The Case for Space VIII: Nuclear Pulse Propulsion

11/15/2014 Note, this post got scrambled by Blogger.  I'm resurrecting it from the Internet Archive but still haven't fixed the all of the links.
11/16/2014 I think I've fixed all external links
11/17/2014 I think I've fixed all the internal links -> please report broken links in the comments section if you find any.

The Case for Space VIII: Nuclear Pulse Propulsion
Nuclear Pulse Propulsion
 
1. History
2. Design
3. Description
4. Advantages
5. Disadvantages
6. Economics
7. Summary

1. History

In 1954 during the Operation Castle nuclear tests at Bikini Atoll, two graphite covered spheres placed near the nuclear detonation not only survived the nuclear explosion but also flew quite a large distance away from the detonation.

Later in 1957 the Pascal-B test (one of the Plumbbob series of tests) placed a steel pusher plate over a single nuclear explosion. It was calculated that the massive (nearly 1 ton) pusher plate achieved a velocity of nearly 150,000 miles per hour (which exceeds solar system escape velocity) from that single explosion! However, rather than leaving the atmosphere, it is thought that the plate vaporize due to the massive frictional heating with the air. The acceleration experienced by the pusher plate in this test applied enormous accelerations that, if applied to a space capsule, would have left a human body as a pool of Jell-O in the bottom of the capsule.

In 1959, the Project Orion team performed a scale test on a one-meter test article using chemical explosives instead of nuclear bombs. The test flew for 23 seconds and the vehicle reached a height of 56 meters. Here's a series of photographs from the test:

and a video that shows how this would work:

 These tests clearly demonstrated that nuclear pulse explosions could very effectively propel vehicles to high velocities.

I'm including George Dyson's TED talk about the Project Orion concept but am not particularly impressed with his treatment of the subject.

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2. Design

The main problem experienced by spacecraft designers was determining how to create a pusher plate capable of absorbing the impact of a nuclear explosion at close range and figuring out how to translate that extreme impact acceleration into an acceleration within the threshold of human tolerances.

Unfortunately no single approach resolves this problem. Fortunately, using multiple approaches does resolve the problem.

F = m * a

Since this basic physics equation describes the problem (force equals mass times acceleration) the primary method of reducing acceleration for a given force is to increase the mass. Every other spacecraft ever built requires the launched mass to be as small as possible to make the vehicle economical. In a nuclear pulse propulsion drive craft, the designers need to make the payload very large.

However, just making the craft massive does not, by itself, solve the problem. Because making the space craft very massive reduces the acceleration so much that a single nuclear explosion becomes insufficient to achieve low Earth orbit. In fact a craft massive enough to not kill the crew from the acceleration of a single nuclear explosion will reduce acceleration enough to require the ship to explode 300 bombs to achieve low Earth orbital velocity (and more would be carried so the craft could complete some other mission). These bombs would have to be detonated between once very second (for 3 gravities of acceleration) to once every other second (for 2 gravities of acceleration). Any slower, and the ship wouldn't accelerate enough to climb through the Earth's gravity well.

The true irony about this sort of vehicle is that the quantity of fissionable materials (Uranium 235 or Plutonium 239) required for a launch is almost completely independent of the amount of mass launched into orbit. Whether we launch a 2000 ton vehicle or an 8,000,000 ton (16,000,000,000 lb.) vehicle we will disperse the same amount of radioactive materials through the atmosphere.
This is because all nuclear weapons require a "critical mass" (the minimum amount of fissionable material) in order to detonate. This means low yield nuclear bombs are very inefficient. Most of their fissionable material gets flung out of the bomb before it's able to participate in the chain reaction. The more efficiently the fissionable material is used, the larger the explosion. By simply ensuring that most of the fissionables are consumed, the spacecraft can be increased in size enough to launch a small city into orbit. This produces the side benefit of producing much smaller amounts of radioactive fallout.

