July 26, 2007

Gaseous core nuclear design, the Liberty Ship




The liberty ship is a Gaseous Core Nuclear Reactor design, of the Nuclear Lightbulb subvariant. This design is from nuclearspace.com

It has an ISP of 3,060 and leverages existing technology to conservatively deliver 1000 tons to low earth orbit, 33% of its takeoff weight.

It flies to space with a thousand tons of cargo, and flies back using some gentle aero-braking and its thrusters with another thousand tons of cargo.

I have more on safety below. Chemical rockets are likely fancy balloons filled with fuel. A nuclear rocket has more performance so it can carry more weight, so it can be like a flying armored tank.

I am saddened by the Scaled Composite deaths. However, why is it more scary for anyone to risk dieing from a nuclear rocket than from a car accident (1.2 million deaths per year), air pollution (3 million deaths per year), chemical rocket (300+ deaths total mostly civilians and ground crew) ? Do we go 3 people died in car accident so lets stop driving cars ? We did not say the Titanic sank or there were boating accidents so let us not make nuclear powered submarines or air craft carriers. Far fewer deaths happen from all our nuclear powered reactors and vehicles because their risks are different and we just tend to be more cautious with things nuclear. Why is better for people to die doing mundane matters as part of mundane lives than to risk dieing trying to achieve something audacious like really establishing offworld colonies ? The Liberty ship does not even have to be a manned vehicle. We just use them as the best way to send up 1000+ tons of cargo in one go. I bet it would put fewer lives at risk to send up a fully assembled international space station than to put up 40 space shuttle flights and a dozen EVAs assembling the ISS. You could do more and it would be safer or at least as safe.


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It has safety systems, redundancy and is re-usable.

It is based on technology and ideas that are decades old. Modern technology just makes it possible to make the design more powerful while keeping it safe, but the concepts and design principals are all well established.



The nuclear waste can be dumped into the sun during orbit.
In a traditional chemical rocket, the circularization burn is used to add a tiny bit more speed to the spaceship, making the orbit nicely round. In this nuclear system, we have so much power to burn that we deliberately 'overshoot' on the way up, so the circularization burn is a lot larger than normal.

Now, if you will remember, up above I mentioned that the exhaust of this nuclear spaceship shoots out at a whopping fast 30 kilometers per second. If you add this 30 kilometers per second to the 8.5 kilometers per second the whole rocket is moving while in orbit, and you point your rocket in just the right direction, you can literally shoot the exhaust right away from the planet so fast that it never comes back. You can then aim it to drop into the Sun without too much trouble.

Now, the radioactive spent fuel of this rocket is gaseous, remember? So, if we only use one of the seven big rocket engines to perform the circularization burn, it is a trivial chore to pump the gaseous waste from the other six rocket engines into the rocket chamber, heat it super hot, and shoot it into space forever.



Gaseous core nuclear light bulb engine schematic

FURTHER READING:
External pulsed plasma rockets

A prior review of different nuclear thermal rockets

ADDED: On the question of safety:
You would launch this from the middle of the Pacific Ocean.

A nuclear rocket if this type would have in the range of 10 pounds of radioactive nuclides in it. The Ivy Mike nuclear bomb test which took place on November 1st, 1952. (see the link to wikipedia on Ivy Mike) Released 1023 pounds worth of radioactive nuclides. No one died or was injured.

The gas cored reactor has several potential "scram" (emergency shutdown) modes, both fast and slow, and the speed of the reaction is easily "throttled" by adding and removing fuel or by manipulating the vortex. A 'scram' is an emergency shutdown, usually done in a very fast way. For example: a gas cored reactor can be fast scrammed by using a pressurized "shotgun" behind a weak window. If the core exceeds the design parameters of the window, which are to be slightly weaker than the silica "lightbulb," then the "shotgun" blasts 150 or so kilos of boron/cadmium pellets into the uranium gas, quenching the reaction immediately. A slightly slower scram which is implemented totally differently is to vary the gas jets in the core to instill a massive disturbance into the fuel vortex. This disturbance would drastically reduce criticality in the fission gas. A third scram mode, slightly slower still, is to implement a high-speed vacuum removal of the fuel mass into the storage system. Having three separate scram modes, one of which is passively triggered, should instill plenty of safety margin in the nuclear core of each thruster.

Because we have so much performance margin we can make the nuclear core very heavy and strong so that even if it falls out of the sky it will not leak. We have the means to prevent it from exploding with the emergency shutdown modes. You launch from the Pacific Ocean so you have a lot of time to abort so that it does not hit anything. With the power and performance this thing can fly pretty much straight up.

I think with the performance margin this can be made far safer than chemical rockets. The space shuttle weighs 2,029,203 kg (4,474,574 lb) and puts 24,400 kg (53,700 lb) into LEO. A little over 1% is cargo. It is why there is so little safety margin. You had trim in order to launch anything and reach orbit. The Liberty ship only has 40% rockets and fuel. 25% of the weight beyond that is devoted to building this thing like a flying armored tank able to resist damage and contain problems.

The Space Shuttle generates about 100 gigawatts of power when it is launched, or as much as 50 big nuclear power plants. Plus, the exhaust gases left behind by those huge rockets are not very safe to breathe, either.


There is risk in everything that we do.
As of 2007, in-flight accidents had killed 18 astronauts, training accidents had claimed 11 astronauts, and launchpad accidents had killed at least 70 ground personnel.

There have been 230+ ground crew and civilian casualties.

