As science and technology advance, we’re asking our space missions to deliver more and more results. NASA’s MSL Curiosity and Perseverance rovers illustrate this fact. Perseverance is an exceptionally exquisite assemblage of technologies. These cutting-edge rovers need a lot of power to fulfill their tasks, and that means bulky and expensive power sources.Continue reading “Exploring the Outer Solar System Takes Power, Here’s a Way to Miniaturize Nuclear Batteries for Deep Space”
On August 25th, 2012, the Voyager 1 spacecraft accomplished something no human-made object ever had before. After exploring the Uranus, Neptune, and the outer reaches of the Solar System, the spacecraft entered interstellar space. In so doing, it effectively became the most distant object from Earth and traveled further than anyone, or anything, in history.
Well, buckle up, because according to NASA mission scientists, the Voyager 2 spacecraft recently crossed the outer edge of the heliopause – the boundary between our Solar System and the interstellar medium – and has joined Voyager 1 in interstellar space. But unlike its sibling, the Voyager 2 spacecraft carries a working instrument that will provide the first-ever observations of the boundary that exists between the Solar System and interstellar space.
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To celebrate the 45th anniversary of the Apollo 13 mission, Universe Today is featuring “13 MORE Things That Saved Apollo 13,” discussing different turning points of the mission with NASA engineer Jerry Woodfill.
For our final installment of this series of “13 More Things That Saved Apollo 13,” we’ll look at an event that has not been widely addressed, but it may have been one of the most crucial scenarios which might have ended in disaster and death for the crew in the final minutes of the rescue.
It starts with an atomic electrical power generator called SNAP-27.
These devises enabled the Apollo Lunar Surface Experiment Package (ALSEP) to operate on the Moon for years after astronauts returned to Earth. They were deployed on Apollo 12, 14, 15, 16 and 17 and included seismometers, and devices to detect lunar dust and charged particles in the lunar environment.
SNAP stands for Systems Nuclear Auxiliary Power and its fuel was plutonium-238 (Pu-238). It was a type of Radioisotope Thermoelectric Generator (RTG) that provides electrical power for spacecraft by converting the heat generated by the decay of plutonium-238 fuel into electricity. Approximately 8 lbs of plutonium was used for each mission and it was it was transported to the Moon in a thermally insulated cask attached to the side of the Lunar Module.
“The cask was so strong and impervious that firing the container with a cannon into a solid brick wall would not break it,” said NASA engineer Jerry Woodfill.
Unfortunately, Woodfill added, as the political climate for anything atomic has grown acrimonious, the application of atomic energy to space exploration has been thwarted.
“Despite a remarkable atomic safety record, a small but powerful political coalition has successfully opposed such harmless devices as NASA’s Apollo SNAP-27 generator,” said Woodfill. “The scare-factor attributed to NASA’s Apollo atomic power generator was based on the threat of a launch pad explosion or exaggerated claims that an accident would contaminate Earth’s atmosphere and ultimately bring death to many. It is amazing that such groups can ignore obvious day-to-day deaths in automobiles yet alarm the public with false atomic threats.”
Woodfill said that the opposition to RTGs has been most unfortunate for the sake of human and robotic exploration of the solar system.
“The limitation of traditional rocket fuels handicaps improvement in propulsion,” he said, “and for the past five decades, little progress has been made in rocket engine specific impulse improvement known as the ISP.”
Additionally, for several years NASA has been facing a shortage of RTGs for powering robotic spacecraft limiting the scope and lifetime of missions going to the far reaches of our Solar System.
For Apollo 13, the SNAP-27 device should have ended up staying on the Moon, but of course, the lander did not land so it, along with the atomic generator, was going to reenter Earth’s atmosphere and end up somewhere on our planet.
It wasn’t long after the accident on Apollo 13 that NASA was contacted by the Atomic Energy Commission (AEC) about where the LM would be reentering and burning up in Earth’s atmosphere.
However, as Apollo 13 approached Earth, their flight path kept deteriorating, despite the crew’s efforts. As we discussed in Part 9 of this series about the potentially fatal gimbal lock, without the Command Modules thrusters and computer navigation system to steer, only the lander’s were available, and manually flying the crippled Apollo 13 spacecraft stack and keeping it on the right trajectory was a huge challenge.
