Thursday, June 22, 2023

Nuclear Thermal Rocket Engine

Searching through my memory hut, the most comprehensive essay I had on this subject was in this post, Stuck In the 20s. I'm very disappointed in myself. I thought I had gone into more detail than I did.

Recovering from umbilical hernia surgery, I stopped at the library prior to the operation to pick up reading materials, some new and some favorites. I looked for Atomic Accidents by James Mahaffey which had entertained me before and found it checked out. Fortunately, Atomic Adventures by the same author was available, and lo and behold, he had a chapter on the thermal nuclear rocket program. Now, there is any amount of declassified material available on the internet, but few that is understandable to the average US citizen (read: morons). As this was a worthwhile chapter (entitled the Lost Expedition to Mars) I figured I'd provide some of the goodies.

As mentioned in my prior essay, Elon Musk is wasting our time with chemical rockets. Not only is the Solar System out of our grasp with this technology, but simple Earth to Low Orbit transportation is also wasteful and long-term cost ineffective as well. If humans want to exploit space, we need nuclear booster tugs to get stuff out there. Damn the radation and crashes, there is no other way to do it.

So, I have to assume I am stuck in this dark timeline where, rather than dying and frying investigating refractory tehcnologies on Venus, I must watch a catfight between Elon Musk and Mark Zuckerberg in a Las Vegas cage.

Pathetic. Nuclear rockets could have been so easy, that, in some brighter timeline, even the Romans could have made a nuclear rocket, had they the knowledge*. Starting in the the 1950s, Los Alamos scientists got to work building nuclear thermal rockets. This consisted of an atomic pile of graphite and uranium fuel used to heat liquid hydrogen to propel itself. Not to demean the genius and talents of scientists and engineers that worked on it, but, like the atomic pile itself, the thing practically willed itself into existence, given the ease with which matter, arranged properly, obeyed the simple wish to fly up into the sky.

(Seriously, the time from the startup of Enrico Fermi's Chicago Pile 1 until the final experiments which solidified atomic pile reactor design was 90 days. The patent for a neutronic reactor, US Patent number 2,708,656, would not be made public until 1955).

A little known fact is that the nuclear engine, or if you will, the proton rocket engine (given that heated and ejected hydrogen gas particles are protons) was to be the third stage of the Saturn V rocket. Werner von Braun approved it without a flinch, and had not the program been pressed for time, they'd have used it.

Back in the 1950s, the atomic rocket was developed in conjunction with the atomic jet plane, the problem being how to lob these massively huge hydrogen bombs down upon Soviet heads. The bomb was shrunk down enough that conventional rockets would work. (In fact, the thousands of pounds of Sputnik II and Yuri Gargarin's capsule were flung into space atop arrayed banks of WWII V2 rocket engines, much as Musk plans to do with his Buck Rogers dildo Starship). The atomic jet plane was killed off, but work on atomic rockets continued until cancellation in 1973. Imagine what we would have now with proven 50-year-old nuclear rocket technologies.

Doctor Mahaffey: "The advantage of a nuclear rocket over chemical rockets is the efficient use of fuel, as designated by its specfic impulse (SI). SI which is expressed in seconds, is the "hang time" of a rocket, or the maximum number of seconds it can accelerate, balancing against the pull of  Earth's gravity and hanging still above the ground. The SI depends on many factors, such as the weight of the fuel which must be carried and the speed of the mass exhaust leaving the engine. The faster the flying gas exits the nozzle, the more reaction is derived, and the speed of the gas is due to its temperature and weight of the gas particles. The lightest possible gas is hydrogen which is the perfect propellent for a nuclear engine. For a chemical engine, the lightest possible (LH2/LOX) combustion product is steam, which is 18 times the weight of a hydrogen particle or proton. The F-1 engines used in the Saturn V, burning kerosene in liquid oxygen, had an SI of 350 seconds. The theoretical limit to a steam exhaust rocket is 450 seconds. A SpaceX Raptor engine has an SI of 380 seconds. The SI for a nuclear rocket starts at 900 seconds and can increase, in theory, into the millions of seconds."

The Rocketdyne F-1 engine, burning through olympic sized pools of propellant, lasted 165 seconds on the first stage of the Saturn V. It could only be started once and its only throttle setting is full thrust. A NERVA  nuclear rocket engine can be run for 10 hours, stopped and restarted sixty times (chilled down by space to near absolute zero an brought up to 2750F in minutes, and can throttle from full thrust to barely moving. Reactor core designs, U235 and graphite, pinned together with stainless steel rods and tungsten ranged from 4000 megawatts to 600 megawatts. In May 1971, the smallest refined nuclear engine, Peewee, weighed just 11 pounds and had a SI of 1000 seconds. It ran for two hours at a blistering 4145F degrees.

Doctor Mahaffey: "(from 1955 on, Los Alamos developed) Five reactor (U235) core designs with power ratings ranging from 600 to 2000 megawatts using graphite as a neutron moderator. Their aggressively odd code names were Uncle Tom, Uncle Tung, Bloodhound, Shish, and Old Black Joe".

(Clearly racist names but hand waved away due to working with black-as-ink graphite would quickly turn technicians into black faced, black handed minstrel show characters. Racial sensitivity being nonexistent then, but I can't help feeling there was a grudging recognition that if you wanted sheer brute strength and stamina, old negroe men were an archetype for a nuclear rocket). 

