Save the Uranium-233, Explore Space, Save Lives

Google TechTalk Slides: “Save the Uranium-233″

Please direct comments over there.

As near as I could tell, after Columbia happened, nobody kept working on the AG-NEP design, or even on Mars studies for that matter. If they did, I certainly didn’t know about it.

But for some reason, the whole idea kept rattling around in the back of my head. There were a few reasons that the JSC guys had given me that were compelling for AG-NEP as a Mars vehicle.

1. You solve the muscle and bone loss problem through artificial gravity. You don’t have to worry about hours of exercise or fret whether their bones will snap when they re-enter the Earth’s atmosphere. They’re going to be good and strong when they get home because you made sure that their bodies felt a normal level of gravity throughout the trip.

1a. Because you’ve solved the muscle and bone loss problem, the pressing need to fly the mission quickly is tremendously diminished. You can go to Mars and come back in the three-year time frame that is more astrodynamically “natural”, in other words, the time frame that aligns with the Earth and Mars’s movements around the Sun.

2. By using nuclear-electric propulsion, you actually have a credible propulsion system to execute a mission abort if you need to, for some reason along the way. You’re not going to get back quickly, but you can get back.

3. By using nuclear-electric propulsion, you actually have a credible story for vehicle reuse. You could refuel the vehicle and go again. Or you could go somewhere else like an asteroid. You have a lot more flexibility than in a chemical or NTR vehicle.

I liked the basic idea. Here was a vehicle that might actually be a true “spacecraft” as we like to think of them, with the ability to go and come from a variety of destinations and be reused. I imagined that this might be the kind of vehicle that would be in Captain Picard’s ready-room a few centuries from now as a little model, with him saying, “This is a model of the vehicle man used to explore the solar system in the early days.”

But there were definitely residual technical problems with the design as it stood when I was exposed to it. The biggest one had to do with getting the body-mounted electric thrusters to point in the right direction as the vehicle moved around the Sun, and the problem got so bad when you got to a spiral-in, spiral-out scenario around a planet that it was practically a no-go. It came down to the architectural decision to orient the thrusters so that they were firing in the same direction as the vehicle’s angular momentum vector (orthogonal to the rotation plane). That approach certainly solved any problem of plume impingement, but since the inertial direction of the thrust vector was going to change by >180 degrees during the transit to Mars, and by that much or more on the way back, you had to continuously move the angular momentum vector of your spacecraft around, and there was a non-trivial cost associated with doing that. During spiral-in or spiral-out the cost became prohibitive.

The other problem concerned spin up and spin down of the system. The JSC design assumed that spinup and spindown would be done by dedicated thrusters on the habitat module end of the vehicle. That meant a duplication in thrusters and tankage for a capability that you would want to utilize as little as possible.

Despite these problems, I recognized that the JSC design as it stood had also solved a great many problems, and that perhaps it represented a minimum in the design space of overall difficulty. I’m fond of saying “you have to pick your pain” when it comes to system optimization, and that the “best” system always involves residual problems. Perhaps this was as good as it got.

Or maybe it could be even better.

One day I was driving down the street in the pouring rain and a simple sequence of thoughts formed in my brain:

1. I had spent a whole bunch of time trying to figure out how to get solar panels on a MXER tether to point at the Sun while the tether rotated.

2. I had been lucky enough to meet Steve Canfield and had figured out how to use the Canfield joint to fix the problem of pointing the panels at an inertial target (the Sun) while the overall structure (the tether) rotated.

3. The basic problem that the AG-NEP vehicle faced was the need to point its electric thrusters at an inertial target (its thrust vector) while it rotated, much like the MXER tether needed to do with its solar arrays.

4. The reasons that JSC had rejected rotating machinery for the AG-NEP vehicle had to do with the difficulty of moving propellant and electric power across a rotating connection like a rotary joint or slip ring, and these were good and valid reasons.

5. The Canfield joint had no such problems because provided that propellant lines or power cables were flexible, they could transmit fluids and power across a Canfield joint.

thus…maybe a Canfield joint was the answer to the problems of the AG-NEP vehicle!

I couldn’t believe that I had known about the Canfield joint for so long and hadn’t put these utterly compatible ideas together.