Woodfill said that any ‘tinkering’ with the reentry geometry was altogether ill-advised considering how very ‘iffy” the angle and entry path had become, but AEC officials were pressuring the retro officers about the orientation required for the LM’s reentry to put it into a deep trench in the Pacific Ocean.
Woodfill said that from his perspective of decades of study about the mission, the need to “deep-six” the SNAP-27 generator was almost responsible for having the Apollo 13 rescue end in tragedy. There was confusion among those in Mission Control as well as the crew about the orientation the spacecraft at reentry. However, Woodfill said, an inadvertent ‘mistake’ by Lovell may have actually saved the crew.
“There was a significant debate between the two most knowledgeable retro officers about jettisoning of the lunar lander,” he said. “So uncertain was the scenario of positioning the command ship for LM jettison that the men held exactly opposite views of the result of selecting the position wanted by the AEC scientists. Added to the peril was Lovell’s brush with ‘running the ship aground’, i.e., into gimbal-lock trying to please the AEC.”
A 2009 research paper for AIAA adds insight into the danger of these moments prior to LM jettison and Lovell’s error. “Attempts to perform rapid analysis in a high pressure, time critical spacecraft emergency can lead to errors in analysis and faulty conclusions,” the paper reads. “For example, the spacecraft was maneuvered to the wrong LM/CM separation attitude, ~45 degrees on the north side of the CM ground track rather than the desired 45 degrees on the south side of the CM ground track. This attitude was close to CM IMU gimbal lock and complicated manual piloting.”
Mission transcripts reveal the confusion and the difficulty the crew faced. As Lovell was trying to maneuver the stack into the correct orientation for LM jettison he radioed:
Lovell: We’re having trouble maneuvering, Joe, without getting it in gimbal lock… You picked a lousy attitude, though, to separate.
Capcom: Well, we apologize. Just take your time. Jim, we’ve got time now.
Lovell continued to struggle as the ship continually approached gimbal lock and he questioned the procedure:
Lovell: Houston, why can’t I stay in PGNS ATT HOLD for the LM attitude hold?
Capcom: Stand by on that, Jim.
Lovell: I want to get way over here, Joe, to prevent going into gimbal lock. I have the yaw at about – I’d say about almost 50 degrees.
Capcom: Roger that. Just stay out of gimbal lock and that 45-degree isn’t critical – the out of plane, that is.
Nonetheless, an Apollo 13 post-mission report reveals that shortly before LM jettison the Retro Officer Chuck Deiterich advised the Flight Director that the LM was not in the correct orientation for separation. “The telemetry indicated that we were yawed 45 degrees North instead of 45 degree South,” the report says, so the ship was 90 degrees out of yaw attitude prior to LM jettison.
However, the LM closeout was underway, and there was no chance to use the thrusters to change attitude. The report continues, “No correction action was taken, because the separation was a minimum of 4,000 feet at entry interface, and more likely was going to be 8000 feet or greater. Therefore, no attempt was made to change the attitude.”
“Because the LM’s guidance computer was maintaining the jettison attitude, the crew could no longer steer the assemblage until the LM release,” explained Woodfill. “And then a terribly threatening event arose. In order to preserve the desired attitude to assure that the SNAP-27 plutonium landed in the ocean, the LM’s computer was moving the command ship’s platform into gimbal lock. It was too late to re-enter the LM. The time to unlatch the hatches would be too great.”
But despite the likely loss of control, somehow the LM was jettisoned just prior to the Command Module reaching gimbal lock.
“Had not, it was later discovered, Jim Lovell actually have mistakenly placed the attitude 90 degrees from the desired jettison position, a potentially fatal gimbal lock would have happened,” Woodfill said. “It was as though despite the disagreement between the retro experts and the resulting confusion between Mission Control and the crew, and then Lovell’s error, neither of the miscues of the entire scenario resulted in the dreaded gimbal-lock. Plus, the SNAP-27 ended up in an optimum location in the Pacific Ocean. Indeed, two mistakes made a right. The entry capsule’s guidance platform became stable and ready for reentry.”
However, Deiterich told Universe Today that with respect to the LM jett attitude, the landing point was not greatly affected by north or south. But to assure maximum separation during entry, the southerly direction was actually opposite the northerly direction the crew would fly.
