I’m not so certain that they aren’t against space launches. Between the animosity against Musk himself and “Earth First” critics of the entire concept of the Mars colonization who condemn the entire project as elitist and even fascist, I would need to see plenty of positive evidence that SaveRGV really are environmentalists acting in good faith.
I’m a bit confused by this ‘storage depot’ language.
How is this different from a refuelling tanker starship?
As I understand it, six payloads-worth of fuel/oxygen are needed to fully refuel an orbiting starship for missions beyond LEO. And my assumption was that there would be six tanker launches, each of which refuels the mission ship.
Surely they are not planning to do some sort of ‘double transfer’ where the tankers load up a ‘depot’ which then refuels the mission ship?
Me neither. Looking at their web site, it seems that they are an ‘environmentalist’ organization leaning towards the fanatic end of the spectrum. Not completely woo-woo, but I suspect that their public material is toned down from their actual agenda?
Again … My point is not to defend SaveRGV.
My point is that late 2024 is a dangerous time to be hoping judges get big for their britches and start making declarations that certain organizations are simply to be ignored as evil or legally illegitimate.
I don’t disagree at all. We are in a situation where legitimate dissent may become dangerous to those who express it.
On the other hand… when does legitimate dissent shade into tinfoil hat woo woo?
But this is in danger of straying into politics. I’d respectfully suggest we keep it out of this thread (or move it elsewhere). I won’t comment further here.
Agreed. Not on-topic. My fault.
I thought it was twelve; at least that’s the naïve ratio of expected payload to Starship fuel load.
And yeah, now I realize I don’t know if SpaceX envisions a fueling depot or using the final to-go ship. The later would be workable for the projected Mars missions if SpaceX can achieve the tanker launch cadence they want to; then a few day’s shakedown in low earth orbit won’t be much of an additional burden on the crew anyway.
Current plans are to have a separate depot. Tanker flights fill the depot, then when the depot is full the HLS docks and gets a full propellant load.
Previous statements were that it would require about a dozen tanker flights to fill, but that was based on Starship v1. Starship v2 and v3 are still in the pipeline. It’s also highly dependent on exactly how much mass they can shave from the system. Since Starship is still under development, they don’t really know exactly how it’ll turn out.
I think it’s possible they’ll eliminate the depot, but for now that’s the plan. That may itself depend on how many flights there are. The fewer flights, the less risk to the vehicle, and the less boiloff compensation they need. Flight rate also plays a part–they need fewer flights if they can do them more rapidly (unless they achieve zero boiloff).
Boiloff is an interesting question. Just how much of a problem will this be?
I would have thought that since the tanks are sealed, it should be negligible.
Though perhaps there might be a certain amount of leakage during the transfer process?
If there is a significant amount of ongoing loss, one wonders how that affects a flight to Mars, which will take quite some time.
Aside: does anyone have a figure for the total amount of delta-v achievable by a fully-fuelled starship?
The total amount of delta-v available to a fully refueled Starship upper stage is still conjectural because the final design hasn’t been worked out yet. Everything depends on how much the heat shielding will weigh that will be necessary for the upper stage to not merely survive reentry once but to do so repeatedly with minimum refurbishment between flights.
A typical flight time Earth to Mars is 260 days; so control of boil off is crucial. It’s one reason why liquid hydrogen was never considered for Starship; 90° Kelvin (the temperature of liquid oxygen) is a hell of lot easier to store than 20° Kelvin. I haven’t heard much about how this is going to be addressed, so presumably it’s one of the things being deferred for the time being until Starship is fully proven out. I would suppose that the engineers consider it probable that some solution can be worked out; most likely a combination of sunshades and a heat radiator.
Understood delta-v is conjectural, but presumably there are some back-of-the envelope estimates?
Hydrogen is notorious for leaking through almost everything, of course.
Though a main reason for choosing methane was the possibility of generating fuel on Mars, I thought.
That’s another reason. Although plans to generate fuel in situ on Mars still depend on locally sourced hydrogen (water ice, etc.), methane is only 20% hydrogen by weight. Since water ice is presumed to be the bottleneck of fuel production, using methane stretches the supply considerably.
For outer solar system probes, forget worrying about hydrogen boiloff. Starship has enough lift you could design a probe kick stage comprised of a nuclear reactor and ion drive, and just accelerate (very, very slowly) all the way to your probe’s destination.
Right. There’s carbon as CO2 in the atmosphere of Mars, of course.
And obviously there is some water on Mars, as a hydrogen source.
I’m not sure we know yet how much or how accessible it is likely to be, though?
If you can get past the ‘no nukes in space’ objection.
Mind you, if someone in the US doesn’t do it, China probably will…
ISTM they could build 2 Starship variants. One for rapid reuse with heavy robust thermal protection and one for not-quite one-way trips with little.
The latter would also work to do a Mars out and back. If it’s gone for 3 years, whether the refurb takes one day or 3 months doesn’t much affect cadence or costs.
It might even be a cadence gain overall if the reduced TPS weight buys enough extra fuel capacity to add enough delta-V.
That seems very reasonable.
After all, are the moon (or Mars) lander variants ever going to need to do a re-entry through Earth’s atmosphere? Don’t see why…
The tankers have to, of course. One would hope this would be obvious to competent engineers… and SpaceX seems to have a few of those…
Seal the tanks and they’ll just explode (which has happened to many defunct upper stages, causing debris fields). The propellant has to stay cold. If you don’t do that yourself, via some combination of insulation and active cooling, then nature will do it for you. The propellant will evaporate, leaving the rest under the boiling point. A rocket’s propellant tank is nowhere near strong enough to resist this pressure.
But how much this is a problem, I don’t know, because it’s very sensitive to insulation. They could, in principle, have some extremely complex deployable multi-layer system like on the JWST. Or they might just have white paint. Or somewhere in between.
And then there’s active cooling. Solar power is plentiful, and cryocoolers exist… so why not just recycle the boiloff back into liquid? It’s possible. But I haven’t run the numbers on the practicality.
Interplanetary flights will have a somewhat easier problem since there’s only the sun to worry about, not a warm planet. Earth doesn’t put out as much heat as the sun, but in LEO it covers half the sky, even at night. The Sun only subtends half a degree and is covered half the time.
If you strip it to the bone, about 8.6 km/s. That’s assuming 120 tons dry mass (100 t ship + 20 t cargo) and 1200 t propellant.
A v3 might be as high as 11.7 km/s. That’s 2300 t prop and 170 t dry mass. We don’t know the actual dry mass but it looks like about a 50% stretch, so I went with 170 t total.
The moon doesn’t. But Mars does. Aerobraking is still very significant there. That’s true even if you don’t land–you still want to use aerocapture to reduce delta-V requirements. Maybe doesn’t need to be quite as robust as the landing version, but not trivial.
I am afraid those satellite launches and re-entries are doing something to the atmosphere, but it is not clear what it is and the people doing the launching do not really care. Do you?