If we were to use the Canfield joint on the AG-NEP vehicle, the overall geometry would change substantially. The logical location for the thrusters moved from the center of the vehicle, on a cross-brace, to the reactor end of the vehicle. This kept the high-power lines short since they didn’t have to run all the way to the middle of the vehicle to reach the engines. You could also place the propellant tanks on the reactor end of the vehicle as well.

This in turn led to several other vehicle advantages:

1. The moment-arm from the reactor module to the hab module is shortened (or alternatively the moment arm from the CM to the hab module can be lengthened) because now there is much more mass counterbalancing the hab module. The thrusters and the propellant constitute a lot of mass.

2. The truss between the reactor module and the hab module now doesn’t need any “cross-brace” on it or any other body-mounted structures. It can be a strong but simple extensible structure, like a CoilABLE boom, with nothing more than the power connection between the reactor and the hab module integrated into it.

3. The main thrusters can be used for spin up and spindown operations. By placing them on the end of the moment arm, they now have the ability to change the angular momentum of the vehicle, by simply remaining fixed relative to the vehicle during spinup and spindown. In fact, spin rate can be changed during thrusting simply by changing the fraction of the spin arc during which the thrusters fire.

4. The angular momentum vector of the vehicle doesn’t have to point along the thrust vector (like in the JSC design) but can point orthogonal to the spacecraft’s orbital plane. This means that the angular momentum vector’s direction doesn’t have to be altered during flight. This also means that spiral-in/spiral-out maneuvers at planets are no problem.

5. If you wanted to use the AG-NEP vehicle for asteroid missions, the electric thrusters might even be able to be used as “descent engines” provided some “landing gear” were provided on the habitat module.

6. Propellant could be used for additional reactor shielding during the flight.

Over the years since this realization, I’ve developed the capability to show how such a vehicle might look as it rotates. Here’s the link to a Java code that will show the vehicle rotating along with the Canfield joints. You can click and drag to rotate the view around and zoom in and out with the mouse wheel.

For reference, here’s the original set of slides from JSC describing the problem and their original design solution.

So in the fall of 2002 that’s what I did, and before long I was writing NRAs (National Research Announcements) to solicit universities and corporations to bid on technology work for tethers. We put the first NRA out for tethers and got responses and had a meeting where a committee picked the winners in March 2003. After that things started getting serious. We had real money for the first time to do momentum-exchange tether work, and there were still so many unanswered questions that needed to be solved.

Sometimes fate or luck or serendipity drops things in your lap. In the summer of 2002, I met one of the most clever and hard-working people I’ve ever had the good fortune to meet–Dr. Stephen Canfield of Tennessee Technological University. The next summer he was down at MSFC and I was in the middle of trying to figure out the answer to a very thorny problem: if you have a tether that’s spinning, how do you keep the solar panels pointed at the Sun? My friend Kyle Frame and I would sit in my cubicle for long stretches of time with pieces of paper pretending to be solar panels and pencils and sticks standing in for the tether, trying to figure out some way to do it that wasn’t totally foolish.

One day Steve Canfield stopped in and asked us what we were up to. We described the problem and he asked a simple question:

“Do you care what orientation your solar panel is in so long as it is pointed at the Sun?”

I said no, we didn’t care, and then he showed me something he’d been working on since he was a grad student. It looked like this:

He called it a “Trio-Tristar Carpal Wrist Joint.” I thought that sounded like a real mouthful so I just called it “Canfield’s joint” and eventually everyone (except Canfield) began to call it a Canfield joint. It was kind of a crazy looking thing that you couldn’t figure out what to do with it unless you held it in your hands and started playing with it. Unfortunately, in a blog post I can’t reach out of your screen and hand you your own Canfield joint to play with, because if I could you’d figure out in a few seconds what I’m talking about, but the real magic of the Canfield joint is that you can point the joint anywhere in a hemisphere without winding up anything.

The joint has several parts. There’s the “base plate” which stays attached to whatever the joint is mounted to, like your spacecraft, and then there’s the “distal plate”, which points to whatever it is that you want to point at. There are six legs on the joint, in three units. The joint is called a “parallel structure” because there’s more than one load path for the loads to follow, and this is what gives it its potential strength. Where the legs mount to the plates is a simple revolute joint. I didn’t know what that meant so I asked Canfield and he said that it just meant that it was a simple, one-degree-of-freedom (one way to move) joint or hinge. Where the two legs come together you could have a spheric joint (a ball and socket with two degrees-of-freedom) or you could have three revolute joints in series. That’s what we usually do.

