What's in the engine room? The control console for the reactor and the control console for the propellant pumps. Plus the controls to the remote waldoes/robot who fix things in the radioactive section. Damage control equipment and related items.
Remember, outside the engine room hatch will be a decontamination booth. And I'm sure over the hatch will be mounted an alarm with a red rotating light, so you don't have to put your ear on the bulkhead to hear Astro say "Oh SH*****T!!!". Past the hatch will be a radiation-shielded corridor, with a dog-leg bend in it, so you can get in but radiation cannot get out (radiation has to travel in straight lines, but crewmen can zig-zag). Be sure you are wearing your dosimeter. And you might want to get a raycat as a mascot.
The shadow shield will be in the floor, with the engine(s) below that.
Around the engine deck will be auxiliary propellant tanks. The main tanks provide propellant, which is also used to cool the reactor and keep it from melting. In case something happens to the main tanks, the auxiliary tanks give Astro a few precious seconds of coolant time so he can SCRAM the reactor. The engine deck will also have some kind of Geiger counter to warn the crew of a radiation leak. There will also be controls for the ship's power plant, whether the power is tapped from the propulsion system or from a separate unit.
There will also be controls for the flow rate of propellant, and pyrometer (temperature) gauges with helpful red bands labeled "DANGER - MELTDOWN IMMINENT!". There will be a large red "discoverer" button which activates the "there's been a nuclear oopsie" alarms, right next to the even bigger button marked "SCRAM", which is the emergency reactor shutdown. Some say the term is an acronym for Safety Control Rod Axe Man, i.e., the guy who axes the ropes in order to plunge the cadminum damper rods back into the pile. Other say that acronym is a bunch of baloney, you decide.
But there will be one even bigger red button, marked "PANIC!". This one activates the explosive bolts and JATO units that jettison the reactor out the stern of the spacecraft, hopefully removing the spacecraft from the lethal radius. The reactor isn't going to explode like a nuclear weapon, it will just act like a midget neutron bomb, utterly destroying all life on the ship except for the cockroaches.
This amusing example of 1960's style user interface design is from NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965). This complements the Pilot's console from the same book. This design assumes that it is for a solid-core nuclear thermal rocket.
Mr. Crouch decided that controlling the rocket's trajectory while simultaneously juggling the power levels of the nuclear reactor was a little too much to ask of a single human being. Each subsystem has too many displays, control functions, and automatic interlocks. So he split it into two jobs: Pilot and Engineer. The Pilot will send the Engineer a request for a certain thrust level. The Engineer will do their best to adjust the engine to produce said thrust; without the engine melting down, exploding, or doing anthing else drastic,
Presented here is the control console for the engineer.
According to Mr. Crouch, there are four independent subsystems involved with flying a nuclear thermal rocket:
The pilot will be controlling the thrust vectoring and spacecraft orientation, the engineer will be controlling heat generation and propellant flow. So the pilot is flying the rocket, while the engineer is flying the reactor and turbopump.
Front and center is the Engine Scanner. This would display the outputs of the various closed-circuit TV scanners, infared mappers, microphone pickups, lights, and periscopes built into the propulsion system to allow the engineer to keep tabs on everything. In addition, multichannel recorders would provide visual displays of engine data, calibration curves, and nuclear radiation profiles. The data plotters would allow the engineer to spot any dangerous trends. The engineer would be particularly interested in displaying computations of fission product inventory and the changing center-of-mass due to propellant sloshing and consumption.
Below the scanner is the thrust demand indicator. This displays the thrust mode order given by the pilot via their thrust mode selector. Again, this is basically a glorified Engine Order Telegraph from the age of steam. The pilot uses it to tell the engineer what sort of thrust is required. It is then the engineer's job to juggle the reactor control rod and the propellant turbines to produce what is requested.
The engineer has two control levers. The right directs the reactor, the left directs the turbopump. These are one-axis levers with stepped notches for shim settings and vernier knobs for trim settings (i.e., a dial on the top of the lever for fine tuning). What it boils down to is that the right lever controls the rocket's exhaust velocity, and the left lever controls the rocket's propellent mass flow. The constraint is that the engineer cannot set the two levers to values that will cause the reactor to melt or the turbopump to explode. Or "flood the engine", that is, feed so much propellant that the reactor can't heat it all so the exhaust is lukewarm.
As mentioned before, you generally don't care what the ship's acceleration is, except during lift-off and landing. But it is also important if you are in a combat situation, or docking, or otherwise have to change the ship's velocity in a hurry.
Associated with the right lever reactor controller, there would be readouts of reactor period, neutron flux level, control drum setting including vernier tube setting, net reactivity worth, rate of temperature change, and temperature levels.
Associated with the left lever turbopump controller, there would be readouts of pump speed, discharge pressure, propellant flow rate, amount of propellant onboard, core bypass valve positions, and quality of propellant going into the reactor core.
Finally would be the dire warning of the "risk-of-excursion" light, at the very top ("excursion" is a technical term for "nuclear reactor criticality accident" ). This alerts the engineer that the reactor is at risk of a criticality and they had better do something about it fast! The "reactor period" readout displays the dangerous decreasing reactor period. The smaller the reactor period, the more rapid the change in reactor power level. The neutron flux readout shows the current reactor power level. If it goes too high, the reactor emits a deadly burst of neutrons and everybody dies.