As a comparison to current launch systems, the 2000 ton vehicle equals 1/2 of the combined mass of the space shuttle launch stack - shuttle, payload, external fuel tank, and solid rocket motors. However, over 1/2 of the total vehicle mass (1000 tons) would be the payload delivered into orbit. So a vehicle less than 1/2 of the mass of the Space Shuttle could deliver nearly 1000 tons (2,000,000 lbs.) of payload into orbit. As a comparison, the shuttle delivers about 20 tons (40,000 lbs.) of payload into orbit or 100 tons (200,000 lbs.) if you include the mass of the shuttle orbiter as payload.

These numbers were for the "small" version of this vehicle. For the same amount of fissionables (and much less radioactive fallout) we could launch the 8,000,000 ton vehicle. This vehicle carries the mass equivalent of over 40 US aircraft carriers into orbit. Considered another way, if only humans were carried, this craft could carry 40,000,000 people into orbit! Of course it would be silly to use such a vehicle to launch people into orbit, instead one or a few launches of a vehicle like this could easily provide everything a self-sustaining space colony would need.

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3. Description

So, what does this thing look like?

Well start with a curved and large steel or aluminum pusher plate (each of these materials has advantages and disadvantages).

Attach this plate to the main body of the spacecraft with a giant airbag - similar to the airbag used in your car to protect you from the sudden deceleration of a car crash. The airbag absorbs the sudden acceleration of the pusher plate and translates it into more gradual acceleration for the spacecraft.

In addition to the airbag, attach the pusher plate to the spacecraft main body with strong cables wound around spools with lots of slack. In the event of a misfire, these cables will prevent the pusher plate from pulling away from the spacecraft (and ripping the airbags).

On top of the airbags, just about any desired spacecraft design could be mounted.

NOTE: My description does not match the above diagram. My concept does not use the large & heavy mechanical shock absorbers.

Note, however, that there's no need to continue using the tall skinny rocket format for these vehicles. In fact, in some ways a short squat structure would serve space crews better.

A related concept called Medusa catches the explosion in a sail before the craft and then reels the craft in.  This configuration would not work for space launch but would be far more economical as an interplanetary spacecraft.

  (From Wikipedia) "Conceptual diagram of a Medusa propulsion spacecraft, showing: (A) the payload capsule, (B) the winch mechanism, (C) the main tether cable, (D) riser tethers, and (E) the parachute mechanism"

Also from Wikipedia: "Operating sequence of the Medusa propulsion system. This diagram shows the operating sequence of a Medusa propulsion spacecraft (1) Starting at moment of bomb / pulse unit firing, (2) As the bomb's explosion pulse reaches the parachute canopy, (3) Pushes the canopy, accelerating it away from the bomb explosion as the spacecraft plays out the main tether with the winch, braking as it extends, starting to accelerate the spacecraft, (4) And finally winches the tether back in."

Remember the largest of these designs can launch craft the size of small cities from the surface of the Earth to anywhere else in our solar system.

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4. Advantages

We had the technical expertise to build these space craft back in the 1960s.  This technology is ready and waiting for us to use at any time.

Reusable launch vehicle

Unlike most other technologies mentioned here (like the light gas gun and ram accelerator) nuclear pulse propulsion does not require massive ground installations. In fact nearly all of the cost for nuclear pulse propulsion goes into the construction of the spacecraft and the bombs used for propulsion. However, the craft can be so overbuilt and performance of the rocket of such minor concern that the craft can be built for durability and reuse. Even the pusher plate which will be exposed to hundreds or thousands of nuclear explosions can be built to suffer minimal erosion.

There are also two separate applications for which this technology can be used. The first is launching a massive amount of materials into orbit for establishing a permanent space settlement (or saving the human civilization from destruction due to an imminent comet or asteroid strike. The second application for this technology would allow humans to travel regularly to any destination in the solar system and beyond. For any of these applications, the ship itself can easily be reused with only the nuclear bombs requiring any special manufacturing capability - meaning having to get resupplied by the Earth.

Rapid turn-around time

I originally scored nuclear pulse propulsion as having a long turnaround time. However, I elected to change my assessment based upon the facts I list in this section.

