Since the thruster described is less than 1m in diameter, you can attach a serious number of them to a ship the size of a Starship.

Also, with such a high specific impulse and the ability to use light ions like helium or maybe even lithium, you may not need to use a 5-kiloton craft to deliver a few tons of payload to Martian orbit. Nuclear fuel has absurdly high power density, even with all the shielding, reactor, turbines, and generators factored in.

The largest electrical power delivered in space from nuclear sources was 3000 watts, which is far less than the 10,000,000 watts they call for just to generate 100 newtons of force.

Generating power in space isn't easy.

Source: https://en.wikipedia.org/wiki/Nuclear_power_in_space

This is the question of launching a conventional fission reactor into space. Nobody just needed 10MW of electric power at space yet.

Of course, a nuclear reactor produces heat, not electricity directly, so we're limited by the Carnot cycle, and will need large radiators, defeating some of the mass savings. OTOH space is very cold, and a suitable gas can be made to work with a seriously higher temperature difference than practical on Earth, giving better utilization of the heat. OTOH reaching the asteroid belt and Jupiter in reasonable time is only possible with non-chemical rockets, and nuclear fission power is the only realistic option now.

> OTOH reaching the asteroid belt and Jupiter in reasonable time is only possible with non-chemical rockets, and nuclear fission power is the only realistic option now.

Nuclear fusion is another feasible option, if you can get around the Partial Test Ban Treaty because an asteroid is going to exterminate humanity or something.

It's not very viable yet because the only form of it we reliably achieve is a bomb. This makes the only useful spacecraft to use it something like Project Orion.

Such a device, while possible, is very fuel-inefficient: you need several kg of plutonium just to ignite the tritium. Peak mechanical and radiation loads are very high.

A thruster which could produce much lower thrust for a much longer time seems much safer and cheaper to implement, even if detonating nuclear bombs in space were not an issue. And a issue it is: you need to do it rather far away from all other spacecraft, which means some weird orbital plane.

Your math seems right at first glance. But keep in mind that it will only expel 25kg of working gas in those 14 hours. For a short trip to mars with a heavy rocket this kind of drive might not matter, but imagine a probe that can run this engine for a decade.