“When I realized they were closing out, I told Kranz we would buy the current attitude,” Deiterich said via email. “The inplane separation velocity was enough to assure reasonable down range separation. We were just being thorough. Knowing is why we accepted the jettison attitude. I remembered the A10 Ascent stage jett and how the pressure between the CM and ASC pushed the ASC away so I picked this as a way to jett the LM on A13.”
Both during the mission and the crew debriefing the puzzling topic of that SNAP-27 disposal caused confusion. Days later during the debriefing, the crew seemed at a loss to understand what was going on with regard to ground control’s insistence on assuming such a particular jettison orientation for the lunar module. Somehow, they didn’t seem aware of the issues with the SNAP-27 atomic generator, an issue that likely would not threaten Earth but in every way threaten the lives of Lovell, Swigert and Haise.
“We were very close to gimbal lock,” Lovell said in the mission debrief. “I questioned whether the LM SEP attitude was that critical. Was it so critical to be at that attitude, or would it have been better to stay away from gimbal lock in the CM?”
Lovell was worried that they didn’t have any backup help of navigating — the Body Mounted Attitude Gyros, or BMAGs. “We didn’t have the BMAGs powered up,” Lovell said in the debrief. “If we had gone into gimbal lock, we would have had to start from scratch again.”
Deiterich agreed, especially since the crew was pressed for time as time for reentry was rapidly approaching. “Maneuvering the LM with the CSM attached was not easy,” Deitrich said via email, “thus Jim tried to keep any maneuvering out of plane to a minimum, once there he was reluctant to move away and also the whole process was brand new and time could then become a factor.”
Woodfill said the entire team in Mission Control helped save the crew – the EECOM (Emergency, Environmental, and Consumables Management) and the lander’s TELMU (Telemetry, Electrical, EVA Mobility Unit Officer) dealing with the spacecraft environmental and power systems, and the ‘Trench’ team of the FIDO flight dynamics officer who was responsible for the trajectory, the GUIDO guidance and navigation officer who was charged with assessing the crafts’ ability to steer itself under astronaut control, and finally, the RETRO whose responsibility was entering Earth’s atmosphere via retro-rocket firing.
“Considering Apollo 13’s myriad of challenges, it would be a toss-up between the groups if a vote were taken akin to voting for the outstanding “player” in a Monday Night Football game,” he said. “But there is no doubt with regard to the final minutes of the contest who would win the vote. It would be the latter group dealing with guidance and reentry. This is especially so considering the number of times the group thwarted loss of guidance. Without them, Apollo 13 would have lost the game to the formidable adversary gimbal-lock.”
And what happened to Apollo 13’s SNAP-27? In the book “Thirteen: The Flight that Failed”, Henry S.F. Cooper said that the plutonium apparently survived reentry and landed in the Tonga Trench south of Fiji in the Pacific Ocean, approximately 6-9 kilometers underwater. It exact location is unknown but monitoring of the areas has shown that no radiation escaped.
Previous articles in this series:
In the past four decades, NASA and other space agencies from around the world have accomplished some amazing feats. Together, they have sent manned missions to the Moon, explored Mars, mapped Venus and Mercury, conducted surveys, and captured breathtaking images of the Outer Solar System. However, looking ahead to the next generation of exploration and the more-distant frontiers that remain to be explored, it is clear that new ideas need to be put forward on how to quickly and efficiently reach those destinations.
Basically, this means finding ways to power rockets that are more fuel and cost-effective while still providing the necessary power to get crews, rovers, and orbiters to their far-flung destinations. In this respect, NASA has been taking a good look at nuclear fission as a possible means of propulsion.
In fact, according to a presentation made by Doctor Michael G. Houts of the NASA Marshall Space Flight Center back in October of 2014, nuclear power and propulsion have the potential to be “game-changing technologies for space exploration.”
As the Marshall Space Flight Center’s manager of nuclear thermal research, Dr. Houts is well-versed in the benefits it has to offer space exploration. According to the presentation he and fellow staffers made, a fission reactor can be used in a rocket design to create Nuclear Thermal Propulsion (NTP). In an NTP rocket, uranium or deuterium reactions are used to heat liquid hydrogen inside a reactor, turning it into ionized hydrogen gas (plasma), which is then channeled through a rocket nozzle to generate thrust.