Nuclear fission reactors are throttled by controlling the amount of neutrons available in the core. "Thermal neutron" reactors need slow moving neutrons, "Fast neutron" reactors use fast ones. Basically this means that a thermal neutron reactor has the fission fuel elements embedded in a "moderator", which turns fast neutrons into thermal neutrons. In both designs, the reactor is generally encased in a neutron reflector. This intercepts neutrons on their way out of the reactor and sends them back in, "kicking them back into play" so to speak. This lowers the amount of fission fuel you need in order to sustain a chain reaction.
The amount of free neutrons available is controlled by the dampers. In a NERVA engine, these are rods of cadmium or other neutron poison inserted or removed from the reactor to control the chain reaction. Make sure they are not warped. A non-automated set-up will have the dampers positioned by hand using a "multiplying vernier" and a "danger gauge". The gauge tells you how hot the reaction is. Verniers tell how far a given damper has been inserted into the reactor. Pull the dampers out slowly until the danger gauge tells you the reaction is at the desired level.
Side note: a vernier is what they used to use back in the ancient days before digital read-outs. Standard gauges have a pointer that runs along a scale. A vernier is a clever way to increase the accuracy of the scale.
In some NERVA designs, instead of rods of cadmium inserted into the body of the reactor, they instead have drums imbedded in the neutron reflector around the body of the reactor. One side of the drums are composed of neutron reflector, the other side is composed of neutron absorbers (AKA "neutron poison"). If the reflector side of the drums are facing the reactor, enough neutrons are reflected to sustain the chain reaction. If the absorber side is facing, it sucks up the neutrons and there are not enough neutrons to sustain the reaction. The control drums are rotated to partial positions in order to throttle the reaction to desired levels. The drums are rotated by the "control drum actuator", which look like a metal mushroom, one atop each control drum.
In both cases you want a damper fail safe, so if anything happens the failure modes will tend to make the dampers slam in to the full "quench reaction" position.
There will be damper safeties. When no acceleration is expected for some time, these lock the dampers in the full-quench position.
The following description is for a solid core fission engine, and is adapted from NUCLEAR SPACE PROPULSION by Holmes F. Crouch (1965).
Say the engineer gets the command from the pilot to prepare for a burn. The engineer "preconditions" the turbopump and the reactor so they are able to generate thrust on command, they are in the state called "idling." The turbopump is idling with its discharge bypassed back to its suction side. The propellant lines into the reactor have been chilled down and bleed close-loop flow is maintained. The reactor is nuclearly critical but the core is "dry", it has no propellant in it.
The fun starts when you try to actually make this contraption generate thrust.
You are changing from a dry core to a wet core, one that has propellant flowing through it. The core is both very hot with heat and very hot with neutron radiation. And the liquid hydrogen propellant is both very cold (we are talking a freaking minus 252° Celsius here) and also a pretty good neutron moderator. So when the cold propellant hits the hot reactor two things happen, and both are rather drastic. There is a nuclear disturbance and a thermal disturbance. Both create transients oscillations that can destroy the engine if they are not controlled. See Figure 1.
The nuclear and thermal disturbances are linked because both are caused by the liquid hydrogen propellant. The more LH2, the more it moderates worthless fast neutrons into fission-causing thermal neutrons, thus drastically increasing the fission rate. Also the more LH2 the more the cryogenically cold stuff thermally cools the red-hot reactor core.
This is the single most deadly problem in nuclear thermal rocket design. Among designers, more attention has been given to this subject, "Engine Dynamic Stability," than to any other aspect of thrust control. This is why the ship's engineer has prematurely gray hair.
The nuclear transient is due to the unfortunate fact that the liquid hydrogen propellant is a pretty good moderator. The dry reactor has the control drums set so the core has enough thermal neutrons to sustain a moderate chain reaction. The flood of propellant abruptly skyrockets the number of thermal neutrons, which abruptly skyrockets the chain reaction. The chain reaction "thermostat" automatically reacts to the sudden rise in neutron flux, and quickly rotates the control drums to cool off the reaction (in Figure 1 at point "Control Drums Set Back").
This causes a flux undershoot as the drums over-correct (in Figure 1 at point "Flux Undershoot").
The thermostat then over-corrects while attempting to bring the chain reaction up to the desired level, which causes a flux overshoot. If the engine designer has done their job correctly, these oscillations die down to a steady flux at the desired level. If they have not done their job correctly, the runaway oscillations increase until the reactor melts or has a criticality accident. The runaway nuclear oscillations would also make runaway thermal oscillations, compounding the disaster.
The thermal transient occurs because the initial flood of sub-zero propellant extracts heat from the idling reactor core faster than the heat can be resupplied. This lowers the core temperature. The temperature drops until the heat from the rising chain reaction increases enough to stop it (in Figure 1 at point "Minimum Temp"). If the temperature oscillation is not controlled, they could create pressure oscillations in the propellant. These pressure oscillations can be transmitted upstream where they could damage the turbopump. And the pressure oscillations can affect the moderating characteristics of the propellant, which would affect the nuclear oscillation.