I asked Canfield what the joint was for. He said that he originally wanted to use it to replace the CV joints in cars, since if it had all revolute-joints then it wouldn’t need a boot. If I hadn’t had to replace the boot on the CV joint in my car when I was in college and dirt-poor, I wouldn’t have had any idea what he was talking about, but the loss of money was still burned in my mind, so I appreciated that application.

Well, to make a too-long story shorter, I learned how the Canfield joint worked and figured out how to solve my little problem on the tether. Tell me if you like the result:
Canfield Joint on MXER Tether
Medium View of Canfield Joint
Closeup of Canfield Joint

My writings have elicited strong responses, both here and over on the NSF forum. People have asserted that I am simply wrong, or that I have gone into the analysis with a bias that has somehow compromised my results, or that I have ignored some obscure advanced version of an NTR that they assert can solve the problems that I have identified. Of these criticisms, it is the criticism that my bias compromises my results that troubles me the most. As an engineer, we want to believe that we are immune to opinions and biases but my experience has been that that is not the case at all. They creep into our judgement, and sometimes they sit flat on our face. I have seen for years now at NASA how even the most clever engineers can be seduced or bullied into accepting terrible vehicle designs, and in a perversion of the Stockholm Syndrome, eventually come to “love” the fatally-flawed design that they would have initially rejected.

I cannot be certain that my biases do not affect my work, but I am always striving to reduce them as much as possible. The most effective way I have found to do this is to get to “the numbers” as quickly as possible. We can talk all day long about how much better this technology is than that technology, but when we get to the numbers we can begin to improve the signal-to-noise ratio of our discussions more quickly than anything else I know.

Let me tell a story about a time when my opinion was changed significantly through solid engineering analysis. When I first got to NASA in 2000, I was part of a study called by a number of different names. Some called it “Decadal Planning”. I thought of it as “go to Mars and back in a year”. Within a few months of getting there, I was running a part of the study looking at propulsion technologies, and we were comparing a number of them. I met a lot of people in the field and began to develop a distaste for nuclear electric propulsion (NEP). The study concluded quietly and in my opinion, was a failure. Some of that was my own fault. But that’s some other post.

Near the end of 2002, my boss asked me to be a part of an MSFC response to a JSC study on a new and different NEP vehicle. As I recall they were interested in launch vehicle options. So in January 2003 several of us went to JSC to talk to them. I was very impressed by what they had done.

Essentially, they had asked “what are the big barriers towards sending people to Mars?” and then “how do we deal with them?” The biggest barrier they had identified was uncertainty about what happens to people after many years in microgravity. So they decided from the outset to design a vehicle that incorporates artificial gravity, thus cutting the “gordian knot” that had driven previous mission planners to “fast” trip times. My own experience on the one-year-round-trip Mars study had convinced me that it was utterly foolhardy to try to go to Mars and back too quickly. But I know the idea still rattles around in the Internet and is carried by the astrodynamically misinformed.

To accomplish the artificial gravity approach, the JSC study anticipated using the natural countermass of the nuclear reactor that would power the NEP vehicle to counterbalance the mass of the inflatable crew habitat. The boom that would be present anyway to keep the reactor away from the crew would now double as the separator needed for artificial gravity. Any time you can get a “two-for-one” value like that in space vehicle design, you want to take it.

The persistent problem in the design was the need to point the engines along an inertial direction while the vehicle was rotating. The JSC planners had rejected the idea of rotating slip or roll rings, for good reason and based on their experience with the ISS. To keep the engines body-mounted and yet pointing along an inertial direction required rotating the vehicle in inertial space, nearly 180 degrees during the transit to Mars. Once they got to Mars, the “spiral-in” proved very difficult, since now they would need to move the rotation axis of the vehicle through 360 degrees on every orbit. They came up with a compromise, called “minor-axis rotation” that mitigated some of the issues associated with this maneuver, but I don’t think they were terribly satisfied with it.