"Old Black Joe, designed to run at 1200 megawatts of power, was approved in Novermber 1956 for continued development. The design was upgraded to 2700 megawatts and plans were to use it as for a range-extending second stage for the Atlas missile. This Super Atlas would be capable of parking a heavy H-bomb in geosynchronous orbit hovering above Moscow ready to pounce at a moment's notice. It would be 9.6 feet in in diameter and 96.6 feet high, and to the delight of the Air Force, it would seem a better idea than carrying missiles around in submarines. The bad news was it would take an eye-watering one billion dollars to develop."

(Keep in mind a decade later the US of A would be spending 2 billion dollars a month in Vietnam).

"At this point in 1956, no nuclear rocket engine had ever been built and the technology consisted of designs on paper and a few computer simulations."

(in 1956 95% of all electronic computer calculations were devoted to simulating nuclear processes. Without  a burgeoning need for computers from the defense sector, there woudn't have been enough demand for commercial development and you, gentle reader, would not be reading this on a magical glass box)

"The mind numbing list of impossibilities didn't seem to bother the engineering climate of the time. The fuel pump would take a frozen hydrogen slush at -434F, near absolute zero, and push it at a rate of 70 pounds per second into the top of a nuclear reactor running at two billion watts. No such pump existed. No nuclear reactor had ever run at that power level. In 52 inches, from the hydrogen intake to the nozzle, the liquified fuel would go from near abosulte zero to 3682F through multiple mechanically chaotic phase changes and a severe pressure drop that would try to suck the core out of the end of the engine. The fuel, liquid hydrogen, was the most corrosive substance known, and while sittting quietly in the storage tank would diffuse into all the metal structures it touched, rendering them more brittle than mere freezing would make them. Nothing was know about how stray neutrons from the reactor would interact with hydrogen slush in the fuel tank, whether two nuclear rockets sitting side by side would cause each other to go supercritical via neutron exchange, or how to keep the hot unsupported end of the reactor, glowing incandescent, from following the hydrogen gas out the rocket nozzle."

"(Rocketdyne was contracted to design the fuel pump and nozzle (the nozzle jacketed to flow liquid hydrogen so it didn't melt) and Aerojet General Nucleonics did the plumbing. The Soviets had their own secret nuclear rocket program in their RD-0410 engine. Their project began in 1965 after clandestine observation of American efforts, but was stopped after Chernobyl in 1986. Their engineers were never able to master the intricacies of pumping liquid hydrogen or even keeping it in a tank. They never put a comrade on the Moon. Think of the money they saved)".

What about radiation? Fuck that noise we're going to Mars. To mollify the proles, talk was always that the nuclear engine would be launched by conventional means, and should an oopsie occur it would be outside the contamination-sensitive atmosphere. But oopsies do occur and it was recognized that stuff would fall to Earth eventually. Studies showed that activation at 100,000 feet or in LEO would result in the same amount of radiation as blasting off from ground zero nuclear engines ablaze. Hydrogen exhaust is not radioactive. An added bonus of ground launch is that the thousands of degrees hydrogen exhaust would combust with atmospheric oxygen, boosting the rocket further. Besides, daily radioactive fallout from cosmic rays hitting Earth's atmosphere far exceed the danger of nuclear rocket exhaust.

What about crashes? During the decades of tests of the various nuclear rocket configurations at Jackass Flats Nevade, yes, chunks of the reactor would occasionally break loose and fly out. Often times no big deal and they would continue operating the rocket regardless. KIWI-B1B, in September 1962 operated at 965 megawatts almost to the target 1000 megawatts.

"It hung there for 100 seconds, despite the fact that the core was still leaving in chunks. It ran for a few more minutes, control drums rotating to maintain criticality, until a nozzle sensor blew out and a fire started. All in all, it was seen as a successful run. Back in Washington, details of the KIWI-B1B test were interpreted differently, and key people in charge of the budget had to be peeled off the walls by Werner von Braun at a hearing. These nuclear rockets were to be used for Earth-Moon shuttles to and from a Moon base, and we could not have pieces of engines flying willy-nilly hither and yon. Even wihout a Mars mission, the nuclear rocket program was important if the manned spaceflight program was going to be something other than an isolated moment of glory".

Again, what about crashes? Oh, well, if you are going to get snippy about it. Development of the nuclear engines was turned over to Westinghouse for construction, but in 1965 Los Alamos decided to end its KIWI program with a bang. 

"NASA, always safty conscious had asked what's the worst that can happen? That would probably be toppling off the top of the Saturn V booster, where it was scheduled to be the last of three boost stages for the manned Moon shot. Los Alamos dropped a KIWI 75 feet onto a concrete pad, to see if it would somehow throw the reactor into uncontrolled criticality, which they knew would not happen. That's interesting said NASA but but it's a 300 foot drop off a Saturn. Los Alamos took the challenge, bolted a KIWI to a rocket sled and slammed it into a concrete barrier at high speed. Then, to top it off, they put together a special KIWI, cleverly named KIWI-TNT, having controls that could be slammed into the full-on position with pneumatic cylinders, putting the reactor into prompt critical mode all of a sudden. The reactor exploded in a blue flash with a blast equivalent of 300 pounds of black gunpowder, scattering its remains over a 1700 foot radius. Over half of it was found and picked up eventually (by hand)." No problem, I guess, considering Jackass Flats was surrounded by nuclear test sites. NASA canned the engine as a third stage for Apollo, feeling that development was moving too slowly.  What if the engine had been dropped from space? Oh, well, the Soviets routinely did that with the Kosmos series. Ho hum.

So there you have it. DARPA and NASA are working on nuclear rockets again, but I can't help but feel that by this time, given real world use of nuclear rocket boosters, we could be putting aircraft carriers moving back and forth from orbit at this point. Hi ho!