If everything goes according to design, the twin oscillations die out and level off. If the engineer's luck has run out, the twin oscillations reinforce each other and the engine has a simultaneous criticality accident and a reactor core melt down.
What makes this so deadly is the fact that the thermal oscillations can affect the nuclear oscillation and vice versa. If the oscillations start to amplify each other you are doomed. This also means that as the engineer tries to damp out one oscillation the procedure might make the other oscillation worse.
There are equations for these transients, but most of them are differential equations so I'm not going to bore you with them. You can find them in Dr. Crouch's book.
A Kiwi nuclear thermal rocket test (1965). This was a deliberate destruction test. The safety experiment was designed to obtain basic reactor shutdown information for use in predicting the behavior of nuclear rocket reactors under a wide range of accident conditions. It detonated with a nuclear explosion yield equivalent to 2.1 tons of TNT, and reached a maximum temperature of 4,250° K, which vaporised 5% of the reactor core fuel rods, of which 68% was dispersed as fallout with a specific activity of 1015 fissions/gram for refractory nuclides like Zr-95.
(ed note: Apparently this is a preliminary document. Many of the performance values are indicated as TBD or value To Be Determined at a future date)
This part of this specification defines the requirements for the performance, design, and qualification of equipment identified as the NERVA Nuclear Rocket Engine, Contract End Item (CEI) No. 90290 as established by the NERVA Program Requirements Document, SNPO-NPRD-1. This CEI, hereinafter referred to as the engine, is used as a source of primary propulsive power for both manned and unmanned space vehicle applications. The engine is designed to operate at a vacuum thrust level of 75,000 lbs (330,000 Newtons) and a specific impulse of 825 seconds (exhaust velocity 8,090 m/s) and shall be man rated. The engine requires externally supplied liquid hydrogen, command signals, and electrical power. Rated thrust is achieved at a nominal thrust chamber pressure of 450 psia and a nominal design thrust chamber temperature of 4250° Rankine and with a nozzle having an expansion ratio of 100:1. Endurance at rated temperature shall be 600 accumulated minutes. The operating time is utilizable in multiple cycles up to 60 with durations of varying lengths up to 60 minutes.
1.1 Mission Definitions — The following missions are used in the definition of NERVA requirements. Payloads shall be maximized consistent with the engine performance requirements of this specification.
22.214.171.124.1 Operational Modes — The engine shall be capable of performance as specified in 126.96.36.199.5, (Impulse and Controllability Requirements) while operating in the following modes: (See Section 6.2 for definition of operational modes.)
- Reusable Interorbit Shuttle — To shuttle payloads (manned and unmanned) between a 262 nautical mile earth orbit (485 km) and a space station in lunar or geosynchronous earth orbit and return for reuse.
- Unmanned Deep-Space Injection — To place a large unmanned payload on a deep space trajectory using the reusable nuclear shuttle from 262 nautical mile earth orbit and returning the shuttle vehicle to earth orbit for reuse.
- Normal Mode
- Malfunction Mode
- Single Turbopump Operation
- Component Malfunction
- Emergency Modes
188.8.131.52.2 Vacuum Performance Rating — The engine performance rating is based on nominal vacuum thrust using liquid hydrogen as specified in MSFC Specification 356 with a 100:1 nozzle area ratio as follows:
- Thrust — 75,000 ± 2000 lb which includes a ± 1500 lb controllability tolerance. (Thrust considered parallel to the pressure vessel axis).
- Specific Impulse — 825 sec ±0.75% (which includes ± TBD % controllability tolerance but does not include allowable operating H2 leakage nor H2 required for tank stage operation) Minium Specific Impulse — 819 s with flight vehicle Isp trim signal.
- Nominal Chamber Pressure - 450 psia (The nominal chamber pressure is the stagnation pressure).
- Nominal Chamber Temperature - 4250°R (The nominal chamber temperature is the stagnation temperature).
- Normal Mode Endurance - 600 minutes at rated temperature (accumulated in up to 60 cycles of varying duration up to 60 minutes maximum per cycle).
- In meeting the thrust and impulse requirements all components must perform within their specified tolerances.
184.108.40.206.3 Operational Constraints — The engine shall be capable of operating at any selected point within the operational constraint map shown in Figure 1.
During thrust buildup and retreat, the engine shall be capable of chamber temperature ramp rates of 150 ± 25°R/sec and chamber pressure ramp rates of TBD psi/sec at a pressure less than 293 psia and at a rate of 50 ± 10 psi/sec at a pressure greater than 293 psia.
220.127.116.11.5.1 Normal Mode Impulse — The engine shall be capable of meeting the following performance requirements when operating in the normal mode. The normal operating cycle shall be initiated by a vehicle command signal to depart from a coast (shutdown) condition or previous operating cycle and is completed upon termination of cooldown flow, post operational status checks and coast preparation, i.e., system power-down, or the receipt of a command signal for restart. The normal operating cycle is shown in Figure 2.
18.104.22.168.5.1.1 Prestart — The engine shall be capable of performing function and status check operations as commanded to assure readiness for startup. There shall be no propellant flow other than permitted by 22.214.171.124 (Leakage) during prestart operations except as required for cooldown during restart. Prestart time shall be TBD + TBD minutes.