The design wasn’t complete or perfect, but it was a real step forward. And I was very impressed by their willingness to challenge their pre-conceived notions about Mars travel and examine a design completely different from what they had looked at before. And it changed my mind about the value of nuclear electric propulsion technology. In a future post, I’ll describe how a design innovation I came up with could solve the remaining architectural concerns with the AG-NEP vehicle and make it far more feasible.

My contrast, the response of some of the MSFC personnel I was travelling with was not so open minded, at least with regards to launch options. Before we went over to the meeting, we met at a Denny’s for breakfast. There were two senior personnel, a mid-level manager, and me. One of the senior folks (who’s no longer with MSFC) began to lay down the “MSFC position” for the meeting, which he said would be Shuttle-C. I gently began to demur, saying how the use of Shuttle-C would commit us to using the expensive Shuttle infrastructure for decades to come. He quietly but firmly cut me off and said:

“The answer…is Shuttle-C.”

I understood from the tone of his voice that this decision wasn’t technical, it was strategic. Shuttle-C was based on propulsion hardware developed and controlled at MSFC. If the Mars program went forward, if this vehicle was developed, if Shuttle-C was baselined for its launch, then MSFC would be supporting it for many years to come. I looked over at the other senior person and he was nodding his head in agreement. I looked at the mid-level manager and he wasn’t saying anything. He and I were like enlisted men getting our orders from the officers. And so Shuttle-C it was…the only option we were permitted to present to the JSC study team.

As it was, several weeks later the Columbia was destroyed on reentry. Everything changed at the agency and everyone forgot all about Mars studies. But I still remember what I learned and think that there are lessons to draw from it.

Wrong. Super wrong.

Nuclear-thermal SSTO turns out to be one of the worst ideas anyone has ever come up with, for two simple reasons: hydrogen and the lousy thrust-to-weight ratio of nuclear thermal rockets. Those are the same two reasons that make NTR lousy or marginal for nearly any other space application as well, but this post will focus on the issues surrounding NTR SSTO.

In the case of any earth-to-orbit vehicle, you’ve got to have the thrust to get off the ground in the first place. Let’s assume that we’re dealing with a vertically-launched NTR SSTO. It has to have a vehicle thrust-to-weight ratio greater than one, and probably a fair bit better than that in the first place, just to get off the ground. So we can take those expressions that I derived before, assuming hydrogen as a propellant and the engine thrust-to-weight ratios that have been quoted by NTR proponents like Stan Borowski to quickly try to figure a payload fraction for an NTR SSTO.

We find the propellant-mass-sensitive term (derivation here) assuming the liquid hydrogen has a density of 71 kg/m3, ullage of 3%, a mixture ratio of zero, and a tank structural mass factor of 10 kg/m3. This gives us a value of 0.1452 for this term.

We find the gross-mass-sensitive term (derivation here) by assuming that the engine has a vacuum thrust of 15000 lbf, a weight of 5000 lbm, and vacuum thrust-to-weight of 3 to 1. I’m not even going to “ding” the engine for sea-level performance, since as we’ll see, it won’t even matter. With a vacuum T/W of 3 and the same for the sea-level T/W and an initial vehicle thrust-to-weight ratio of 1.25, and we’ll just say that the thrust structure doesn’t weigh anything either, the gross-mass-sensitive term comes out to be 0.4167.

We’ll also ignore any recovery hardware (wings, landing gear, TPS, etc) and say all that weighs nothing. We’ll assume that the engine has a vacuum Isp of 900 seconds and that it takes 9200 m/s of delta-V to get to orbit. Plugging those numbers in the rocket equation gives us a mass ratio of 2.835 (very good!) and a propellant mass fraction of 64.73%. Next we use the prop-mass-sensitive and gross-mass-sensitive terms, along with the propellant mass fraction to get the payload fraction (derivation here).

We start out with the final mass fraction (1 – prop mass fraction) of 35.27%. It doesn’t get any better than that. Then we subtract the gross-mass-sensitive term (41.67%). Now we could stop right here, because we’re already negative (-0.064). That is to say, even before accounting for the issues with tankage, we’re already out of performance. The engines weigh too much. But we’ll keep going and subtract the product of the prop-mass-sensitive term and the propellant mass fraction (0.6473*0.1452 = 0.0940) and we end up with a payload fraction of -0.1579.

