Preface: I’m no rocket surgeon.
Wish they weren’t scribbling off SpaceX’s design so hard. It’s entirely possible that examining the conditions and dynamics of re-entry would lead to a very Starship-like design in the end, but there’s merit in exploring the possibilities. Especially since this is supposed to be a smaller vehicle, so it won’t be experiencing the same exact conditions as Starship at any stage of flight. Fingers crossed that it really is just a render for publicity and the actual design will be informed by physics, not FOMO.
But it certainly puts a smile on my face to see ESA gradually pulling its head out of its ass and realizing that reusability isn’t a fad.
If they have to copy something, I wish they would pick the Stoke upper stage. I feel like the capsule reentry has to be way easier than the Starship one. I’m sure the aerospike would add a bunch of issues. And the architecture is still unproven. It just seems like a less insane vehicle that will scale up and down better.
A capsule design will likely never be fully reusable, since it doesn’t allow you to shield the second stage engine(s), propellant tanks and anything else that usually goes in the trunk section. For a fully reusable design, you need to go with either a spaceplane or whatever it is that we’re calling the Starship’s cylinder-with-control-surfaces design.
EDIT: Looked into Stoke’s design a little more. It’s a fascinating idea and I will be looking forward to test launches!
There’s one more option - propulsive re-entry. You point your big engines retrograde and burn until your velocity is effectively zero, then descend on a <1g burn. Won’t need heat shields if you don’t have to endure re-entry heat. But this ain’t happening until we develop ludicrously more efficient high thrust engines and orbital fuel production.
If they figure out orbital refueling, they can reduce the launch weight, reload, and have controlled, powered descent.
Long-term, even better if they figure out orbital fuel generation out of atmospheric CO2 and greenhouse gases. Win, win.
SciFi now, but most of this stuff was a couple generations ago.
I don’t see a way to make powered descent work while we’re still bringing fuel from dirtside. There’s just no point, cause we can’t lift enough fuel to bring down one vehicle, let alone two (the tanker and whatever it’s loading up). A second stage with more than 7.8km/s of delta-V is going to be absolutely massive, as long as we’re using chemical rockets. Electric engines don’t work for a powered descent, they don’t have enough thrust.
The Chinese appear to have figured out orbital refueling (at least for satellites): https://interestingengineering.com/space/china-refuels-satellites-in-earths-orbit
It’s not clear where that tanker fuel comes from, if it’s being replenished, how long the tanker stays in orbit, and how long it takes to refuel (minutes, hours, days?)
If the tanker is a one-time disposable vehicle, deorbited once the fuel runs out, it likely won’t be cost-effective. Refueling a satellite then pushing it back into service, you’re not under the same time limit you would be for a reusable rocket that needs to get back down and into circulation ASAP.
There are a lot of interesting, hard problems. But theoretically, a self-replenishing, reusable tanker could be the most cost-effective way to get to reusable upper stages. Of course, you would need to also solve where to park it so it stays up while still being reachable by LEO rockets. Again, SciFi now.
I just hope if it’s ever built, they stick a Jetson’s style neon “Gas ⤵️” sign on it.
Tanking up a satellite to extend its lifetime is quite a different beast from tanking up a whole-ass second stage for a powered descent. You’re looking at hundreds of tons of cryogenic liquid fuel and an enormous vehicle. For context, a Starship carrying no cargo has about 8km/s of delta-V.1 That’s with 1500 tons of LOX and liquid methane. That’s barely enough to lose the orbital velocity. Assuming that prop load doesn’t include the header tanks (but it does), a fully tanked up Starship could maybe pull off a powered descent and landing from LEO. And that thing is huuuuge.
1: I ran the numbers for a block 2 Starship and came out with 11km/s of delta-V. That sounds like too much, so I’m going to assume I got the math wrong and go with the latest “official” numbers I could find.
Wonder if the calculus changes if they use a different type of fuel for descent vs ascent? That way a hypothetical tanker would only need a supply of descent fuel, whatever that is.
That would make things worse, because now you’re carrying the extra mass of fuel, tanks, plumbing and engines for the descent. Can’t run a rocket engine on two different types of fuel and oxidizer.
The rocket equation is a harsh mistress. As long as you’re limited to chemical rockets, you’re not gonna have enough spare propellant for a powered descent. The energy density just isn’t there. We don’t do direct burns to pretty much anywhere farther than Mars and a Mars Hohmann transfer (the most fuel-efficient trajectory) burn takes ~3.6km/s. Less than half of LEO velocity.