126.96.36.199.5.1.2 Startup — Startup consists of temperature conditioning, nuclear startup, bootstrap, and thrust buildup. Startup is initiated upon receipt of a command signal to initiate propellant flow or nuclear startup and is completed when rated performance has been achieved within the specified controllability limits. The engine shall be capab1e of accomplishing temperature conditioning and nuclear startup simultaneously or sequentially depending on prior operating history. The engine shall be capable of meeting the following requirements during normal startup operations.
188.8.131.52.184.108.40.206 Temperature Conditioning and Nuclear Startup.
220.127.116.11.18.104.22.168.1 Temperature Conditioning — Temperature Conditioning is initiated at engine startup and consists of non-nuclear component and reactor thermal conditioning. These operations may be conducted separately or simultaneously depending on engine thermal conditions at startup. The engine shall be capable of being temperature conditioned for the initiation of bootstrap within TBD seconds and shall consume less than TBD lbs of propellant during this time. The time required for this function shall be predictable within + TBD seconds and propellant consumtion shall be predictable within + TBD lb.
22.214.171.124.126.96.36.199.2 Nuclear Startup — Nuclear startup may occur separately or simultaneously with temperature conditioning. During nuclear startup reactor criticality shall be achieved and temperature control shall be established. The time required for this function shall not exceed TBD sec and shall be predictable within TBD sec for each nuclear startup.
188.8.131.52.184.108.40.206 Bootstrap — Bootstrap startup begins with initiation of flow through the turbines, and ends when program control has been achieved to initiate thrust buildup. Temperature control shall be maintained during bootstrap, and the engine shall be brought under program control when chamber pressure has increased to TBD + TBD psia. The engine shall be capable of completing bootstrap startup within TBD sec and shall consume less than TBD lb of propellant during this time. These parameters shall be predictable to within + TBD sec and + TBD lb propellant for each bootstrap startup throughout the engine operating life.
220.127.116.11.18.104.22.168 Thrust Buildup — The engine shall be capable of thrust buildup through the engine throttle point, and shall maintain rated specific impulse from the throttle point to steady state operations. For each thrust buildup cycle during the engine useful life, the thrust and specific impulse shall be predictable as a function of time and engine history. These parameters shall be controllable to + TBD percent thrust and + TBD percent specific impulse of instantaneous predicted values.
22.214.171.124.5.1.3 Steady State Operation — Steady State Operation is initiated when rated performance has been achieved within specified controllability limits, and is terminated by receipt of a command signal to begin retreat from this condition. During Steady State Operation the engine shall be capable of providing the vacuum performance specified in 126.96.36.199.2 (Vacuum Performance Rating).
188.8.131.52.5.1.4 Shutdown and Cooldown — Shutdown consists of throttling, throttle hold, temperature retreat and pump tailoff, and is initiated by a command signal to depart from rated conditions and is completed upon termination of powered pump operation. During shutdown the engine shall be capable of steady-state hold at the engine throttle point. Cooldown is initiated upon completion of engine shutdown and is completed upon termination of propellant flow or the receipt of a command signal for restart. Cooldown propellant is supplied at tank pressure conditions as defined in 184.108.40.206.8, (Propellant Conditioning). The total delivered impulse during shutdown and cooldown shall be predictable within + TBD percent of the total startup and steady state impulse as a function of engine operating history and shall be controllable as a function of time after initiation of shutdown. Provision shall be made for a TBD sec steady state hold at the throttle point, and for each shutdown cycle during the engine operating life the thrust and specific impulse shall be controllable to + TBD percent thrust and + TBD percent specific impulse of instantaneous predicted values from initiation of shutdown to termination of steady state hold at the throttle point. The total impulse delivered after termination of the steady state hold at the throttle point shall be controllable to within ±20,000 lb sec at termination of cooldown. The time of termination of cooldown impulse shall be predictable within ±15 sec. Cooldown thrust shall be not less than 30 lb and average cooldown specific impulse shall be not less than 400 sec.
220.127.116.11.5.1.5 Post Operations — The post operation period begins with the termination of cooldown and ends when the engine is powered-down for coast or space storage. During this period, the engine shall be capable of functional and status check operations. The time for this operation shall not exceed TBD minutes. There shall be no propellant flow other than permitted in 18.104.22.168 (Leakage).
22.214.171.124.5.1.6 Coast — The coast operation period is initiated upon completion of the post operation period and continues until receipt of a signal to initiate restart operations. During coast operations the engine thrust (due to allowable non-operating leakage) shall not exceed TBD lb.
126.96.36.199.5.2 Malfunction Mode Impulse — The engine shall be capable of meeting the following performance requirements when operating under the following malfunction conditions. The engine shall be capable of direct transition to the malfunction modes during any phase of engine operation.
188.8.131.52.5.2.1 Single Turbopump Operation Impulse — The engine shall be capable of operating with one Propellant Feed Subsystem leg inoperative. Operation in this mode shall be initiated or completed as specified in 184.108.40.206.5.1 (Normal Mode Impulse), or by receipt of a command signal demanding the engine to switch to this mode of operation from the normal mode startup, steady state or shutdown functions.