So it’s a no-go with these engines. Our payload fraction is grossly negative and we’ve got nothing. It’s clear from the magnitude of the numbers that the engine thrust-to-weight ratio is the main culprit, although the “fluffy” liquid hydrogen tanks don’t help much either.

So what kind of engine performance would you have to have to get even a zero payload fraction? Well, I ran some rough calculations based on a variety of speculative vacuum T/W ratios for some putative NTR engine, at a few different values of specific impulse and plotted the results here:

The graph tells the story. To get payload fractions of zero (a launch vehicle of infinite size) you have to have a T/W at 900 sec Isp of over 10. That’s more than three times the T/W that Stan Borowski projects for his sporty 15K NTR design, which he says will have a T/W of 3. So if you think that Stan or others can design an NTR that only weighs a third of what he thinks it will weigh, then you can dream about an NTR SSTO of infinite mass.

As for me, I’ve thought for some time that NTR was a really bad idea for almost every application for which it is considered. The SSTO application is probably the worst.

Then we moved on to see work on a Merlin engine underway. Despite the fact it was the evening, there were quite a few people at work, building engines, writing code, running tests and so forth.

We moved on and saw a nine-engine cluster of Merlin engines, that if I’m not mistaken will be part of the second Falcon 9 launch vehicle. I’ve seen the Saturn V and Saturn 1B engine clusters up close, and I was amazed how “tight” the F9 engine cluster is. I’m a little worried about the gas generators all exhausting into the freestream rather than being ducted into the nozzles as they were on the F-1 and J-2 engines, but time will tell whether that is an issue for F9.

Also there were some very impressive friction-stir welding equipment that are used to manufacture the propellant tanks for the F9. I saw a circumferential friction-stir welder, and Brian explained that two people can make an F9 tank in 19 days. That is very impressive and part of how they keep costs down. I also saw milling machines used to mill isogrid patterns in the metal stock used for the tanks.

In the rear of the building I saw huge cube-looking structures covered by translucent deep-blue sheets of plastic. Brian explained that that was where they did welding on upper stage engines that use refractory metals (niobium) that must be welded in inert gas atmospheres. I also saw tanks of argon that I figured were used in the inert-gas welding.

SpaceX was kind enough to treat us to hors d’oeuvres afterward and we could mingle and talk about what we had seen. Next to the cafeteria area of the plant was the Falcon control room with huge screens and computer consoles. A video of Falcon launch highlights and F9 launch preparations was playing, and gave you a sense of the excitement that was building as the first Falcon 9 launch was approaching. In the cafeteria area were two statues, one of “Iron Man” and the other of a Cylon that sure gave you a sense that you were in the cool “alt.space” world rather than in a stodgy, cost-plus government contractor facility.

I looked around at the employees that would walk by. Almost all of them were younger than me (35) and I couldn’t help but contrast that with the demographics I experience at NASA, where I’m practically a baby compared to my co-workers, most of whom are in their 50s and 60s.

There was an excitement and buzz in the air at the SpaceX facility. People are designing, building, and testing rockets. They’re going to launch soon. And I think they’re going to succeed. Even if it doesn’t happen at first–I think they’re going to succeed.

This is why President Bush and Sean O’Keefe knew that we would have to bring the shuttle program to an end in order to have any hope of going forward with NASA’s use of space. Michael Griffin knew it. President Obama and General Bolden know this too.

Source: Money key to more space shuttle flights

Continuing on this vein, an article today in The Space Review: “Costs of US piloted programs”

“each day spent onboard by an ISS crewmember costs about $7.5 million (compared to $5.5 million for Skylab.)”

“I find great comfort in knowing that President Obama has seen fit to put his faith in us to develop a game-changing strategy in our four mission areas, and that he has given us a $6 billion plus up on our FY10 budget as a show of support and trust. I fully believe in the plan that this budget has allowed us to set out for NASA’s road ahead, and unlike many of our detractors, I do believe it will very likely allow us to reach exploration destinations sooner and more efficiently than we would have been able to while we were struggling to develop the Constellation Program.”

I completely agree with this statement.