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Metastable Helium Rockets
Tech Level: 14
Metallic Hydrogen Rockets
Tech Level: 15

Tech Level: 14

Metastable substances are materials that exist in a relatively long-lived excited molecular state. Metastable helium is one such substance, comprised of a Helium atom with two electrons, one in the first orbital level and one in the second orbital level. The electrons have parallel spin properties.

The molecule is "balanced" on a knife’s edge of quantum forces. The electrons "want" to enter into the ground state (where both electrons occupy the first orbital level) but are "forbidden" because of the forces acting upon them from their locked spin state. The atom can remain in this state for up to 2.3 hours, but just a small input of energy (from jostling, molecule formation, electromagnetic forces, etc) will send it over the edge into its ground state, resulting in a large release of energy, about 114 kilocalories per gram, or roughly twice that of the most powerful conventional chemical fuel, hydrogen.

Metastable helium can be manufactured by several methods. Absorption of photons (laser) or particle bombardment (electrons, radiation, "hot" atoms) can be used to excite the atom to its metastable state.

But the problem with metastable helium is not with obtaining it, but storing it. The 2.3 hour limitation only applies to a completely isolated atom; metastable helium packed in with anything else, even other metastable helium atoms, will result in jostling and it losing its metastable state in a fraction of a second.

Research is being conducted to see if metastable helium can be formed into a room-temperature solid if bonded with diatomic helium molecules, made from one ground state and one excited state atom. This solid, called Helium-IV-A can in turn be used as a solid rocket propellant. Simply heating the fuel is enough to relese the energy, so an oxidizer is not needed. These rockets would produce a specific impulse of about 2200 seconds, compared to the approximate 450 specific impulse for modern liquid hydrogen-oxygen rockets, and would have enormous thrust capabilities, on the order of 31,000 meters per second, matching types of plasma and fusion rockets.

Robert Forward, in his fiction novel Saturn Rukh, suggested bonding 64 metastable helium atons to a single excited nitrogen atom, forming a stable super-molecule called Meta. Whether this is possible remains to be seen. However, as the Meta fuel is much purer than the Helium-IV-A form above, it would allow a metastable helium rocket’s specific impulse capabilities to jump to around 3150 seconds.

Tech Level: 15

Metallic Hydrogen is another metastable substance. It is formed by compressing hydrogen ice with approximately 1.4 million atmospheres of pressure. It is thought to exist in nature at the cores of gas giants, and has been synthesized in the laboratory.

Metallic Hydrogen releases the stored energy needed to compress it when the pressure is released and it reverts to its normal solid ice form. This stored energy is 30 to 40 times that found in TNT, and if used to propel a rocket would produce a specific impulse of about 1700 seconds at high thrust. Mixing the hydrogen with an oxidizer as its released could add to this.

As with metastable Helium, the big problem with using Metallic Hydrogen as a rocket fuel is with storing it. The metallic hydrogen would have to carried along in ultra-high-pressure tanks capable of exerting millions of pounds of pressure and be cryogeneically frozen as well. One can imagine a dynamic compression and release system aboard a rocket, converting stored hydrogen ice into metallic hydrogen with millions of pounds of focused pressure and then releasing it every few milliseconds. This could possible be done with explosive power generators. However, if the spaceship has that kind of energy available to it, it might be better served to use other, easier engineered propulsion systems.


In Print:

Saturn Rukh, by Robert L. Forward

On the Web:

The article on Metastable Helium from JPL’s Advanced Propulsion webpage:


An article on Metallic Hydrogen from the same site:

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