 8 times the mass for two.)

Of course, the eventual plan at Planetary Resources is to electrolyze the water brought back and store $LH_2$ and $LO_2$ in orbital fuel depots for further use. This has all the advantage of easy refueling between trips and the much better $I_{SP}$ of hydrolox without the larger solar panels on the craft, but requires more complicated engines, routine fully automated docking, and, crucially, long-term storage of cryogenic fuel in space, which has not yet been done and probably relies on cryogenic selective surfaces to be practical.

I used this rocket equation calculator for the initial figures, then automated calculating $\Delta V$ for ten- and twenty-asteroid missions with this PowerShell script:

$dry = 1000$pay = 100
$dv = 250$v_e = 450*9.81
$asteroids = 10 # or 20$mass = $dry 1..(2*$asteroids) | ForEach-Object {
if ($_ % 2 -eq 0) { # Outgoing trip, no payload$mass -= $pay } else { # Incoming trip, payload$mass += $pay }$mass *= [Math]::Exp($dv /$v_e)
}
\$mass

• I honestly don't know anything about this topic, but reading "the Prospectors are intended to refuel as they go" sounds like a pretty good reason to use water. A lot easier than finding some hydrazine. – Steve Aug 15 '16 at 20:52
• Where'd you get the idea that LH2/LO2 engines are only 340s Isp? The SSME has a better Isp than that at sea level, and much better (452s, approximately 33% more) in vacuum. Even much smaller and less-complicated H2/O2 rockets like the RL10 used on the Centaur (Atlas V upper stage) manage at least 450s of Isp. – CBHacking Dec 1 '16 at 8:14
• @CBHacking: I'm not sure, tbh; I was working off memory and, obviously, was mistaken. I've reworked the figures; the main point stands, although it's less impressive. – Nathan Tuggy Dec 1 '16 at 8:21