220.127.116.11.18.104.22.168 Prestart — There shall be no propellant flow other than permitted in 22.214.171.124 (Leakage) during prestart operations except as required for cooldown during restart. Prestart time shall be TBD + TBD minutes.
126.96.36.199.188.8.131.52 Startup — The engine shall be capable of the following requirements during single Turbopump startup operation.
184.108.40.206.220.127.116.11.1 Temperature Conditioning and Nuclear Startup
18.104.22.168.22.214.171.124.1.1 Temperature Conditioning — The engine shall be capable of being temperature conditioned for the initiation of bootstrap within TBD seconds and shall consume less than TBD lb of propellant. The time required for this function shall be predictable within + TBD sec.
126.96.36.199.188.8.131.52.1.2 Nuclear Startup — Nuclear startup may occur separately or simultaneously with temperature conditioning. During nuclear startup reactor criticality shall be achieved and temperature control shall be established. The time required for this function shall not exceed TBD sec and shall be predictable within + TBD sec for each nuclear startup.
184.108.40.206.220.127.116.11.2 Bootstrap — Bootstrap startup begins with initiation of flow through the operational turbine, and ends when program control has been achieved to initiate thrust buildup. Temperature control shall be maintained during bootstrap, and the engine shall be brought under program control when chamber pressure has increased to TBD + TBD psia. The engine shall be capable of completing bootstrap startup within TBD sec and shall consume less than TBD lb of propellant during this time. These parameters shall be predictable to within + TBD sec and + TBD lb propellant for any Single Turbopump bootstrap startup occurring throughout the engine operating life.
18.104.22.168.22.214.171.124.3 Thrust Buildup — For each thrust buildup cycle during the engine useful life, the thrust and specific impulse shall be predictable as a function of time and engine history. These parameters shall be controllable to + TBD percent thrust and + TBD percent specific impulse of instantaneous predicted values.
126.96.36.199.188.8.131.52 Steady State Operation — The engine when operating with one PFS leg shall provide the vacuum specific impulse specified in 184.108.40.206.2 (Vacuum Performance Rating) and a nominal vacuum thrust of 60,000 lb. The engine shall be capable of operation at extended duration as required to deliver a single burn total impulse equivalent to that which would have been required for normal mode operation.
220.127.116.11.18.104.22.168 Shutdown and Cooldown — The engine shutdown and cooldown requirements for this mode of operation shall be as specified in 22.214.171.124.5.1.4 (Shutdown and Cooldown) for normal mode operation.
126.96.36.199.188.8.131.52 Post Operation — The engine post operation requirements shall be as specified in 184.108.40.206.5.1.5 (Post Operations) for normal mode operation.
220.127.116.11.18.104.22.168 Coast — During coast operations the engine thrust (due to allowable non-operating leakage) shall not exceed TBD lb.
22.214.171.124.5.2.2 Component Malfunction Impulse — When operating with component malfunctions other than those which would cause operation with a single PFS leg as specified in 126.96.36.199.5.2.1 (Single Turbopump Operation Impulse), or emergency operation as specified in 188.8.131.52.5.3 (Emergency Mode Operation), the engine shall be capable of operating under the conditions specified in 184.108.40.206.5.1 (Normal Mode Impulse).
220.127.116.11.5.3 Emergency Mode Operation — The engine shall be capable of operation in an emergency operating mode. No more than one emergency cycle shall be required of the engine. The emergency operating modes shall be initiated manually or by a command from the malfunction detection and control system or the trend data system demanding the engine to an emergency mode of operation from any point on the engine operating map. The engine shall be capable of a transition to the emergency operating mode during any portion of startup or steady state operation. For emergencies occurring during shutdown, the engine shall be capable of cooldown for up to five hours prior to entering the emergency operating mode. The engine shall be preserved in a restartable condition if it can be done at no additional risk to the population, passengers or crew. The engine shall be capable of providing emergency mode impulse at selected points (TBD) within the operational constraint map shown in Figure 1. Emergency mode impulse requirements shall be determined during the engine development program. The minimum emergency mode impulse and thrust shall be 108 lb.sec. and 30,000 lb. respectively, as specified in 18.104.22.168.1.3 (Malfunction Operation). Minimum emergency mode specific impulse shall be 500 sec.
22.214.171.124.6 Restart Requirements — The engine shall be capable of entering the engine prestart phase for restart at any time after completing the shutdown phase of a previous operating cycle.
126.96.36.199.10 Thrust Vector Control — The engine gimbal system shall be capable of providing the following thrust vector control in all directions:
Thrust Vector Control Angle from null 0 - 3.0° Angular velocity 0 - 0.25°/sec Angular acceleration 0 - 0.5°/sec2
188.8.131.52.11 Nuclear Radiation Shielding — Engine components shall be protected from radiation emitted from the nuclear subsystem by a shield internal to the reactor pressure vessel. The engine shall be capable of incorporating an external radiation shield to reduce the dose due to engine radiation to permissable levels within manned spacecraft. The engine design shall minimize the sources of radiation and thereby reduce the penalty for meeting crew protection requirements.
