[Inertron] is a synthetic element…It reflects 100 percent of the heat and light impinging upon it. It does not feel cold to the touch, of course, since it will not absorb the heat of the hand. It is a solid, very dense in molecular structure despite its lack of weight…It is a perfect shield…in many ways resembles the fabled hypothetical antimatter. It can co-exist with matter from our universe without mutual destruction, but it doesn’t much like to. Given a choice, it will try and head to the nearest perfect vacuum—which, from a terrestrial point of view, is always straight up. Thus it forms an effective anti-gravity agent. It also has the happy faculty of being a nearly 100% perfect insulator against any and all forms of electromagnetic radiation.
— Philip Francis Nowlan, Armageddon 2419 AD, pp. 32–33
So it repels normal matter ... somehow. It wants to fly up into space. It is not what we'd call negative matter. That stuff pushes back against anything pushing it, obliterates itself and normal matter in mutual annihilation and apparently does a good impression of Azathoth under the influence of any kind of electromagnetic radiation.
Let's look at its lesser known properties. For one thing you can put a layer of it on your hull as a defense against energy weapons. More to the point line your combustion chambers with it and you could theoretically have a high thrust and high impulse drive that wouldn't melt itself. Ditto for reactors (though you'd still need to get rid of the heat somewhere.
Gamma and x-rays would also be shielded making it somewhat effective against nuclear explosions though particles like neutrons could still get through presumably. You could reduce the mass (in more ways than one) of your radiation shield.
Let's get to the anti-gravity aspects. first the stuff is very compact. A jump pack was about the size of a small book bag and negated most of a grown human's weight (say 75 kilos). A human body displaces about a cubic meter. These belts were about 5 liters. So a liter of the stuff negates about 15 kilos. More importantly it doesn't add appreciably to the mass which is a very good thing. Making extraordinary jumps is fine, making them with the equivalent of another person on your back is dicey. People with a belt were fairly easy to knock over since they didn't have their full weight to give them traction.
If its mass was equivalent to its repulsion force then wearers would be even harder to knock over. s. Also rockets would be lugging a lot of inertron around that was only useful for takeoff and landing. Fuel use would increase exponentially for all uses of the rockets though.
Since the inertron's repulsion power was far more than just the absolute of its mass I think the repulsion force was not gravity but paragravitic (using the definition Winchell Chung gives in Atomic Rockets). It's like gravity but not gravity.
Why not? Some naturally occurring materials give off various radiations. Maybe inertron is like that and it might even have a half life. In the initial strips they mention it is not common and used only for the warriors' packs and aircraft. Maybe it degrades every couple of years.
I'd like to point out the design of the pack is a little wonky. Supporting a person by straps under the armpits and across the waist can't be comfortable and the main lifting force is above the center of gravity. Something more like a parachute harness with straps under the legs and across the butt would probably feel better and be more stable. I'd also arrange the lifting element in pouches around the waist allowing you to add or subtract from your lifting ability to account for large loads or a big lunch.
Assuming such an element is possible we have an interesting economic set up. It's pretty easy to sell paragravity to undeveloped planets by the box. Sell the belts or packs or whatever and that's it, no upkeep necessary (unless the stuff does have a half life, then you're set by life selling it.)
Add a flying suit and you can glide all day or fall from the edge of space safely. It's a perfect parachute that you can always wear. You needn't fear floods either. Your reduced weight would allow you to walk on water. You could step out of the window of a burning building or jump above the chaos of an earthquake.
Tornadoes would still be a bitch. But 3 out of four classic elements is not too bad. Later the stri introduced moto tubes. You held them they pulled yu through the sky. You could probably get the same effect from using a leaf blower. The belts eventually became rocket packs and inertron was largely ignored. Given the aerodynamics of a human body I'd rather use the moto tubes. You only had one hand free but it seemed way easier to control than a rocket on your back. Not to mention less flammable. The rocket belts also had the straps under the arms.
Inertron was a fine element in the construction of spacecraft. A ship would pile enough in the hold to behave like a dirigible and could perform an unpowered climb to the edge of space for less mass than the rocket propellant it would otherwise use. It could also get by with far less thrust meaning smaller rocket motors and more room for propellant.
It was a good idea to spread your inertron through out the ship. The engineers were usually pretty good about this but you still had scenes where part of a ship would get shot off and the rest would rise uncontrollably. This was probably inspired by numerous dirigible accidents back in the day.
Even with the dangers inertron would be a great export item for struggling colonies. Think about paragravity with no power or other machinery involved. Building aircraft becomes a breeze and they don't need landing strips! Huge loads can be moved without need of construction equipment or cranes. Sure a couple of people drift off into the stratosphere occasionally. That's OSHA's problem, not ours. Having several kilos of inertron in quick release pouches can cut down on that. People might also want several lead anchors for tying off their jumping devices at night. Having to pull your jump pack off the ceiling can be a pain. Inertron based aircraft also want anchors or tie down points to keep from blowing away in a strong wind.
Salvaging lost inertron from low orbit might be lucrative for enterprising young people with an aircraft. Best not to be too greedy or you will have a lot of trouble landing. Some ships had air pumps to suck in air and liquify it for use as ballast in an emergency (like having the bad guys disintegrate your tail section).
If you manage to mass produce inertron all bets are off. Including it in building materials would let you increase building height, bridge spans and for the truly wealthy and romantic you have your castles in the air. Why should psychiatrists be the only ones to charge rent for those living in them?