The gas selected as the propellant to be used in the Gen1 Enterprise’s ion propulsion engines will depend mainly on three things:
- Propellant efficiency usage in the engines (known as the specific impulse of the engines)
Setting safety aside for the moment, there is a trade off to be made between the first two items. It may be a classic case of cost vs. performance.Most ion propulsion engines today use Xenon as the propellant. But Argon can be used as an alternative propellant, and it’s much less expensive. Argon may not work as efficiently as Xenon in some engine designs, but it provides a very high cost savings. Argon is $18 per pound while Xenon is $2273 per pound. If 220 million pounds of propellant need to be sent into space to fill the propellant tanks on the Enterprise (55 million pounds) as well as the tanks in three propellant depots (55 million pounds each), then the cost savings when using Argon is enormous. Here is the cost comparison:
- Xenon: 220 million pounds x $2273 = $500 billion
- Argon: 220 million pounds x $18 = $4 billion
Xenon is simply not an option for a ship as large as the Enterprise. Buying the Xenon gas would take up half of the entire $1 trillion budget for the Gen1 Enterprise. Argon, on the other hand, uses less than 1% of the total Enterprise budget. If the purchasing of Argon is spread over 15 years, only $.27 billion per year is spent on buying propellant. This is quite manageable given that around $50 billion is spent in an average year on the Gen1 Enterprise program.
220 million pounds of propellant may sound like a lot. But when you consider that every Saturn 5 rocket used 5 million pounds of propellant, 220 million pounds becomes more comprehendible. The Gen1 Enterprise and the three propellant depots combined need about the same mass of propellant as forty Saturn 5 rockets. The tanks on the Enterprise alone carry about the same mass of propellant as ten Saturn 5 rockets.
Hydrogen might also be considered as the propellant, and its cost is even lower than Argon. It’s also attractive because hydrogen could be mined on the moon, Mars, and on an asteroid. For example, water could be collected on Mars and converted into hydrogen and oxygen after processing onboard the Enterprise. This fits with the spirit of doing experimental mining and manufacturing aboard the Gen1 Enterprise as well as fitting with the goal of the Enterprise acquiring some of its supplies on its own. Hydrogen may also well be the preferred propellant of future generation Enterprises. Later Enterprises might use scoops or electromagnetic nets to capture hydrogen that is scattered thinly throughout interstellar space. So gaining experience with hydrogen as a propellant in the Gen1 Enterprise has some value.
But Hydrogen, if not handled properly, raises safety risks (think of the Hindenburg). Hydrogen is odorless, and leaking gas, if ignited, will cause an explosion. This safety issue is a big negative. For the Gen1 ship, where safety is an overriding concern, using hydrogen presents a significant risk. Above all else, the Gen1 ship must be safe. After all, this will be the first USS Enterprise spaceship ever constructed, and it must prove the worthiness of the Enterprise program so that future generation ships will be built.
However, if the hydrogen is stored as water (H2O), and the hydrogen is created on a as-needed basis, this would likely eliminate the safety concerns. Electrolysis can be used to break the water down into hydrogen and oxygen.
In the end, the development team will have to make a decision regarding what propellant is to be used based on all of the factors mentioned above. If the safety concerns of hydrogen can be overcome, hydrogen would be the preferred propellant when the ship’s propellant is send up from Earth.
It should be noted that Argon can also be mined on Mars. The Mars atmosphere is 1.6% Argon. Furthermore, the VASIMR ion propulsion engine, one of the most advanced engines today, can already use Argon as its propellant. If the designs of the Gen1 Enterprise engines end up being based on a scaled up version of the VASIMR engine, this is one more reason in favor Argon.
On the other hand water is on Mars and on asteroids where it can be mined and transferred to the Enterprise. After electrolysis hydrogen would be available as propellant.
An additional goal is to design the engines to be able to work with more than one propellant gas. A single type of propellant might be used most of the time, but other gasses could be used when they are more available or at least on an experimental basis to gain learning for future Enterprises. For example, the engines might use hydrogen during many missions, but Argon could be used some of the time instead.
Furthermore, it is desired that the engines can use other gasses, mainly nitrogen and CO2. Nitrogen can be harvested via air scoops from the Earth’s thin upper atmosphere by the Enterprise when does aerobraking when returning from a mission. Likewise, when the Enterprise arrives at Mars or Venus, aerobraking can be used to harvest CO2.