While theoretical modeling of these systems suggests an advantage over bipropellant motors, several factors limit their practical implementation, including the difficulty of injecting solid metal into the thrust chamber; heat, mass, and momentum transport limitations across phases; and the difficulty of achieving and sustaining combustion of the metal.
For example, injecting a small amount of liquid hydrogen into a kerosene-burning engine can yield significant specific impulse improvements without compromising propellant density.
[4] Although liquid hydrogen delivers the largest specific impulse of the plausible rocket fuels, it also requires huge structures to hold it due to its low density.
These structures can weigh a lot, offsetting the light weight of the fuel itself to some degree, and also result in higher drag while in the atmosphere.
With light enough engines this might be reasonable, but an SSTO design requires a very high mass fraction and so has razor-thin margins for extra weight.
He concluded that tripropellant engines would produce gains of over 100% (essentially more than double) in payload fraction, reductions of over 65% in propellant volume and better than 20% in dry weight.
His last full study was on the Orbital Rocket Airplane which used both tripropellant and (in some versions) a plug nozzle, resulting in a spaceship only slightly larger than a Lockheed SR-71, able to operate from traditional runways.