NASA's Artemis rocket makers explain that it's a marathon and a sprint
SLS rocket is a mix of old and new
NASA's Space Launch System might look like a mishmash of heritage Space Shuttle parts but it's all new hardware, and the team who built the twin boosters and engines have been talking about the challenges of bringing designs from the 1970s into a lunar future.
Northrop Grumman is the company behind the boosters used by the SLS. The work is overseen by NASA but, as the agency's Bruce Tiller (the SLS Booster Manager) puts it, "it's all one team."
One team it might be, but, as Tiller also admitted, "I have a small team on the government side, and they have a big team on the contractor side." This is not surprising, because Northrop Grumman (like a number of other big aerospace businesses) is recipient of quite a bit of US taxpayer money and is key to numerous aspects of the US space program.
The Register spoke to the teams on both the NASA and NG sides, responsible for the twin solid rocket boosters which, coupled with four RS-25 engines, are still sitting on the Florida launchpad after two scrubbed launch attempts.
It is hard to avoid thinking of a Shuttle stack (just without the Shuttle) when looking at the SLS, although the orange tank sitting between the boosters is a very different beast to the previous generation. The twin solid rocket boosters are, however, very much heritage parts. That said, there are changes. "We did two big things," said NASA's Tiller, "and a lot of small things."
The most visible of the big things is the addition of an extra booster segment: "That gives us another 20 per cent more power on those boosters" explained Tiller. "I like to kid my engine buddies: 'You can light all four of your engines and you're not going anywhere. Once you light those boosters, you're gone!"
It isn't really a joke, although a manager responsible for the engines described the boosters as the "sprinters" while his power units were more a "mile runner".
The solid rocket boosters provide around three-quarters of the initial thrust of the SLS at launch. Each consists of five segments, the casing designs of which are derived from the Space Shuttle era. "Two were on STS-135 … I flew with them 11 years ago," Northrop Grumman senior director and pilot of the final Space Shuttle mission, Doug Hurley, told us.
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However, unlike on Hurley's final Shuttle mission, these casings will not be floating back to Earth via parachute, and those RS-25 engines won't be gliding back on a Shuttle either.
"It's mostly performance," Tiller explained regarding the fate of the boosters and the deletion of the parachutes – one of the big changes. "We saved eight flight sets … knowing we were going to develop a more powerful booster [Northrop Grumman's Booster Obsolescence and Life Extension (BOLE) boosters]" That, coupled with a low anticipated flight cadence compared to the Space Shuttle, means the spent casings will be left to fall into the ocean.
"The more you fly," explained Tiller, "the more recovery makes sense. And it really saves you a lot of money. I wish we were flying that frequently," he added ruefully, "but we're not… so…"
So the boosters become expendable. Or go out in a blaze of glory. "It's just great to see them used in this manner," said Hurley, "as we go back to the Moon."
The show must go on
The same applies to the engines, formerly Space Shuttle Main Engines and now set to be dropped into the ocean after a final hurrah. Douglas Bradley, RS-25 deputy program director at Aerojet Rocketdyne, who had a hand in designing the units back in the 1970s, said production would account for the loss. "We've got 16 total engines from the Shuttle program," he explained, "so we can get to Artemis IV."
The SLS uses up four engines per vehicle (up from the three for the Shuttle) and will be run at 109 percent of rated thrust for Artemis I. This is something that was possible in the Shuttle days but, as Hurley and Northrop Grumman's director of business development Rick Mastracchio – also a former Shuttle 'naut – noted, that would take place only in the event of an emergency.
There is scope to push things further with Artemis engine thrust, and Aerojet Rocketdyne's Bradley added: "We've run lots of times at 109, 111 and 113, so we knew they were capable…"
Why so high? "We know that sooner or later we're going to run at 111, and so our process at Aerojet Rocketdyne has always been to go two percentage points above that … you usually have to run a little bit higher to ensure that every part has seen that type of strength."
Those engines (of which 14 were used in Shuttle missions) have been paragons of reliability during the life of the Shuttle, he said, but confessed to some mixed emotions knowing that the end is rapidly approaching. "We used them for so long, we kind of get to know them. You know - this one runs a little warmer. This one runs a little more specific impulse (ISP) … and so on the one hand it's sad, but it's the coolest thing ever to have some engines I worked on taking us back to the Moon."
While Northrop Grumman is working on the BOLE project to replace the heritage SRB booster parts, Bradley detailed the challenges of restarting the production of the former Space Shuttle engines. "Some of the things we made in-house – but people have retired. Some of the suppliers that we got our hardware from – they're out of business. So we've had to somewhat relearn how to make the parts."
And more cheaply too – Bradley told us that since the engines were to be expendable, the expectation was that they would be less expensive, although without degrading either reliability or performance. "In some cases," he said, "we had changes we were going to make during the Shuttle program, but ran out of time. So we incorporated those."
In other cases, modern manufacturing techniques can be used to cut costs - Bradley is quite the fan of 3D printing of rocket engine parts, something many other manufacturers are using to cut costs without sacrificing reliability.
As for other changes, the Aerojet engineer explained that an adaption program had been needed – the Artemis stack is considerably taller than the Shuttle, for example, and the engines are clustered at the base. This means the operating pressures will be higher and so on.
While the RS-25s will come to life seconds before the SRBs, as in the Shuttle days, their position means that the familiar twang of the Shuttle stack, which would lean slightly upon ignition, will be gone.
"The twang," said Bradley, "was strange to see when I first saw it. It'll be weird to see it not [happen]."
Arguments over keeping the reusability of the heritage Shuttle parts have been rendered somewhat moot thanks to the lower flight cadence of the SLS and the need to wring every last bit of power from what is lurking in the storerooms.
While the engineers and ex-Shuttle astronauts all confessed to some mixed feelings over using and then dumping the equipment, all were also visibly delighted that rather than gathering dust in a museum, the hardware will be used one final time for a long-hoped for return to the Moon.
NASA said at the weekend that it has decided not to attempt another launch in early September, and will have to roll both the rocket and spacecraft back into the Vehicle Assembly Building after it decides on whether to perform work to replace a seal at the pad itself, where it can be tested under cryogenic conditions, or inside the Vehicle Assembly Building.
Because the launchpad is required for a flight to the International Space Station in early October, according to NASA director Bill Nelson, it is likely that Artemis-1's next launch attempt will not take place until the second launch window next month – so any time from mid-October. ®