Project Orion (nuclear Propulsion) - Potential Problems

Potential Problems

Exposure to repeated nuclear blasts raises the problem of ablation (erosion) of the pusher plate. However, calculations and experiments indicate that a steel pusher plate would ablate less than 1 mm if unprotected. If sprayed with an oil, it need not ablate at all (this was discovered by accident; a test plate had oily fingerprints on it, and the fingerprints suffered no ablation). The absorption spectra of carbon and hydrogen minimize heating. The design temperature of the shockwave, 67,000 °C, emits ultraviolet. Most materials and elements are opaque to ultraviolet, especially at the 340 MPa pressures the plate experiences. This prevents the plate from melting or ablating.

One issue that remained unresolved at the conclusion of the project was whether or not the turbulence created by the combination of the propellant and ablated pusher plate would dramatically increase the total ablation of the pusher plate. According to Freeman Dyson, during the 1960s they would have had to actually perform a test with a real nuclear explosive to determine this; with modern simulation technology, this could be determined fairly accurately without such empirical investigation.

Another potential problem with the pusher plate is that of spalling—shards of metal—potentially flying off the top of the plate. The shockwave from the impacting plasma on the bottom of the plate passes through the plate and reaches the top surface. At that point spalling may occur, damaging the pusher plate. For that reason, alternative substances (e.g., plywood and fiberglass) were investigated for the surface layer of the pusher plate, and thought to be acceptable.

If the conventional explosives in the nuclear bomb detonate, but a nuclear explosion does not ignite (a dud), shrapnel could strike and potentially critically damage the pusher plate.

True engineering tests of the vehicle systems were said to be impossible because several thousand nuclear explosions could not be performed in any one place. However, experiments were designed to test pusher plates in nuclear fireballs. Long-term tests of pusher plates could occur in space. Several of these tests almost flew. The shock-absorber designs could be tested at full-scale on Earth using chemical explosives.

But the main unsolved problem for a launch from the surface of the Earth was thought to be nuclear fallout. Any explosions within the magnetosphere would carry fissionables back to earth unless the spaceship were launched from a polar region such as a barge in the higher regions of the Arctic, with the initial launching explosion to be a large mass of conventional high explosive only to significantly reduce fallout; subsequent detonations would be in the air and therefore much cleaner. Antarctica is not viable, as this would require enormous legal changes as the continent is presently an international wildlife preserve.

Freeman Dyson, group leader on the project, estimated back in the 1960s that with conventional nuclear weapons (a large fraction of yield from fission), each launch would cause statistically on average between 0.1 and 1 fatal cancers from the fallout. That estimate is based on no threshold model assumptions, a method often used in estimates of statistical deaths from other major industrial activities, such as how modern-day U.S. regulatory agencies frequently implement regulations on more conventional pollution if one life or more is predicted saved per $6 million to $8 million of economic costs incurred. Each few million dollars of efficiency indirectly gained or lost in the world economy may statistically average lives saved or lost, in terms of opportunity gains versus costs. Indirect effects could matter for whether the overall influence of an Orion-based space program on future human global mortality would be a net increase or a net decrease, including if change in launch costs and capabilities affected space exploration, space colonization, the odds of long-term human species survival, space-based solar power, or other hypotheticals.

Danger to human life was not a reason given for shelving the project – those included lack of mission requirement (no-one in the US Government could think of any reason to put thousands of tons of payload into orbit), the decision to focus on rockets (for the Moon mission) and, ultimately, the signing of the Partial Test Ban Treaty in 1963. The danger to electronic systems on the ground (from electromagnetic pulse) is insignificant from the sub-kiloton blasts proposed.

Orion-style nuclear pulse rockets can be launched from above the magnetosphere so that charged ions of fallout in its exhaust plasma are not trapped by the Earth's magnetic field and are not returned to Earth.

From many smaller detonations combined, the fallout for the entire launch of a 6,000 short ton (5,500 metric ton) Orion is equal to the detonation of a typical 10-megaton (40 petajoule) nuclear weapon, if pessimistically assuming the use of nuclear explosives with a high portion of total yield from fission. Historical above-ground nuclear weapon tests included 189 megatons of fission yield and caused average global radiation exposure per person peaking at 0.11 mSv/yr in 1963, with a 0.007 mSv/yr residual in modern times (superimposed upon other sources of exposure, primarily natural background radiation which averages 2.4 mSv/yr globally but varies greatly, such as 6 mSv/yr in some high-altitude cities). Any comparison would be influenced by how population dosage is affected by detonation locations, with very remote sites preferred.

With special designs of the nuclear explosive, Ted Taylor estimated that fission product fallout could be reduced tenfold, or even to zero if a pure fusion explosive could be constructed instead. A 100% pure fusion explosive has yet to be successfully developed according to declassified US government documents, although relatively clean PNEs (Peaceful Nuclear Explosives) were tested for canal excavation by the Soviet Union in the 1970s with 98% fusion yield when 15 kilotons total each (only 0.3 kilotons fission).

The vehicle and its test program would violate the Partial Test Ban Treaty of 1963 as currently written, which prohibited all nuclear detonations except those conducted underground, both as an attempt to slow the arms race and to limit the amount of radiation in the atmosphere caused by nuclear detonations. There was an effort by the US government to put an exception into the 1963 treaty to allow for the use of nuclear propulsion for spaceflight, but Soviet fears about military applications kept the exception out of the treaty. This limitation would affect only the US, Russia, and the United Kingdom. It would also violate the Comprehensive-Nuclear Test-Ban Treaty which has been signed by the United States and China, as well as the de-facto moratorium on nuclear testing that the declared nuclear powers have imposed since the 1990s. Project Orion however would not violate the Outer Space Treaty which bans nuclear weapons in space, but not peaceful uses of nuclear explosions.

It has been suggested that the restrictions of the Treaty would not apply to the Project Daedalus fusion microexplosion rocket. Daedalus class systems use pellets of one gram or less ignited by particle or laser beams to produce very small fusion explosions with a maximum explosive yield of only 10–20 tons of TNT equivalent.

The launch of such an Orion nuclear bomb rocket from the ground or from low Earth orbit would generate an electromagnetic pulse that could cause significant damage to computers and satellites, as well as flooding the van Allen belts with high-energy radiation. This problem might be solved by launching from very remote areas, because the EMP footprint would be only a few hundred miles wide. The Earth is well shielded by the Van Allen belts. In addition, a few relatively small space-based electrodynamic tethers could be deployed to quickly eject the energetic particles from the capture angles of the Van Allen belts.

An Orion spacecraft could be boosted by non-nuclear means to a safer distance, only activating its drive well away from Earth and its attendant satellites. The Lofstrom launch loop or a space elevator hypothetically provide excellent solutions, although in the case of the space elevator existing carbon nanotubes composites do not yet have sufficient tensile strength. All chemical rocket designs are extremely inefficient (and expensive) when launching mass into orbit, but could be employed if the result were viewed as worth the cost.