Due to some of the negative consequences of using this design in the atmosphere (nuclear fallout on the Earth and radiation hazard to the crew) and the ease of getting materials back to Earth through aero braking, then any and all nuclear pulse propulsion launches will be one-way trips. We will use these to launch mass into orbit and they'll never land on Earth again.

If the craft won't be returning to Earth, you might ask "how can I say the ship can have a rapid turnaround?" In order to withstand nuclear detonations in close proximity and the pulse nature of the acceleration pulses, the craft will need to be very over-built. To "turn-around" the craft, you just need to get it back to high Earth orbit (you won't want to go lower - see the disadvantages for the reason why), send up replacement nuclear bombs, reload the bomb magazine & other consumables, and you're ready to go.

Cheap

Although this technology would be the cheapest by far, some of the disadvantages of the technology will probably restrict its use to just once but definitely no more than a few times. Because most of these costs will be incurred regardless of the size of the launch, it certainly makes sense to maximize the return by maximizing the vehicle launch size. As mentioned later in the economics section, the cost could be as low as $0.70 per kilogram for the largest craft. More realistically it might cost as little as $10 per kilogram (provided the large craft option is used) - making this the cheapest method of launching mass into space by far.

Consider that a 16,000,000,000 lb. craft might cost as much as $160,000,000,000 ($160 billion) but that launching that mass into orbit would cost 10,000 times more!

Requires no technological breakthroughs

The original concept, research, and design for this technology was all conducted in the 1950's. We've progressed technologically more than 60 years since then, have more advance materials, control, computers, etc. These technological advances would allow us to better utilize the launch mass - meaning we could get more capabilities out of the mass we launch into space.

We would use more advanced technologies but there's absolutely no reason to do so.

Space launch

Like the previously mentioned technologies, this propulsion scheme can launch payloads from the surface of the Earth into space. It would be the most effective means of doing so. In the event of an imminent threat from space, it is likely to be the only launch system likely to enable a rapid response to threats from space.

I have more to say about space launch but I'll cover that in detail under the disadvantages section.

Gentle acceleration

This technology can be tuned to provide the accelerations required by the cargo - including humans. Tuning of the acceleration can take the form of simply modifying the mass of the craft, the size of the nuclear explosion, the frequency of the explosions, or any combination of these things.

However, one thing that should be considered is that even with the best damping systems, a ride on a nuclear pulse propulsion vehicle will involve a "rough ride".

Large payloads

The smallest possible craft (based upon the smallest nuclear bomb explosive yields) would be 880 tons (about 1/3 of the mass of the Space Shuttle). As mentioned earlier, nuclear pulse propulsion can not only launch large payloads into space, it is the only technology capable of launching CITIES into space.

A number of different payload sizes and mission types have been studied – to which I’ve added the Space Shuttle and Saturn V by comparison:

Category	Ship mass	Payload to LEO	Craft Diameter	Propellant Velocity	Number of bombs for mission	Bomb weight	Bomb yield	Thrust	delta V
	tons	tons	meters	km/sec		tons	ktons	millions lbf	km/sec
Minimum	300	51	20	30	540	0.22	0.01	1.8	12
Midsize	2,000	1,080	40	60	1080	0.75	0.11	12	24
Inter planetary	4,000	1,600	40	40	800	1.5	0.14	24	30
Advanced Inter planetary	10,000	6,100	56	120	800	2.5	0.35	60	100
Maximum	8,000,000	4,320,000	400	60	1080	3000	420	48,000	24
Saturn V	3,350	131	10	2	N/A			7.6	8
Space Shuttle	2,030	27 (120)	3.7+8.4+3.7	3.5	N/A			6.9	8

 

Interplanetary flight

ALL of the disadvantages of nuclear pulse propulsion do NOT apply for interplanetary travel. The radioactive bomb debris does not make space any more hazardous since high energy charged particles (like the solar wind) regularly blast through interplanetary space. Furthermore nearly all of the exhaust debris will possess a velocity greater than the solar systems escape velocity - meaning it will blow out into interstellar space and not remain a hazard for future navigation of our home system. Of course some care must be taken to ensure that exhaust debris does not intersect the Earth.

