New Rocket Fuel

Solid nitrogen could pack double the punch of existing space propellants

Solid nitrogen rocket fuel could make for smaller, lighter spacecraft, chemists calculate.

It should be possible to make a new form of nitrogen called N5+N5-, say Rodney Bartlett and colleagues of the University of Florida. The atoms will be linked in groups of five. Half of these groups will be positively charged and half negatively charged, like the sodium and chlorine atoms in table salt. N5+N5- should be a stable crystalline salt, not a gas, the researchers reckon.

N5+N5- would contain twice as much energy as the same volume of hydrazine, the nitrogen-rich compound that propels many spacecraft today, Bartlett's team estimate1. So N5+N5- fuel could be packed into a smaller tank, allowing smaller rockets to be used - much of their volume is currently taken up with fuel.

Nitrogen has a long association with rocket fuels and explosives, such as nitroglycerine and TNT (trinitrotoluene). This is because nitrogen gas, N2, is extremely stable. Compounds with more than two nitrogen atoms per molecule release readily decompose to N2, releasing a lot of energy as they do so. Hydrazine - a blend of nitrogen and hydrogen - works in this way.

Bunch of fives

N5+, half of the compound predicted by Bartlett and colleagues, was first made three years ago2. It is a chain effectively made up of two N2 molecules linked end-to-end by a nitrogen-atom bridge.

No one has made N5- yet. But it should be a ring of five nitrogen atoms, say the Florida chemists. Carbon atoms form a similar negatively charged molecule, called a pentadienyl ring.

Metal compounds might offer a route to synthesizing N5-, suggest Laura Gagliardi of the University of Bologna in Italy and Pekka Pyykko of the University of Helsinki in Finland.

They think that the N5- ring could be made, along with a ring of seven nitrogen atoms (N73-), in a compound with a metal atom, such as titanium or zirconium, sandwiched between the rings3. Carbon-ring compounds like this already exist; ferrocene, for example, is an iron atom sandwiched between two five-atom carbon rings.

Source, 11 June, 2002