A second possible method, known as Nuclear Electric Propulsion (NEP), involves the same basic reactor converting its heat and energy into electrical energy which then powers an electrical engine. In both cases, the rocket relies on nuclear fission to generate propulsion rather than chemical propellants, which has been the mainstay of NASA and all other space agencies to date.
Compared to this traditional form of propulsion, both NTP and NEP offer a number of advantages. The first and most obvious is the virtually unlimited energy density it offers compared to rocket fuel. At a steady state, a fission reactor produces an average of 2.5 neutrons per reaction. However, it would only take a single neutron to cause a subsequent fission and produce a chain reaction and provide constant power.
In fact, according to the report, an NTP rocket could generate 200 kWt of power using a single kilogram of uranium for a period of 13 years – which works out to a fuel efficiency rating of about 45 grams per 1000 MW-hr.
In addition, a nuclear-powered engine could also provide superior thrust relative to the amount of propellant used. This is what is known as specific impulse, which is measured either in terms of kilo-newtons per second per kilogram (kN·s/kg) or in the amount of seconds the rocket can continually fire. This would cut the total amount of propellent needed, thus cutting launch weight and the cost of individual missions. And a more powerful nuclear engine would mean reduced trip times, another cost-cutting measure.
Although no nuclear-thermal engines have ever flown, several design concepts have been built and tested over the past few decades, and numerous concepts have been proposed. These have ranged from the traditional solid-core design to more advanced and efficient concepts that rely on either a liquid or a gas core.
In the case of a solid-core design, the only type that has ever been built, a reactor made from materials with a very high melting point houses a collection of solid uranium rods which undergo controlled fission. The hydrogen fuel is contained in a separate tank and then passes through tubes around the reactor, gaining heat and converted into plasma before being channeled through the nozzles to achieve thrust.
Using hydrogen propellant, a solid-core design typically delivers specific impulses on the order of 850 to 1000 seconds, which is about twice that of liquid hydrogen-oxygen designs – i.e. the Space Shuttle’s main engine.
However, a significant drawback arises from the fact that nuclear reactions in a solid-core model can create much higher temperatures than conventional materials can withstand. The cracking of fuel coatings can also result from large temperature variations along the length of the rods, which taken together, sacrifices much of the engine’s potential for performance.
Many of these problems were addressed with the liquid core design, where nuclear fuel is mixed into the liquid hydrogen and the fission reaction takes place in the liquid mixture itself. This design can operate at temperatures above the melting point of the nuclear fuel, thanks to the fact that the container wall is actively cooled by the liquid hydrogen. It is also expected to deliver a specific impulse performance of 1300 to 1500 (1.3 to 1.5 kN·s/kg) seconds.
However, compared to the solid-core design, engines of this type are much more complicated and therefore more expensive and difficult to build. Part of the problem has to do with the time it takes to achieve a fission reaction, which is significantly longer than the time it takes to heat the hydrogen fuel. Therefore, engines of this kind require methods to trap the fuel inside the engine while simultaneously allowing heated plasma the ability to exit through the nozzle.
The final classification is the gas-core engine, a modification of the liquid-core design that uses rapid circulation to create a ring-shaped pocket of gaseous uranium fuel in the middle of the reactor that is surrounded by liquid hydrogen. In this case, the hydrogen fuel does not touch the reactor wall, so temperatures can be kept below the melting point of the materials used.
An engine of this kind could allow for specific impulses of 3000 to 5000 seconds (30 to 50 kN·s/kg). But in an “open-cycle” design of this kind, the losses of nuclear fuel would be difficult to control. An attempt to remedy this was drafted with the “closed cycle design” – aka. the “nuclear lightbulb” engine – where the gaseous nuclear fuel is contained in a series of super-high-temperature quartz containers.
Although this design is less efficient than the open-cycle design and has more in common with the solid-core concept, the limiting factor here is the critical temperature of quartz and not that of the fuel stack. What’s more, the closed-cycle design is expected to still deliver a respectable specific impulse of about 1500–2000 seconds (15–20 kN·s/kg).
However, as Houts indicated, one of the greatest assets nuclear fission has going for it is the long history of service it has enjoyed here on Earth. In addition to commercial reactors providing electricity all over the world, naval vessels (such as aircraft carriers and submarines) have made good use of slow-fission reactors for decades.
