David Fourman: So right now we're joined by Ben Brockert, who was put in touch with us through Chris Radcliff, and he's going to tell us a little bit about what's going on with SPX and the LOX loading problem. This is why you were specifically brought here, so thank you for joining us because I know you're currently on vacation in Viet Nam.
Ben Brockert: Yeah, thanks for having me.
First off, tell us a little bit about who you are and what you do.
I've actually worked for four different New Space companies now, all much smaller than SPX, but I've done hands-on work with LOX, training people to use that and other fun stuff. I have some experience there. Propulsion Engineer is my main work, but I also do ground support equipment and photo/video and I've work for Masten Space Systems and Armadillo, and I've had my own company called Able Space, and I've worked for Moon Express, which was a GLXP [Google Lunar X-Prize] team.
So specifically, about this LOX loading problem, we've been guessing that's all that Ben and I can do. Sort of speculating about exactly what's going wrong, but you have some insight into that. If you could explain, starting I guess with the first launch, which was scrubbed... Or first off let me say that I think it was you who was saying that each laugh abort was related to this LOX loading problem? IS that right or is that not correct?
Yes, so it's kind of a roundabout thing. Essentially, the challenge that SPX is facing all comes down to the sort of root cause; that they don't want to increase the diameter of the stage. Which sounds like an odd reason to scrub for LOX temps, but SPX manufactures their stages (their first and second stages are the same size), they manufacture them in LA and then they ship them to TX to test them, and then they ship them out to FL to launch them. And to be able to do that, you have to be able to haul them down the road which sets a maximum size you can practically do just to ship them across country, plus they have all their machines, friction stir welding and all the other stuff they use to actually build the stages, so for them to actually increase the size of the stages would require a bunch of money or a bog change int he way they manufacture and move around their stages like putting them on a barge or something like that. So they're fixed in diameter, you know, the rocket equation is based on how much mass of propellent you have in your rocket, so the other thing they could do is make the rocket longer, which they did, and now the rocket is one of the longest, in rocketry, it's called a fineness ratio; the length to the diameter. The F9 is one of the finest rockets that's ever been built by that terminology where it's really really long compared to its diameter. So essentially, the diameter is fixed, the length is as long as it can get. If you want to put even more propellent on the vehicle, you can change the density of the propellent. That's what they've done with this whole sub-cooled propellent thing, they're chilling down the kerosine down to about, close to the freezing point of water. They're trying to cool the LOX down to near it's solidification point, which is very very cold. Normal LOX is down at -380°F, and they're pulling this down even further. That requires a lot of ground handling. Normally with LOX, what you do is just have it atmospheric pressure, and it's at -300°F, you pump it into your rocket, and and it can sit there and it can sit out venting and the density is good, but to build to do this sub-cooled thing, you have to build to pump it and refrigerate it at the same time. If you fill the thing up and then you have, say, a tug boat sitting in the launch range, and then the vehicle sits there, heat is coming in through the walls of the tank, and that heats up the LOX and then, you know, it warms up and loses density and expands, and suddenly you don't have as much mass of propellent on the vehicle as you want. The connection to the one one scrub they had for wind shear is a side effect of the fineness ratio of the rocket. If it had been a shorter rocket, it makes it structurally stronger. Essentially, the wind shear flying through it makes the rocket bend, and since the rocket is so long, that's why they have a lower wind shear limit than is true of other rockets. So it all comes down to SPX wanting to put the maximum amount of propellent they can on the rocket, and the sort of challenges that are involved with that. There has been a lot of other difficulties in there too, like the seals being designed to work in specific temperature ranges. If your kerosine suddenly goes from being room temperature to near freezing, then you have to change out a bunch of seals. Same is true for the LOX. Things that worked previously at one temp that is fairly cryogenic, and you're getting deeper into cryogenic. So in general, it's added a lot of complexity. There's a reason that other companies haven't done this ,but it's understandable that SPX is trying to get the maximum performance out of a fixed size that they can.
That's basically how I did understand things. I thought that maybe there was something else going on that perhaps we didn't quite understand. But essentially what you're saying is that they have to maintain some very low temperatures and they can't do that because the LOX keeps boiling off or heats up. That sounds fairly intuitive.
One handling hiccup of that that I didn't get into is that the rocket is only capable of standing up when full of propellant when it's under pressure. With normal LOX, that's not that hard. You put the LOX in and you keep the tank pressure up somewhat while you're going. However, with sub-cooled, that makes it somewhat harder because you get all sorts of stratification issues where you have warm LOX in one part of the tank and cold LOX in another. There are some interesting dynamics that go on when you try to fill up a massive tank with a million pounds of super cold propellent while trying to keep it from collapsing under its own weight at the same time.