184.108.40.206.11.1 Unmanned Configuration — In the unmanned configuration (no external shield) the internal shield (internal to the pressure vessel) shall be no larger than is necessary to prevent radiation damage or heating of engine components which would preclude meeting their specified performance requirements. The internal shield shall limit Pressure Vessel and Reactor Assembly (PVARA) radiation leakage through a plane located at a height of 63 inches forward of core center, perpendicular to the engine axis, to the levels shown in Table I, within the radius defined by the pressure vessel outside radius. Additionally, the PVARA leakage radiation at critical locations in the engine system shall be limited to the levels shown in Table II.
The internal shield shall be limited to an envelope within the inside radius of the pressure vessel and an overall thickness not to exceed 18 inches (including structural and coolant regions).
220.127.116.11.11.2 Manned Configuration — In the manned configuration the engine shall be capable of providing external shielding which in conjunction with vehicle and spacecraft shielding reduces the dose per round-trip to 10 rem at the location of each passenger and 3 rem at the location of each flight crew member in the spacecraft. The manned shield (external shield) shall be capable of being removed in space for unmanned flight and replaced for manned flight. Variations in crew shielding attenuation. capability, based on mission requirements, shall be possible with minimum redesign.
18.104.22.168.3 Nuclear Safety — The engine shall include provisions for preventing the inadvertent attainment of reactor criticality through any single or credible multiple failures, malfunctions, or operations during all ground, launch, flight, and space operations in accordance with the following:
- During reactor assembly and for all subsequent shipping, storage, engine assembly, and handling operations prior to movement to the launch pad, the reactor shall be provided with a poison wire system such that the effective neutron multiplication factor shall not exceed 0.95 if the reactor is flooded with water or liquid hydrogen.
- The poison wire system shall remain effective if the reactor or engine as packaged for shipment is subjected to the Hypothetical Accident Conditions set forth in Annex 2 of AEC Manual Chapter 0529 Appendix.
- The central poison wires alone shall be capable of retaining the reactor in a subcritical state if all control drums are rotated to their most reactive position or if the reactor is subjected to a compaction accident.
- For ground operations which are conducted with poison wires removed, as well as for engine use in space flight, protection against inadvertent criticality shall be provided both by safety measures that prevent inadvertent roll-out of control drums; and safety measures that prevent valve operations that could permit LH2 to flow from the propellant tank to the reactor through either the normal flow path or the cooldown flow path. The safety measures applied to the PFS valves shall also prevent inadvertent flow of LH2 to the reactor following propellant loading and during launch, boost, and space operations.
- The engine shall provide a means of destruct during launch and ascent so as to assure sufficient dispersion of the reactor fuel upon earth impact to prevent nuclear criticality with the fuel fully immersed in water. The destruct system shall be capable of removal prior to initial reactor startup in space.
- The engine shall include provisions for the safe determination of sub-critical multiplication when all poison wires have been removed.
- The engine shall have a minimum reactivity shutdown margin (no H2 flow, poison wires out, control drums full-in position) of 1.50 at 540°R at all times.
- The engine shall include provisions for monitoring the neutron flux during engine non-operating periods during space flight. The monitor shall provide an appropriate alarm signal to indicate an abnormal increase in the neutron level.
- Normal mode — The operation of the engine when all subsystems and components are capable of being operated as designed.
- Malfunction Mode
- Single Turbopump Operation — The operation of the engine with only one leg of the propellant feed subsystem.
- Component Malfunction Mode — The operation of the engine when a component has malfunctioned, other than one which would require single turbopump operation or emergency operation or results in a Category IV failure effect. This mode of operation allows the engine to operate the same as in the normal mode, but without the advantage of the normal mode redundancy.
- Emergency Mode — The operation of the engine at a level to effect safe crew return or to prevent danger to the earth's population subsequent to a failure effect Category III failure.
- Single Failure Point — Any single mode of failure occuring at the part, component or subsystem level that can be attributed to a specific internal failure mechanism at the part level and that results in inability of the engine to meet its normal-mode performance or service life requirements.
- System Power Down — Reduction of engine power to a minimum level consistent with the operating requirements for that particular phase of operation.
- Prestart — The phase of an operating mode where all functions of the engine are performed prior to initiating propellant flow other than that required for cooling from a previous operation.
- Startup — The process of conditioning and bringing the engine to the first steady state operating point.
- Failure Effects
- Category I — Failures which produce no significant performance or safety degradation of the system, allow continued operation in the normal mode throughout the rated engine life, and do not result in an increase in the number of Single Failure Points.
- Category II— Failures from which the engine can recover and still meet its normal mode performance and service life requirements by switching to or reverting to a recovery mode, but which do result in an increase in the number of Single Failure Points. Failures in this category are further subdivided as follows:
- IIA — Failures which degrade the safety of continued operations but which do not produce transient effects and, at the time of failure, do not require automatic or manual action for the recovery mode. Failures of safety systems and standby-redundant components fall within this category.
- IIB — Failures which are compensated for automatically by the normal control mode or which produce transient effects which can be tolerated by the system and which permit time for human judgement to be exercised on the method and desirability of the recovery mode. Failures which require the functioning of safety systems or redundant components to preclude Category IIIB effects fall within this category.
- IIC — Failures which require immediate malfunction detection and subsequent action to remove or lessen the transient effect and to preclude system damage. Switching to the recovery mode is usually accomplished automatically by the malfunction detection system or by the engine control system. Failures which require the automatic functioning of safety systems or redundant components to preclude Category IV effects fall within this category.