Driving cars kill 1.2 million each year.

http://en.wikipedia.org/wiki/Aviation_accidents_and_incidents#Safety

Trains have .04 deaths for every 100 million miles
Air travel has .01 deaths for every 100 million miles traveled.
Automobile has .94 deaths per 100 million miles

UPDATE:
Computational work has been done to model the superheated gas which is confined within a vortex.

Los Alamos researchers believe they can come up with a stable flow configuration

Heat transfer to the working fluid (propellant) is by thermal radiation, mostly in the ultraviolet, given off by the fission gas at a working temperature of around 25 000°C.

GCNR TECHNOLOGY STATUS
In the forty years since the Rover program, hundreds of millions of dollars have been spent in plasma research and in developing powerful computational modeling capabilities. The most notable efforts in these areas were the fusion energy programs and the nuclear weapons programs. Both of these large programs relied heavily upon benchmarked computational models to examine stability, operations, and technical feasibility prior to executing expensive experiments. Similarly, the concept of a gas-core nuclear reactor can now be examined computationally before large, expensive and hazardous test facilities must be constructed.

Recently, a new, small effort was initiated to seriously assess the feasibility of the gas core concept using the computational tools and expertise at Los Alamos. By applying the knowledge developed over fifty years as part of the nuclear weapons program, the question of developing a rocket that truly opens up the solar system to manned exploration might finally be answered.

From the inception of this project, the complexities and difficulties inherent in the GCNR concept have been recognized. This is a hard problem. Initially, the research has focused on modeling the cylindrically symmetric configuration wherein an annular injection of hydrogen forms a recirculation vortex in the chamber. Once formed, the vortex is replaced with a uranium vortex which will go critical, heat up to around 5 eV, and radiatively couple to the surrounding hydrogen to produce thrust. So far, five different computer codes have been exercised to assess their capability to model vortex formation and stability in a cylindrically symmetric geometry. From the past few months we have ascertained the following for the cylindrical configuration:

1) flow through the base plate can alter the location of the vortex allowing for active control but can actually destroy the vortex if too high a mass flow is injected;

2) the strength of the vortex, the vorticity, depends almost wholly on the inlet velocity for annular injection;

3) for conditions with high levels of vorticity, no shedding or breakup of the vortex was observed;

4) fuel pellet injection and subsequent evaporation appears to be a viable concept for start-up and fuel-loss recovery;

5) "vacuuming out" the fuel back through the base plate appears to be a viable shut-down concept;

6) diffusion of the fuel throughout the propellant volume appears to occur rapidly for the cylindrical configuration, so that fuel retention is low;

As the result of these studies, we have determined that the cylindrical configuration will not scale to full size because the full-scale mass flow will be between 2 to 6 kg/s which, for an annular injection with a radius of .75 to 1.0 meter, would mean the thickness of the annulus would be quite narrow. A narrow injection results in the thickness of the hydrogen propellant between the uranium and the wall will be narrow and relatively transparent to the emitted radiation. The result is that wall heating will be high, propellant heating will be low, and the configuration is not practicable.

During the short time this project has been underway, the team at Los Alamos has made exceptional progress (Thode 1997) in understanding the physics inherent in an open-cycle gas-core rocket, in developing the computational tools to pursue design of a stable configuration, in identifying strengths and deficiencies of those tools, in testing several computer codes against existing data, and in generating an intrinsic "feel" for what operational conditions will be required to make a gas-core rocket feasible. Eventually, we intend to examine critical issues such as shear-flow-turbulence losses of the uranium, mixing caused by displacement of the vortex due to acceleration, the need for sufficient residence time of the propellant in the chamber, fission product removal, and stability of the vortex.

As a result of our efforts so far, the team is confident that a gas core reactor can be built in a stable configuration and driven critical with substantial power generation. The questions of final performance with regards to fuel-loss rate, specific impulse, and mass will depend upon the integration of many factors into the final design.


Steve Howe reviewed the gas core work for consideration for Mars Missions

As a result of the Los Alamos effort, a new geometric configuration for a gas core rocket has been formulated. Conceptually, a high speed jet of gas is injected axially into the reaction chamber. As the jet expands across the chamber, some of the gas will exit through the nozzle but some will be recirculated along the outer wall. The recirculation creates a toroidal vortex. In a nuclear system, uranium is injected into the vortex. Driving the uranium to criticality will heat the gas along the axis to extreme temperatures providing a very high specific impulse thrust. This configuration has the advantages of having the highest heat flux be at the centerline of the hydrogen flow stream, of having a thick hydrogen barrier around the walls, and of not suffering the uranium migration loss mechanism.

There is little doubt that a gas core reactor can be built in a stable configuration and driven critical with substantial power generation. The questions of final performance with regards to fuel-loss rate, specific impulse, and mass will depend upon the integration of many factors into the final design and must be experimentally investigated. This is the next step.


Gas core reactor rockets at wikipedia

Links to other research

The old 1970's solution to protecting the side walls from the superheated gas:

The basis of the design is the injection of a cold layer of hydrogen around the walls of a spherical cavity. This layer prevented the radiation from the hot uranium in the center to reach the walls. The drawback of this design, though, rests in the fact that as the ship accelerates, the heavier uranium in the center tends to migrate backward toward the nozzle and is vented in the exhaust stream. Although the estimated loss rate of 1 kg-uranium to 400 kg-hydrogen is deemed acceptable from a mission performance point-of-view, this configuration is no longer under consideration.


A 1992 study of the nuclear gas core light bulb concept

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