Unlike the previously mentioned technologies, this propulsion scheme can not only be used to launch craft into space, it can be used to send the craft to anywhere in the solar system. More adventuresome programs could use this technology to travel to nearby solar systems, provided the crew could survive a 100 year voyage.

This propulsion technique is so powerful that by allocating 66% of the ship's mass to carry bombs, a human crew could easily complete round trip travel to anywhere in the solar system in about 4 months. Meaning 2 month travel to Pluto, perform the exploration of Pluto, and then 2 months home. Using any other technique for this mission would take many decades to complete the trip. As a comparison here are some other trip times to other planets:

 

Planet	Trip time
	(days)
Mercury	4.3
Venus	2.9
Mars	4
Asteroids	7.5
Jupiter	11.6
Saturn	51.6
Uranus	75.7
Neptune	96.2
Pluto	111
Solar Focus	1269

NOTE: The solar focus is an interesting astronomical phenomenon. Gravity bends light and the Sun's gravity is no exception. If you can travel to the correct distance from the sun (550-800 astronomical units) and look back, you can use the sun like a giant telescope lens. With this lens you can get super high resolution images of objects on the other side of the Sun. This mission would have the side benefit of providing a very long baseline for interferometer measurements as well as providing excellent precision parallax (distance to the stars) measurements. It would take about 4 years of travel time (2 years to go out and 2 years to get back) to complete this mission with a nuclear pulse propulsion space craft.

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5. Disadvantages

Despite all of its great advantages, let's not kid ourselves - the disadvantages of nuclear pulse propulsion are very real and large. However, most of these disadvantages are not covered under my typical treatment of launch system disadvantages, so I will add a few sections to cover those "unconventional" disadvantages.

Must carry its own propellant

For chemically powered and even nuclear powered conventional spacecraft, having to lift your own propellant leads to prohibitively large and expensive craft like the Space Shuttle or Saturn V rocket. The actual payload the shuttle delivers into orbit is about 1/100th of the launch mass of the shuttle launch stack (about 1/25th of the stack's mass if you count the orbiter). For a trip to the moon these numbers get much MUCH worse. The Saturn V delivered only 1/70th of the total launch mass of the vehicle to lunar orbit and returned only 1/600th of the launch mass back to Earth!

By comparison, nuclear pulse propulsion with 1950s vintage technology could have sent a mission to Saturn and back and the returning mass fraction would be 1/6th of the launch weight. Modern bomb designs, materials, controls, computers, etc. would not only make the design even more efficient (larger mass fraction returned) it would increase the value of that mass fraction (less dumb wasted structural space more instruments, consumables, and people).

Very unsafe for nearby vehicles

WARNING: nuclear weapons in use!

Seriously, anything not part of this spacecraft had better keep a very safe distance. In space and depending upon the bomb size this could be miles to dozens of miles. In the atmosphere (and depending upon the bomb size) this had better be at least dozens of miles and 50 miles or more might be more appropriate.

Furthermore, nuclear weapons can damage things in more ways than the lasers mentioned in earlier blog posts. Lasers can dazzle (temporarily blind), blind, melt, or vaporize critical components of a satellite or space craft. However, it shouldn't be too hard to calculate the beam trajectory and ensure that no satellites will be in that path during launches.

However, nuclear weapons can damage things through many mechanisms including electromagnetic radiation (e.g. light), neutrons, and blast.

Blast would only be encountered in the atmosphere or in the unlikely event that the unlucky satellite got between it and its propulsion bombs. Blast consists of an over-pressure wave which is like a very powerful sonic boom - but if you get a powerful enough blast it can blow a concrete building to pieces.

Neutron radiation doesn't inflict much damage upon structures. However, they do tend to penetrate very easily through those structures and tend to kill the people inside. The best method of protecting humans from neutron radiation is to place large amounts of light materials in between the neutron source and the human. In the atmosphere, this could be the atmosphere itself.

