assumed not to exceed 100,000 m/s. The thrust of the ramjet d/dt(mv2) was equal to the mass of ions expelled per second multiplied by 100,000 meters per second. In the Zubrin/Andrews design of 1985, this resulted in the condition that d/dt(mv1) > d/dt(mv2). This condition resulted in the drag force exceeding the thrust of the hypothetical ramjet in the Zubrin/Andrews version of the design. Related inventions Ram Augmented Interstellar Rocket (RAIR) The problem of using the interstellar medium as the sole fuel source led to study of the Ram Augmented Interstellar Rocket (RAIR). The RAIR carries its nuclear fuel supply and exhausts the reaction products to produce some of its thrust. However it greatly enhances its performance by scooping the interstellar medium and using this as extra reaction mass to augment the rocket. The propulsion system of the RAIR consists of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. The scoop field funnels interstellar gas into an "accelerator" (this could for example be a heat exchange system transferring thermal energy from the reactor directly to the interstellar gas) which is supplied power from a reactor. One of the best ways to understand this concept is to consider that the hydrogen nuclear fuel carried on board acts as a fuel (energy source) whereas the interstellar gas collected by the scoop and then exhausted at great speed from the back acts as a propellant (the reaction mass), the vehicle therefore has a limited fuel supply but an unlimited propellant supply. A normal Bussard ramjet would have an infinite supply of both. However, theory suggests that where a Bussard ramjet would suffer drag from having to pre-accelerate interstellar gas to its own speed before intake, a RAIR system would be able to transfer energy via the "accelerator" mechanism to the interstellar medium despite velocity differences, and so would suffer far less drag. Laser Powered Interstellar Ramjet Beamed energy coupled with a vehicle scooping hydrogen from the interstellar medium is another variant. A laser array in the solar system beams to a collector on a vehicle which uses something like a linear accelerator to produce thrust. This solves the fusion reactor problem for the ramjet. There are limitations because of the attenuation of beamed energy with distance. Magnetic sail The calculations (by Robert Zubrin and an associate) inspired the idea of a magnetic parachute or sail. This could be important