This week, I was invited by NASA to join them at a static firing test for the motor to power the SLS (Space Launch System), NASA’s next-generation heavy-lift rocket system, intended for a first flight in 2018. Enjoy this 2-minute video of a great big giant kablooey:
Spoiler: the test was a success.
This (June 28, 2016) was the final ground test for the motor before its scheduled flight in about two years from now. There were a number of really quite surprising facts about this motor, so if you’re at all a rocket/NASA/space/aerospace geek, read on. And I’m not just talking about thrust numbers, I’m talking about the hows and the whys of using components from a system that first flew in 1981.
The Space Launch System is often described as a “shuttle derived” system. Look at it. The solid rocket boosters look the same; the main tank looks the same; and the motors on the bottom look the same. That’s because, essentially, they are. In the case of the SRBs (solid rocket boosters) they actually are physically the same hardware that already flew on shuttle missions!
My group had many opportunities to ask detailed engineering questions of many senior execs and engineers from NASA and from Orbital ATK, the contractor that builds the SRBs and at whose Utah facility this test was conducted. We were all “Whoa, whoa, one of these things blew up the Challenger, and it’s now a new century, you mean to tell me we’re not using a new, more modern design? Like SpaceX does?”
NASA and SpaceX are two very different entities that pursue the same essential goal via very different means. Yes, SpaceX designed and will fly their Falcon 9 Heavy in less time than NASA’s shuttle-derived SRBs have been sitting around unused. It’s a far more modern design. SpaceX moves far faster. SpaceX pushes the bounds of agility and efficiency; NASA pushes the bounds of glacial bureaucracy and overcaution. NASA execs tolds us their relationship was like a teenage driver wanting to go out and be crazy, and dad telling him how to slow down and be safer. The cooperation and competition — co-ompetition, if you will — between SpaceX and NASA makes both entities better.
Understanding that gives a feeling of “Oh, that makes sense, then” to the process of constructing these SRBs that we got to observe. Let’s start with the main difference between the SLS SRB that I watched, and those that powered the shuttle. They’ve added one segment. Instead of four, these now have five. And these are all used, recycled segments that were used on shuttle flights! The same steel cylinders. Using five of them means there’s more pressure inside, and the steel cylinders were designed to handle the pressure of four. So they’ve reduced the pressure by widening the nozzle slightly, at a slight cost of thrust from what all five could provide. “What a kludge!” you might say, and well, yes, that’s one way of looking at it. But that’s not the way NASA thinks about it. You see, there are nearly 100 of these used segments sitting around, suitable for flight. We can fly 8 or 9 SLS missions using the cylinders we already have. Why not use them? Of course the SLS team intends a future upgrade — composite cylinders in a future “Block 2” phase — but for now, use what you’ve got. We even gain back a little bit of performance because we don’t intend to re-use these cylinders again. The shuttle SRBs parachuted softly into the ocean for re-use (which is why we still have these segments). We’re going to let them crash and sink this time, and save the weight and complexity of the parachute and recovery systems.
Inside Orbital ATK’s impressive Utah factory facilities, gargantuan hardware is used to handle these segments, which weigh some 140 tons when completed and fueled. After having all their hardware installed, each cylinder is lowered into a giant pit, then a core mold is lowered down into its center. A vacuum chamber comes down over the whole assembly and the fuel is added. These cores are specifically shaped for each segment. The SRBs have a hollow tube down their center, and the geometry of this tube is what defines the thrust profile (once an SRB is lit, they can’t be throttled or turned off). Finally, a black rubbery seal is spread over the fuel on both exposed ends, which keeps it from burning in the crack between segments. A finished SRB segment is a thing of beauty: a massive cylinder, beautiful glossy white on the outside and glossy black on the ends, with a tunnel down the middle big enough to walk through.
The segment shown in this picture is one of three that are completed, fueled and ready to go, scheduled for the first flight of the SLS in 2018, each flight needing ten of the segments. Yes, they are done that far in advance. The fuel is a stable, hard, rubbery compound and can sit for years.
And just as a quick humblebrag, I got to meet and shake hands with Charlie Blackwell-Thompson, NASA’s first female flight director, who will lead the SLS launches. She is classy and awesome, and an inspiring reminder to all young people that the moon is not the only place to leave pioneering footprints.