Electromagnetic radiation (photons) includes the entire spectrum of photons but we mostly need to be worried about the flash of radiation absorbed by the surface of the spacecraft (dangerous to sensors - including eyeballs) and higher energy radiation which can penetrate the skin of spacecraft (such as x-ray and gamma rays). The latter are more dangerous for people as the craft might survive the flash but the radiation would still kill the people on board.

But perhaps the best protection for all three is just keeping vulnerable things (people and electronics) at a safe distance. At a far enough distance no additional protection would be required. So the launch would need to happen at a location far from population centers.

For the crew, a variety of different materials (lightweight ones to block neutrons and dense ones to block X-rays & gamma rays) would protect the crew from the bomb detonation radiation.

Nuclear fallout

During the above ground nuclear test of the 1950s and 1960s most of the radioactivity added to our environment by those bomb tests was originally inert material from the ground transmuted into radioactive isotopes by the particle radiation from the nuclear bombs. A small portion of the radioactivity came from the "unburnt" fissionables (U-235 & P-239) and a tiny portion came from the daughter products of the nuclear reaction.

The amount of unburnt fissionables can be reduced by utilizing high-efficiency bombs specifically designed to consume them all. This will increase the amount of daughter products produced; however, those have much shorter half-lives than that of the fissionable fuels.

The amount of radioactive fallout can be dramatically decreased by launching the craft from the water, rather than on land. Water consists of light elements (oxygen and hydrogen) neither of which can become highly radioactive. Ideally such a launch would occur in/on fresh water, but salt water would be acceptable.

Furthermore as the spacecraft climbs away from Earth, we (the inhabitants of the Earth) would really rather that fallout left the Earth's vicinity and did not fall back to the Earth. Although we cannot ensure all fallout fell back to the Earth, we can maximize the amount that leaves with the craft by launching the spacecraft at one of the magnetic poles of the Earth (North or South). This is because ionized particles (like much of the radioactive debris) tend to follow magnetic field lines. Note how at the magnetic poles of the Earth, those field lines point into space?

NOTE how the magnetic field lines have already trapped particles in the Van Allen radiation belts. A nuclear pulse propulsion launch from Earth would add significant quantities of radioactive particles both here and on Earth if not launched from one of the magnetic poles.

The North magnetic pole offers greater access to the shipyards of the North Atlantic and also a convenient location to get to by ship during the summer (as that location is often ice free). The South magnetic pole offers a far more difficult location for travel, however, the spacecraft could launch off of the ice pack reducing the amount of fallout as well as being much further away from any developed areas (further reducing human exposure to the radioactive debris).

Electromagnetic pulse (EMP)

Launching from one of the magnetic poles would significantly reduce the EMP effects of the third component of EMP although I'm uncertain about whether it would help with any of the other components.

Of the 3 components of EMP, the first (the immediate EMP effect) and the last (the last component) are the portions of the EMP that would be most damaging. Because the vehicle will pass through the zone in which nuclear detonations would cause the greatest EMP effects, it is essential that the by the time the craft reaches 300 miles in altitude that it is also over the horizon of populated areas. The ground distance to ensure minimal EMP damage could be as much as 1500 miles in radius.

Honestly, I am uncertain if launching from the north magnetic pole provides enough distance to European population centers. If launching from the north magnetic pole does not provide a safe launch distance, then the south magnetic pole would have to be utilized. This alternate location provides a completely safe launch site.

Here's a chart showing the EMP effects of a 300+ KM EMP burst - this would be a more powerful bomb than those used for the nuclear pulse propulsion.

Superimpose those ranges on this view of the North Pole:

and you can see that the EMP might affect the far north of Canada but otherwise shouldn't be a problem for heavily inhabited locations in North America (and apparently not in Europe).

Radiation

This hazard is one experienced by the spacecraft crew and not those left on the ground. The crew and payload of a nuclear pulse propulsion craft would be very safe from the radiation released from each explosion when the craft is in space. However, the atmosphere can cause radioactive bomb debris to blow back along the manned portion of the craft.

