Ben W
Chess Grandmaster (Supervised)
This is super informative. Thanks!At the risk of stating the overtly obvious, it’s worth noting that electric propulsion requires a massive amount of DC power to charge the field and huck molecules out the business end. The good news is that it’s a very compatible concept with communications satellites (most GEOs, most megaconstellations, etc) as they already have large power subsystems to supply the power hungry payloads….but for a mission that doesn’t otherwise need a ton of power (imaging sats, a fuel depot, etc) it can result in an EPS materially bigger than is otherwise necessary.
In the case of putting EP on Booster that would mean a massive solar array, batteries, chargers, etc. All totally doable for sure...its just going to be on an unprecedented scale. For reference, PPE--which near as I know is the largest contemplated EP system to-date--has something like a 50kW power subsystem and is going to push around 40T worth of mass. An EP Booster will be probably mass an order of magnitude or more higher.
From the internet engineering perspective Its also worth throwing in here that EP tanks are really high pressure--MEOP is in the 400+ bar range and the load is in the 300's. So...they kinda need their own tanks. In the contemplated re-use concept the pressurant tanks would likely suffice, though volume would likely be an issue.
And finally, just for funsies, its worth noting that both the Xenon and Krypton markets have been MASSIVELY impacted by Vlad's antics, since much of the world's supply--especially Xe--comes from the Ukraine. The price has gone up like 6-10x since he decided to wage not-war, and its super fluctuatey and generally difficult to procure. Plenty of folks are scrambling to develop alternatives but its not easy because the efficiency of Xe is hard to beat--that's why it was chosen in the first place for EP. Krypton is above Xe on the table and so its lower molecular mass makes it that much less efficient. Iodine is probably going to emerge as the winner (Busek is already advertising it) as its pretty heavy (its right next to Xenon on the table) and also can be stored as a solid so the reduction in storage vessels brings a lot of volume/mass upside there.
I sort of pictured that the krypton/xenon would be cryogenically liquefied in the tanks, rather than stored as a high pressure gas. They have similar boiling points as methalox. (I suggested krypton because it's 10x cheaper than xenon, Vlad notwithstanding, though still admittedly very pricey.) The high pressure would mostly come for free as the propellant converts itself from liquid to gas. For big missions, an RTG or very small nuclear reactor (think nuclear submarine but smaller) might work for providing constant power for electric ion propulsion, if not VASIMR directly. Iodine may get around some of these issues, as it's cheaper (10x again cheaper than krypton), and doesn't need to be cryogenically cooled, though I don't see how the methalox tanks could be repurposed for it. Just thinking out loud, and I am admittedly not an expert on this!