- Category III — Failures which result in inability of the engine to meet its normal-mode performance and service-life requirements but which allow Emergency Mode Operation or Single Turbopump Operation. Failures in this category are further subdivided as follows:
- IIIA — Failures which require Single Turbopump Operation.
- IIIB — Failures which require Emergency Mode Operation.
- Category IV — Failures which result in direct injury to the crew, endanger the earth's population, or damage the spacecraft or other stage modules upon which crew survival depends, and/or which preclude Emergency Mode Operation. This category includes failures which produce one or more of the following system effects:
- Uncorrectable thrust vector misalignment.
- Loss of thrust to less than that required to effect Emergency Mode Operation.
- Inability to reduce thrust or unsuccessful shutdown and/or cooldown which precludes engine restart.
- Unsuccessful startup to attain thrust equal to or greater than that required for Emergency Mode Operation.
- Kerma — A term used to describe the energy deposited by radiation. It is an acronym for Kinetic Energy Released in Material. In this document it is used to describe the intensity of gamma radiation.
I have not managed to find details about shutting down an open-cycle gas core fission engine. But there is an alarming possibility that the only way to shut it down is to eject the entire mass of fissioning uranium out the tailpipe. Not only would this be incredibly radioactive, but also a waste of expensive enriched uranium (cost of $100,000 in 1965 dollars, about $778,000 in 2017 dollars).
- Fill hydrogen ducts and neon system from storage to a pressure equal to approximately 20 atmospheres
- turn on neon recirculation pump
- inject fuel until critical mass is reached
- increase power level and adjust flow rates and cavity pressure to maintain criticality and limit component temperatures to tolerable level
- inject propellant seeds when 10 percent of full power is reached
- increase power to desired operating leve
The paper looks at two "power ramps", going from cold to full power in 60 seconds or a more leisurely 600 seconds. Below a temperature of 15,000°R the fusing uranium is heating up the hydrogen propellant mainly by convection. Above 15,000°R the uranium heats the propellant by infrared thermal radiation.
Since convection does such a pathetic job of transfering heat, most of the fission energy goes to heating up the uranium dust instead of the propellant. In about five seconds flat the uranium reaches 12,000°R, and vaporizes from dust into red-hot gas. Then at 15,000°R thermal radiation takes over and the uranium temperature rises more slowly (which you can see by the way the curve starts flattening out). At 60 or 600 seconds (depending upon which power ramp you used) the uranium is at the nominal temperature of 45,000°R. It won't rise any higher unless the engine is exploding or something rude like that.
As previously mentioned the hydrogen propellant is pretty much transparent to thermal radiation, which is most unhelpful. Normally the infrared will shoot right through the hydrogen without heating it up. So tungsten dust is seeded into the propellant to soak up the thermal radiation and heat the propellant by conduction. Any thermal radiation that misses the seeding will hit the far wall of the propellant chamber, which is also the beryllium oxide moderator (BeO) helping to keep the uranium fissioning. The thermal heating of the BeO is nothing but wasted energy but the seeding is doing the best it can. The BeO is designed so it can handle up to 2,400°R.
Since the BeO moderator outweighs the uranium dust by several orders of magnitudue, it takes far longer to heat up. As you can see from the graph the uranium fuel starts heating up after only 0.03 seconds but the BeO doesn't even start heating until 10 seconds, about 300 times longer. The uranium gets up to nominal temperature in 60 seconds but the BeO takes 300 seconds. And the BeO only gets up to 2,400°R while the uranium is smokin' at 45,000°R. That is for the 60 second ramp. The 600 second ramp has both the uranium and BeO all warmed up at the same time, only because 600 seconds gives the BeO time to catch up.
However, the shorter 60 second ramp is desireable, because the 600 second ramp wastes precious propellant. Take the propellant mass required for a standard 20 minute burn at full power. The 60 sec ramp requires an additional 2.7% propellant as startup wastage. The 600 sec ramp requires a whopping 27% additional, which is totally unacceptable. What, do I look like I am made of propellant? The paper says it might be possible to reduce the ramp time down to 6 seconds, in the interest of reducing the propellant startup wastage even further (presumably to 0.27%).
The critical mass of uranium-235 fuel required in the quartz tubes increases during the ramp up. It requires 18.6 pounds at zero power up to 30.9 lbs at full power. For the 60 second ramp up full power initially happens at 28.2 lbs, but rises to 30.9 lbs at 300 seconds. This is because at 60 seconds the BeO moderator has only warmed up about two-thirds of the way to its max temperature. Apparently once the BeO is fully warmed up the critical mass rises.
When the paper was read, one of the attendees was skeptical about pressure. Specifically if the pressure of the uranium/neon mix is not the exact same as the pressure of the hydrogen propellant, the pressure differential will shatter the quartz tube like dropping an old-school incandescent lightbulb on a concrete floor. The paper authors insisted that the two pressures could be balanced rapidly enough to prevent that unhappy state of affairs. They say that a differential of two or three atmospheres will shatter the blasted tube, so they want to keep the diff under 2/3rds atm. Yikes, I didn't know that! That would instantly ruin the propulsion system, and spray everybody and everything close by with fissioning uranium.