I haven't read any analyses about how to protect the crew from this hazard, however, I can think of a series of steps for use during launch by which the crew should be safe during launch.

The first method would be to use an aluminum-beryllium outer skin on the spacecraft and then to place all astronauts into the solar storm "shelter" portion of the craft (a special radiation hardened location required to protect the crew from severe solar storms). The aluminum-beryllium skin is necessary because low atomic mass materials do not absorb neutron very readily - reducing the chances of radioactive particles or neutrons transmuting portions of the spacecraft into radioactive isotopes and making the spacecraft itself radioactive.

The last step would place a thin coating that could be shed after launch. This would ensure any radioactive dust that had been clinging to the spacecraft could be shed.

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6. Economics

I've seen estimates that a large nuclear pulse propulsion craft could launch mass into orbit for as little as $0.70 per kilogram up to $250 per kilogram. As with many other things, increasing the size of the system increases its per launch cost but also decreases the cost per unit mass.

Even if we use a nuclear pulse propulsion craft in the middle of the size range listed above and assume that the per bomb price is $1,000,000 per bomb, then the launch costs still come out as $250 per kg. This beats any conventional launch system, ram accelerator, the light gas gun, and even a light craft.

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7. Summary

Despite all of my glowing discussion of this technology, there remains a huge irrational phobia against anything nuclear. Add to that a healthy respect for the power of nuclear weapons and there is a huge social and political wall that will prevent the use of this technology. Additionally the US joined the nuclear test ban treaty in which we agreed not to detonate nuclear weapons in space.

Politically it shouldn't be too difficult to wriggle out of the nuclear test ban treaty by simply defining the detonations as propulsion units (not a weapons test). Of course this would incur a lot of political fallout with which the US government would have to contend, however, I'm not going to address that problem since I'm not politician.

OTOH, the social aspects will be much more difficult to overcome. So difficult that I can only foresee one scenario in which the use of a nuclear pulse propulsion craft would be completely accepted by the American public - as a method of addressing an impending comet/asteroid impact. In such a situation, the nuclear pulse propulsion would provide a quick, easy, and massive craft to fly out to and deflect the threat. Once we've launched the craft there's no reason to not continue using it. Furthermore, the consequences of the first craft failing in its mission might require the construction of two such craft. In this case, even if the first craft is successful, the existence and successful use of the first craft might provide the proof that the craft can be used safely to achieve amazing goals in space.

Positive attributes of craft	Light craft	Light gas gun	Ram Accelerator	Nuclear Pulse Propulsion	Nuclear Rocket	Chem Rocket
Most equipment reused?	Yes	Yes	Yes	Yes	Yes	No
Doesn't carry its propellant?	Yes	Yes	Yes	Yes1	No	No
Rapid turn around?	Yes	No	No	Yes	No	No
Safe for nearby craft?	No	Yes	Yes	No	Yes	Yes
Can use cheap materials?	Yes	Yes	Yes	Yes	No	No
Requires no breakthroughs?	Yes	Yes	No	Yes	Yes	Yes
Space launch	Yes	Yes	Yes	Yes	No	Yes
Gentle acceration3	Yes	No	Yes	Yes	Yes	Yes
Large payloads (>50 tons)	No	No	No	Yes	Yes	Yes
Interplanetary flight	No	No	No	Yes	Yes	Yes
Estimated launch cost per pound	<$500	<$500	$500	<$5002	N/A	>$10,000
Score	7	6	6	9	6	6

1. Nuclear pulse propulsion has such good performance that this really isn't a drawback for that concept.
2. I'm making a WAG (wild @$$ed guess) at this because I have no idea how much nuclear bombs cost.
3. This is a measure of whether the system could be used for manned vehicles. Many of these systems can be tuned to provide different accelerations.

More information from Wikipedia on Project Orion.


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Proceed to my next space blog, (not completed yet).

Return to my previous space blog, The Case for Space VII: Ram Accelerator.