Back to the Moon! Artemis program follow along (it's going to be a long long time)

Incidentally, hydrogen/LOX first stages tend to be not so great, not because of ground handling, but because the density of hydrogen is so low that it’s hard to get decent thrust out of the engines. Thrust is less important for the upper stages, where you’re going so fast that the vehicle has a lot of time to apply the thrust over (taken to an extreme in the case where you’re already in orbit, and don’t have to worry about gravity losses at all).

It’s why SLS (and the Shuttle) has solid boosters; to give it a bit of initial velocity, so that the main engines have more time to work. The Delta IV Medium and Heavy are the only rockets I know of that had pure hydrogen first stages, and they were very… compromised rockets.

Kerosene is nice and dense, and allows compact, high thrust engines. Solids are even higher thrust. Liquid methane is pretty good, too (especially when chilled well below the boiling point).

In fact they’re the same boosters. Also, the RS-25 engines used in the first stage of Artemis are literally from the Space Shuttle – not just the same design, but the first four SLS missions will use main engines literally taken from retired Shuttles:

Technically, not quite the same boosters–the SLS ones have a fifth segment. But similar, and made in the same factory (in the same Congressional district, of course). And you are correct that the RS-25 engines are literally old Shuttle engines.

The Ares V was basically the predecessor to the SLS. Same basic idea–traditional rocket stack, not like the Shuttle, but using extended solid boosters, a hydrogen core stage, and another hydrogen upper stage. Same diameter as the Shuttle so as to reuse the same facilities (ideally).

They intended to use the RS-68 engine, which was always designed to be expendable (it’s the one on the Delta IV). It has an ablative nozzle, not regeneratively cooled (the RS-25 nozzle is cooled with liquid hydrogen).

They spent a huge amount of money on it before realizing that the solid rocket boosters–which put out an incandescent plume of aluminum particles–would have slagged the RS-68 engines. Partly due to the proximity (the Shuttle engines were farther away, not squeezed between the boosters) and partly due to the lack of regenerative cooling.

So the later designs switched to the RS-25, which was way more expensive (ignoring the freebies from the Shuttle), but at least they wouldn’t melt. The SLS inherited that decision, and now we’re stuck with engines that will cost around $100M each to manufacture.

Obligatory literature recommendation:

Ingition! (history of rocket fuel - hugely funny to read even for the leyman by John D. Clarke)

(link will start a - as I understand it - legal .pdf download of the book)

enjoy your sunday literature

Several more delays, and maybe it really is going to be a long long time. Honestly, no one will remember a few small snafus but everyone remembers explosions.

It really is a great read. I read it several years ago (online) when it was still out of print. It’s actually available again in less-dubious form (though I agree, my understanding is that the circulating PDF is legal):

Unfortunately, the only physical form is an expensive hardback. At any rate, it contains such gems as (referring to chlorine trifluoride):

It is, of course, extremely toxic, but that’s the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water–with which it reacts explosively.

On a slightly different note, I ran across this old (1997) talk by Robert Zubrin about Mars. In it, he talks about the Ares rocket, which is very much like the SLS, and indeed the Ares V that I mentioned, but as far as I know is unrelated. Nevertheless, it takes exactly the same design philosophy:

Zubrin is a smart guy and takes a great deal of realpolitik stuff into account, but nevertheless we can see how his idea for a “junkyard special” was wildly naive. He chose the design because he thought it could be made cheaply and quickly. Didn’t quite work out that way. Though it was pretty clever to put the engines off to the side in a little pod, matching the position of the Shuttle engines, so as to reuse the existing flame trench.

NASA update in summary:

We will repair a seal at the pad
…but maybe we will roll the rocket back to the VAB anyway
…and let’s face it if we want to keep the the current requirements of the launch escape system we would have to roll it back so there is that. We did say current requirements.

Fiendish’s interpretation: we are going to try to fix it on the pad, and if we can do that we will try to extend the usage requirements of the launch escape system.

https://blogs.nasa.gov/artemis/2022/09/06/teams-continue-to-review-options-for-next-attempt-prepare-to-replace-seal/?linkId=180330780

I don’t believe it’s the LES that’s the issue, it’s the Flight Termination System (i.e., " the rocket is out of control, blow it up before it crashes into something" system) that has the batteries in question - although it may be both.

I recall the same was true of the space shuttle: the ride was pretty rough until the SRBs separated. Why do solid rocket motors cause so much vibration compared to liquid-fueled engines?

Answering my own question in a simplified way, based on slide 16 and beyond in this presentation: long, skinny SRBs behave like pipe organ pipes, with acoustic waves ringing back and forth inside them at a frequency determined by geometry and speed of sound.

So when is the next try?

The seals have been replaced and they will conduct a tanking test on 17th Sept before making a decision.

https://blogs.nasa.gov/artemis/2022/09/09/teams-replace-seals-on-artemis-i-moon-rocket-prepare-for-tanking-test/?linkId=180771699

The agency has outlined a new date for the fueling demonstration: Sept. 21. If all goes well, the SLS will launch during a 70-minute window that opens Sept. 27 at 11:37 AM EDT. However, for all the reasons, the agency has a backup launch date on Oct. 2.

The fueling tests were successful, but now there is concern about the weather:

This is peripheral, but worth mentioning: parts of the space geek community are very disillusioned about the whole SLS/Artemis project. I guess they see it as a symbol of the old, pre-SpaceX, cost-plus way of doing things – which it is, of course.
Nasaspaceflight.com hasn’t published anything about SLS since last Wednesday, Sept. 21st, when the latest tanking test was beginning; their Artemis forums mostly focus on the mechanics of budgets between NASA and Congress. But they publish near-daily reports and live feeds and “expert” speculation about SpaceX’s projects in Boca Chica, Texas.

Eri Berger got a large amount of shit for this tweet:

Well, it’s already almost 2023. Certainly from the perspective of someone in 2017. It was far more pessimistic than NASA’s schedule or really anyone’s, and yet here we are.

What little enthusiasm there was has evaporated over the years. Especially with alternative rockets being superior. Never mind Starship; Falcon Heavy has been flying for years now, and while SLS well exceeds its capacity, the Heavy so much cheaper that the SLS seems pointless. This comment by Charles Bolden aged poorly:

“Let’s be very honest again,” Bolden said in a 2014 interview. “We don’t have a commercially available heavy lift vehicle. Falcon 9 Heavy may someday come about. It’s on the drawing board right now. SLS is real.”

And then there’s Starship. SLS doesn’t have a purpose at all outside of Artemis. And yet Artemis is dependent on Starship. Since Starship is a prerequisite, and yet is better than SLS in essentially every way, why does SLS still exist? If Starship was just SpaceX’s little private project, maybe we could dismiss it as not real in the way that Bolden did about Falcon Heavy. But that’s not the case now; Artemis doesn’t exist unless Starship does.

As Randall Munroe points out, if you want rockets to work, the Nazis need to be in charge.

The history of the space race is really interesting. After the Germans were defeated the US raided as much of the Nazi rocket program as they could including taking (maybe making offers to) rocket scientists and other staff, knowing that the Russians were about to take control of the areas where most of the rocket factories were built. The Russians got the factories but the Americans got most of the rest.