Also, NASA has been relying on nuclear reactors to power unmanned craft and rovers for over four decades, mainly in the form of Radioisotope Thermoelectric Generators (RTGs) and Radioisotope Heater Units (RHU). In the case of the former, heat is generated by the slow decay of plutonium-238 (Pu-238), which is then converted into electricity. In the case of the latter, the heat itself is used to keep components and ship systems warm and running.
These types of generators have been used to power and maintain everything from the Apollo rockets to the Curiosity Rover, as well as countless satellites, orbiters and robots in between. Since its inception,a total of 44 missions have been launched by NASA that have used either RTGs or RHUs, while the former-Soviet space program launched a comparatively solid 33.
Nuclear engines were also considered for a time as a replacement for the J-2, a liquid-fuel cryogenic rocket engine used on the S-II and S-IVB stages on the Saturn V and Saturn I rockets. But despite there being numerous versions of solid-core reactors produced and tested in the past, none were ever put into service for an actual space flight.
Between 1959 and 1972, the United States tested twenty different sizes and designs during Project Rover and NASA’s Nuclear Engine for Rocket Vehicle Application (NERVA) program. The most powerful engine ever tested was the Phoebus 2a, which operated for a total of 32 minutes and maintained power levels of more than 4.0 million kilowatts for 12 minutes.
But looking to the future, Houts’ and the Marshall Space Flight Center see great potential and many possible applications for this technology. Examples cited in the report include long-range satellites that could explore the Outer Solar System and Kuiper Belt, fast, efficient transportation for manned missions throughout the Solar System, and even the provisions of power for settlements on the Moon and Mars someday.
One possibility is to equip NASA’s latest flagship – the Space Launch System (SLS) – with chemically-powered lower-stage engines and a nuclear-thermal engine on its upper stage. The nuclear engine would remain “cold” until the rocket had achieved orbit, at which point the upper stage would be deployed and the reactor would be activated to generate thrust.
This concept for a “bimodal” rocket – one which relies on chemical propellants to achieve orbit and a nuclear-thermal engine for propulsion in space – could become the mainstay of NASA and other space agencies in the coming years. According to Houts and others at Marshall, the dramatic increase in efficiency offered by such rockets could also facilitate NASA’s plans to explore Mars by allowing for the reliable delivery of high-mass automated payloads in advance of manned missions.
These same rockets could then be retooled for speed (instead of mass) and used to transport the astronauts themselves to Mars in roughly half the time it would take for a conventional rocket to make the trip. This would not only save time and cut mission costs but also ensure that the astronauts were exposed to less harmful solar radiation during the course of their flight.
To see this vision become reality, Dr. Houts and other researchers from the Marshall Space Center’s Propulsion Research and Development Laboratory are currently conducting NTP-related tests at the Nuclear Thermal Rocket Element Environmental Simulator (or “NTREES”) in Huntsville, Alabama.
Here, they have spent the past few years analyzing the properties of various nuclear fuels in a simulated thermal environment, hoping to learn more about how they might affect engine performance and longevity when it comes to a nuclear-thermal rocket engine.
These tests are slated to run until June 2015 and are expected to lay the groundwork for large-scale ground tests and eventual full-scale testing in flight. The ultimate goal of all of this is to ensure that a manned mission to Mars takes place by the 2030s and to provide NASA flight engineers and mission planners with all the information they need to see it through.
But of course, it is also likely to have its share of applications when it comes to future Lunar missions, sending crews to study Near-Earth Objects (NEOs), and sending craft to the Jovian moons and other locations in the outer Solar System. As the report shows, NTP craft can be easily modified using modular components to perform everything from Lunar cargo landings to crewed missions to surveying Near-Earth Asteroids (NEAs).
The Universe is a big place, and space exploration is still very much in its infancy. But if we intend to keep exploring it and reaping the rewards that such endeavors have to offer, our methods will have to mature. NTP is merely one proposed possibility. But unlike Nuclear Pulse Propulsion, the Daedalus concept, anti-matter engines, or the Alcubierre Warp Drive, a rocket that runs on nuclear fission is feasible, practical, and possible within the near future.
Nuclear thermal research at the Marshall Center is part of NASA’s Advanced Exploration Systems (AES) Division, managed by the Human Exploration and Operations Mission Directorate and including participation by the U.S. Department of Energy.