- Close Fuel Injection Control Valve (turn off the uranium)
- Begin Linear Decrease in Propellant Flow Rate (propellant flow past light bulbs to exhaust nozzles)
- Begin Linear Increase in Radiator Flow Rate (flow from coolant heat exchanger to radiator)
- Maintain Secondary Circuit (flow of hydrogen coolant) and Cavity Neon Flow (buffer gas flow inside the quartz light bulbs) at Full Power Value.
Once the engine shut-down sequence is initiated, it takes six seconds for the power level to drop to zero. It only takes 0.8 seconds for power level to drop to 0.01 of full power, during which time the contained uranium fuel drops from the steady-state level of 13.65 kg down to 11.5 kg.
Gimbals are one technique of thrust vectoring, allowing the thrust to go off center. The pilot uses this to yaw and pitch. If there are multiple engines with enough separation from the spacecraft's axis, gimbaling can be used roll as well.
It can also be used in emergencies if during thrust the rocket "falls off its tail." That is, if the center of gravity unexpectedly shifts, and the engines can gimbal by enough degrees off center, they can compensate.
In a 1969 US Atomic Energy Commission pamphlet on NERVA, it had this to say:
Dr. Crouch is unsure about this. He notes that unlike a chemical rocket, the poor gimbal is trying to swing an entire reactor so the actuation loads will be large. Even worse: the gimbal is subject to cryogenic temperatures, nuclear radiation, and thrust loads. Simultaneously. He is of the opinion that a more optimal solution would be secondary injection, jetavators, or a movable nozzle.
Having said that, here are the gimbals designed for the NERVA project.
In the images here, the gimbals are the network of blue rods emerging from the bottom of a hydraulic piston. The wasp-waist in the center is the pivot. The green "mushroom caps" are the control-drum actuators, perched on top of the control drums protruding from the reactor.
Note that the gimbals are a nasty trouble spot. If one jams, that engine cannot be steered. The proximity to the dangerously radioactive engine makes repair difficult. This is a good place for a waldo.
The various controls, tongs, and remote control "waldoes" will reach around or penetrate the anti-radiation shadow shield, and there may be auxiliary lead baffles. Peeking around the baffles is how Rhysling lost his sight in Heinlein's "The Green Hills of Earth".
The various controls, tongs, and remote control "waldoes" will reach around or penetrate the anti-radiation shadow shield, and there may be auxiliary lead baffles. Peeking around the baffles is how Rhysling lost his sight in Heinlein's "The Green Hills of Earth".
Remember that the shadow shield will be in the floor, with the engine below that. Closed-circuit TV monitor will help Astro see what he is doing, but if they are damaged, he'll have to make do with mirrors and/or doing it by touch. What he really needs is one of Tom Swift Jr.'s Giant Robots, which were designed to do maintenance inside nuclear power plants. There is more about robots here.
For external repairs, the chief engineer might use something similar to the amazing Canadarm 2, which is currently on active duty on the International Space Station. Unlike the first Canadarm, this one is not attached at either end. Instead, either end can plug into special sockets ("power data grapple fixtures") built at strategic spots on the surface of the station. Canadarm 2 can literally walk on the surface of the station to where it is needed, moving end-over-end like a giant metal inch worm. The main limitation is that each "step" must end at a socket, but this is due to power and control signal issues. A more advanced version might be self contained enough to not require sockets, just hand-holds or other protrusions that it could grab.
Canadarm 2 is quite large, 17.6 meters (57.7 feet) long when fully extended. It can move payloads with a mass up to 116 metric tons.
On your atomic rocket, one would use arm(s) long enough to reach any spot on the radioactive engine.
Outside but adjacent to the engine deck will be the maintenance shop full of engineering tools and storage for replacement parts.
On larger ships this might be the place for damage control central.
Joining tools include hammer and nails, screwdriver and screws, soldering gun and solder, socket wrench and bolts, arc welders, and glue. Also included under joining tools is solid freeform fabrication for rapid prototyping.
Since every gram counts, instead of carrying tons of spare parts that may never be needed the ship might carry bar stock for spares to be created as needed. Or input material for the 3D printer. Yes, the 10-meter Orion Drive spacecraft carries 1.13 metric tons of spares, 2.27 metric tons of repair equipment, and half a metric ton of checkout instrumentation; but that monster just doesn't even notice a few extra hundred tons of payload.
The machinery, decks, and walls will have lots of access panels to allow the engineers to make repairs. If the machinery is large, there will be ladders and catwalks. The lighting will also be bright and harsh. The walls will have tool bays, spare parts lockers, fire control gear, and similar items.
The engineering deck is likely to be a noisy place, with all the heavy machinery. Engineers might need ear-plugs, and good lip-reading skills. If the situation is more high-tech, they will wear combination radio/hearing protection headsets.
The engineer has a logbook as well.
If some of the spares are larger than, say, a person, either the corridors or the airlock will have to be large enough to accommodate it.
In the movie Forbidden Planet, there is a small crane mounted over a deck hatch to facilitate moving equipment between decks. It is shown in the scene where the invisible monster enters through the hatch into the bunkroom full of sleeping enlisted men. It is the long metal arm that the invisible monster bumps out of the way.