Delays are ok and to be expected. I just hope Starship doesn’t explode.
We might be hearing about it even earlier. Musk still has concerns about the autogenous pressurization system. He didn’t seem 100% confident that the Raptors can keep up with the demands of keeping enough ullage pressure, given that it is constantly condensing into liquid (the engines are the source of heat for vaporizing the propellant into gas in the first place).
Given that Starship requires sufficient pressurization just for structural integrity, it’s entirely possible that a failure here could result in a big boom.
At this point, an explosion is inevitable at some point in the flight (if only when the stages impact water). Even well before that isn’t really a problem–a boom during staging is by no means out of the cards. That’s ok; there are lots of other test articles lined up with significant improvements. It wouldn’t delay anything unless it was found to be from a significant design issue.
What SpaceX really doesn’t want to happen is an explosion on/near the pad. That would destroy a huge amount of infrastructure and delay development for the better part of a year.
Just to clarify something here:
Ullage is just the non-liquid space in a tank. It can be filled with a gas, whether helium or otherwise, or even be taken up by mechanical means, like via a piston or flexible diaphram.
Maintaining ullage pressure is important in most rockets since they depend on the structural integrity that it provides (like standing on a closed vs. open soda can). The usual means is to use helium, since it’s inert, lightweight, and gaseous at just about any interesting temperature.
But helium is expensive, and getting more so (it doubled in price in the last couple of years). It’s the most expensive consumable on the Falcon 9; a cubic foot of gaseous helium costs more than a cubic foot of liquid oxygen.
So Starship is to use autogenous pressurization instead, which means to use gaseous oxygen to pressurize the liquid oxygen tank, and likewise for methane (this wasn’t possible on kerosene rockets since it doesn’t vaporize at a low temperature).
But as you noted, sloshing or even just normal surface action means the ullage gas can condense, reducing the pressure. That could mean the engines don’t have enough pressure to operate, or even cause structural collapse.
I don’t believe any rocket has used this before, but it’s required for their cost targets. And helium isn’t available on Mars, either.
I think that’s the biggest reason. Even with helium pressurization Starship will be incredibly cheap compared to other rockets. But Helium is hard to get on Mars and the Moon and many other bodies, and a true reusable space transportation system will need to use in-situ materials as much as possible. Shipping helium to Mars would be incredibly expensive. And I’m not sure it can be mined in quantity there, either.
If autogenous pressurization turns out to be a failure point that can’t be easily solved, it will set Starship development back significantly, and if they go to helium pressurization it will not only raise costs and reduce system usability, but it will increase the weight of the rocket and lower its payload.
Yes, but they’re aiming for a ~100x improvement, not ~10x. Starship is something like 250k cubic feet. Helium is about $0.60/cf at ambient temps/pressure, but within the tank it’s at 6 bar and at 1/4 the temperature. So about $14/cf, which comes to $3.5M just for the helium. They’re aiming for $1-2M total launch costs. Even at $10M, that’s a very significant portion.
Then again, the primary appeal of LEO, at least right now, is that it’s cheap. If you’ve got a payload that just needs “somewhere in space”, then you go to LEO. But if we had a space elevator that made higher orbits cheaper, then a lot of what’s currently in LEO could just as well be in one of those higher orbits instead.
As I noted earlier, it actually does take a very significant amount of dV to come down to LEO from GEO, but it’s still far cheaper than reaching orbit.
But it occurs to me we could do even better - couldn’t we use a magnetic accelerator at the top of the elevator to give vessels a retrograde kick (if we want to go to LEO) or a prograde one (if we want to go elsewhere in the solar system)? We’d still need rockets to lower our apoapsis once we coast down to our new lower periapsis, but that should save at least half the dV we need.
And, given how the Rocket Equation works, cutting dv needed in half does more than halve the size of the craft.
It’s easier than that. Just release from the elevator below GEO height, and you’ll be in an eccentric orbit with your apoapsis at release point and periapsis somewhere lower. How much lower is variable depending on how much below GEO you release. And you wouldn’t have to circularize all in one orbit, so could use ultra-high ISP ion thrusters or whatever.
