Episode 53: Ben Hallert correction burn

Editor: Ben sent this correction burn in to us via email on April 5, 2016. We didn't have time to read it on the show, but thought there was enough good info to make it available here.

Greetings, citizens, I hope you are getting enough oxygen!  One tiny correction from this week's show: your guest described Enterprise as a 'mockup'.  A note, it was actually intended from the beginning to be completed for use on orbit after the glide test program was complete.  It (OV-101) was the first hull constructed that was intended for space but after it was done and being used in the landing tests, the engineers were able to make some changes to the design of the frame that would cut more weight off the orbiter.  Each kilo removed from the orbiter was an extra kilo of cargo because the spacecraft is taken all the way up the hill so this was a big deal.

When they began planning the Enterprise's, ah, refit to meet this new specification, they determined that the changes to be made would require such a massive amount of work on this frame because of how far along it was that it would be cheaper to start somewhere else.  To add insult to injury, even after the changes, Enterprise would still have less cargo capacity to orbit than the next spacecraft in the pipeline, Columbia.

So they needed to start, but where?  The answer came from an unexpected direction.  Because of where the state of computer modeling wasn't, Rockwell had built a pair of Static Test Articles for testing.  Essentially space shuttle frames, these were instrumented and then squeezed, squished, pushed, and shaken for hours to try and predict how the real shuttles would behave during launch and landing.  After the 1978 decision not to upgrade the Enterprise to full flight status, STA-99 became the focus.  They determined that it could be more cheaply modified to flight status than the first shuttle because it was just a frame and so they did exactly that.  STA-99 was saved from destruction at the hands of a giant hydraulic test rig and built-out to be a fully operational space shuttle.  The name of that shuttle: Challenger.

When looking at the hull numbers of the fleet, you'll note that Enterprise is OV-101, Columbia was OV-102, Discovery/Atlantis/Endeavour were OV-103, OV-104, and OV-105 respectively, but Challenger held onto a tiny piece of its original history and flew to space as OV-99.

Anyways, ginormous wall of text for a tiny correction burn, but it's a subject I'm passionate about.  The number of well meaning if wrong people who keep calling it a mockup or suggesting it was "never intended to fly to space" is frustrating and this is one of my little missions in life.

Episode 48 SpaceX discussion transcription

Link to full episode audio and show notes.


David Fourman: First SPX story in our lineup of SPX stories. Second FH flight with is apparently called STP-2 slips to march 17. What does STP stand for?

Ben Etherington: STP stands for Space Test... something? Basically it's an USAF protocol where they grab a bunch of hardware they want to test in space and they put it into one big, I guess it's a deployer? Something like that. It's just a bunch of hardware that needs to be tested out. A lot of smallest kind of stuff. There might be some materials [testing], but I don't think so. That's the kind of thing that would go on here, but I don't believe there's actually any materials. I believe there's some communications modules, stuff like that, that they want to get up and do some technology demonstration.

Now, the first flight of FH is scheduled so far for sometime in the first quarter or the first half of 2016.

I think it depends on... Oh, the first one? Yeah, I think it's supposed to be in the Spring. I don't know. It's gonna keep slipping.

Unless that has slipped [already] as well. But I don't think it has, I haven't heard anything about that. This second flight is basically a year following that, I mean a full year afterwards in March 2017. Now what was that you were going to say? You had a Twitter [source] and a Reddit thing here. I guess that's the only information we have about this.

Yep, as far as I've seen.

So there's very little information to go on. I guess this is not official, then.

It's not official, but it's pretty reliable. I mean, it's coming from Stephen Clark, so...

So no word yet from SPX themselves? No tweet from Elon that maybe this has happened? I mean I just want to make sure that the listeners know that this is not official. Maybe more of a rumor than anything.

You can call it a rumor, but rumors of FH flights slipping back in their schedule [are] pretty reliable, even if there's no actual basis, it's probably gonna be true at some point.

That's one way to look at it. You just say it's gonna happen anyway. Okay, so not much to go on there.

Yeah, it's just a quick heads-up, I guess.

So the next bit of SPX news; CRS-8, the core appears to be fixed. I assume you're talking about, because I'm not too familiar with this one, the first stage core? Is that what you're referring to.

Yeah. You know they call it a core because they're doing tests just on the vehicle. It's not CRS-8 until they really put the Dragon on top, at least in my mind. What happened was a week or two ago, I think, we found out that there was some damage to some of the engines on this core.

Oh, that's right.

And what happened was they had it up on the test stand and there was a ground support equipment error. I don't know exactly what happened, but some of the nozzles got banged up. I believe 8 out of the 9 were damaged in some way. Since it's not the vehicle, it's been confirmed by Chris Bergan over at NSF, who's pretty reliable and he's got good contacts within SPX. He says that the damage happened due to the GSE, and not the vehicle, so I'm wondering if that means that they were moving it around, maybe on a crane or something? And they banged up against something? Alternatively, there might have been a heat rejection issue or something. Maybe there was supposed to be a water coolant system that wasn't in place or something like that. Anyway, the GSE damaged a couple of engines. For a while they were thinking about maybe using the engines from the landed first stage and reusing those because they're sitting around and they're ready to go, but it looks like they ended up fixing the engines. For whatever that's worth, it looks like it's all good to go. It looks like they've done another stand test. Somebody got some footage, I believe on the SPX fan Facebook group, it looks like someone got some footage of what is probably this core on a stand doing a burn. Looks like that's all good to go. It doesn't look like the CRS launch date has been pushed back, so it looks like we're good to go.

See, I would be surprised that they would even consider reusing that used stage, but that was something that they were considering?

Not the whole stage, just the engines.

Yeah, but that's the part that I would think would be the least likely to be reused. Because that's the business end of the rocket.

I can see what your'e saying but you also have to keep in mind that the engines are the only thing that we've ever tested before. Because they do full duration burns with these engines, and they do that before the launch, so I'm pretty sure they're familiar with what the lifecycle for these rocket engines look like. What we don't know is what the lifecycle of a tank looks like. Because we're not able to subject those to the same stress as they get during flight, so you know, IDK, it could go one way or the other, but my thinking is that those engines are pretty well understood.

I suppose that sounds reasonable. I kinda wouldn't want them to do it though, because I thought maybe they had some tests or something to do, but they have to make the decisions that they have to make in order to get the job done.

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.

Haha, thanks!