Do they pressurize it with some other gas while it's being filled with the LOX or... Because it can't stand up under its own weight, right?
Yeah, it would be pressurized with He. That used to be the case. They might have made it stronger to deal with this complication. But it used to be the case that they were only making the stages strong enough so they could essentially stand up under their own weight while they were empty, but when you started loading propellent into them they had to be somewhat pressurized. He is the only gas that you can use to pressurize LOX, because essentially every other gas you would use to pressurize it would dissolve into it. If you try to pressurize LOX with liquid N2, it would just dissolve into it and you wind up with a mix of LOX and LN2. You could use gaseous H2, but using a fuel to pressurize an oxidizer is not a good idea.
I don't think that would be good.
Not a good idea.
No, that sounds like the worst single idea anyone could have. I'm sure they never even considered it. So, on the [last unsuccessful launch attempt], the engines turned on and they were just about to launch and then they stopped. There was an abort. DO you know more than what the rest of us would, or can you speculate? Because apparently it was about an He bubble or something like that hat had been run through the engines.
Because these are pump-fed engines... I don't know if you've ever taken apart a vacuum cleaner, it has the actual fan part of it, that is pulling in air. The pump on a rocket looks very very similar. It's essentially a screw on the front and fan blades that stick out. It's designed to work over a very specific range of inlet conditions, that is both the temperature and density of the propellant, but also essentially the vapor pressure of the propellant. What happens is that the pump is running, it's putting high pressure out on the outside, which is a few thousand PSI to run the Merlin engine. On the inlet, it's creating a suction, so your tank has some pressure pushing it towards the inlet. But if the temperature of the LOX going into the engine gets too high, then essentially you can vaporize the LOX as it goes into the pump, and then the pump doesn't work correctly. You get cavitation bubbles and other things. Elon said something about it. If it sits there and warms up so much the pump won't work correctly. Rocket turbo pumps are known for exploding so you generally want to keep them inside their happy operating ranges. They're, you know, very finely balanced, spinning at high rates of speed with hazardous things running through them. He also said that an He bubble, so that's basically the same thing, where something you don't want in the process is getting in there. The pumps are definitely a big challenge. That was probably most of it. You can also get having a bubble of He going actually going through the pump and going into the injector. You can get instability going through the injector when the bubble transitions through there. I'd expect it to be a pump issue rather than a combusted issue. Sadly, they don't let me see their traces.
Obviously. I'm wondering if they had to make any modifications to the turbo pumps when they transitioned to using the super-cooled LOX because it's denser. You're still moving the same volume no matter what I suppose, but there'd be a different mass. How does that effect things? I have no idea, but i'm just wondering. That can't be... They'd have to have done something differently.
So pumping, sort of the efficiency and the amount of power required out of a pump depends more on the volume than the mass. So in theory, the densification should actually make their pumps more mass-efficient, because they can be marginally smaller to run the same mass rate. At the same time, part of the whole upgrade sequence that lead to the densification was also increasing the thrust level of the engines, I don't know what they have, maybe twice as much thrust as they did when they were first designed. That means that either the pumps need to be bigger or you're running them at higher speeds or something like that. The technical term for this sort of the temperature at the inlet and everything that's necessary is called NPSH: net positive suction head. Meaning that when the pump is running, you still have a positive pressure on it because if the pressure goes negative, you get cavitation or other bad things going on with the pump.
Do you know... I know that we're straying slightly off topic, but now I'm just curious. I know for large engines you'll have a centrifugal pump, and then you'll have a smaller axial pump which sits in front of it which pulls the fuel in so as to not create these cavitation bubbles. But for the Merlin engine, is that something that SPX does?
I'm sure that's something. They'd probably have an impeller, a little axial one, but even that one has its limits to what it can operate in. The original pump for the merlin was designed by Barbara Nichols out of CO so I'm sure it's now a couple of generations beyond that. The basic technology would be recognizable to anyone who had worked on Apollo or any of the other pump-fed engines.
I guess that wraps it up. Is there anything else you wanted to talk about?
No, I was glad they finally launched. It was sort of guaranteed because I was on a plane to NY, so I missed it. That guaranteed after going out to the beach for all three previous attempts that this time it would finally go.
Thanks for coming on and talking to us. I know you have a vacation to get back to, so we'll let you do that.