Downside is that this is only useful for equatorial orbits. Inclination changes are hideously expensive, and even with a free equatorial orbit it’s probably easier to just launch into a desired non-zero inclination from the get-go. And for many applications where you want global or near-global coverage - mapping, low-latency comms, earth surveillance (spy, weather, whatever), you need inclined orbits. In fact, there are probably not very many uses at all for equatorial LEO satellites.
As it is right now, inclination changes are sometimes done by first boosting apogee to out past the Moon, changing inclination there, and then lowering the orbit again. A space elevator would make that first step much easier.
Ah, that is a good point! I suppose putting railguns partway up is infeasible since their weight would be pulling down to Earth rather than up (as putting them on the elevator’s counterweight would allow)?
But don’t they get much cheaper the further from the gravity well you go? Couldn’t a mostly prograde but eccentrically angeled railgun at the top of the elevator help significantly?
So, a use for a prograde railgun after all?
Inclination changes are least expensive with the inclination burn at the apoapsis of an extremely eccentric orbit. Basically you want your velocity relative to the parent body to be as low as possible. Probably the way to use a space elevator to get the cheapest inclined LEO would be to release from the elevator with your desired target orbital height at periapsis, then burn prograde at periapsis to add additional eccentricity, then make the inclination change at apoapsis, and finally circularize.
I suspect it’s less delta-V to just launch a rocket at the desired inclination to start, though.
You don’t need prograde railguns. Make the cable go up to \sqrt[3]{2} times geosynchronous height, and you can escape Earth entirely just by going out to the end and letting go.
If Starship achieves its cost-per-kg goals, I can’t see a space elevator matching it.
$10/kg, Musk’s ‘aspirational’ cost to orbit, isn’t much more than the energy cost of getting a kg to orbit. There’s no way a space elevator will beat that. At $100/kg, which is entirely possible, I think it would be very difficult to meet the price, especially considering the transfer costs of anything not going to precisely GEO.
The capital costs of a space elevator and the cost of lifting the materials to orbit would be very high. Maintenance and operation costs would be high. Insurance and mitigation from impacts would be high. It would be a 24,000 mile mechanical monster full of moving parts under high stress. And it would be competing against a steel can that throws material into any orbit necessary, then lands and is reused.
At best, the practicality of a space elevator is now questionable. We’ll know more once we see Starship perform.
Wouldn’t that require exponentially stronger materials for the construction of the elevator though?
Thanks everyone for the info - far more precise information than the news provided.
I just find that the SpaceX/Musk approach is an interesting take on engineering; instead of spending years and a whole lot of engineering and scientific analysis for one or two all-or-nothing test flights, instead build something, try it, and see what happens. Based on those results, make the necessary modifications and try again. When your starting point was the mindset “we throw it away as part of each flight” you have nowhere to go but better.
Not necessarily. That’s about 1.25 times geosync. If the cable will hold for an elevator to earth, it will hold for a launcher without a great deal better strength. As I said earlier, you just need a cable that can support a spool of 15,000 or so miles of itself and you’ve got what you need - or for a launcher, 30,000 miles. (Depending on whether that’s calulated as distance from center of earth or from surface.) It was going to need to go a little past GEO anyway to keep the cable taut.
Even if they have to resort to helium pressurization, it doesn’t actually get consumed during flight. So since both stages are re-usable, would it not be possible to recover at least most of the helium as part of the refuelling process?
A space elevator has to extend beyond GEO anyway. Otherwise all the forces are downwards and the thing won’t stay up.
On the Starship booster it does get consumed. At least currently. One of the optimisations made is to remove the separate compressed gas supplies for the cold gas thrusters used for attitude control during part of the return. Instead the thrusters are fed with the pressurised gas in the tanks. There is a lot of compressed gas just sitting there, and using it for manoeuvring removes a whole lot of additional complexity. At least that is the theory. So with the current design, at least some of the gas used for pressurising is lost. No doubt the design could be reverted to add cold gas storage for the thrusters. But there are always additional knock on effects. Nothing is simple or consequence free.