The Propellant Problem
Franklin Chang Díaz was born in San José, Costa Rica, in 1950, of Costa Rican and Chinese ancestry. He emigrated to the United States as a teenager with minimal English and limited funds, repeated a year of secondary school, and completed a doctorate in applied plasma physics at MIT in 1977. NASA selected him for the astronaut corps in 1980, and between 1986 and 2002 he flew seven Space Shuttle missions — tying the record for the most spaceflights by any individual.
Chemical rockets are constrained by the Tsiolkovsky rocket equation, Δv = v_e · ln(m_0/m_f), which relates the achievable change in velocity to exhaust velocity and to the ratio of initial to final mass. Because the mass ratio enters logarithmically, increasing Δv by carrying more propellant produces sharply diminishing returns. The leverage is in exhaust velocity (v_e), equivalently specific impulse, which appears as a linear multiplier. Chemical combustion caps exhaust velocity near 4.5 km/s, a hard limit set by the chemistry itself.
Chang Díaz's VASIMR — Variable Specific Impulse Magnetoplasma Rocket — sidesteps that ceiling. Propellant such as argon is ionized and then heated by radio-frequency waves to plasma temperatures far beyond what any material nozzle could survive; a magnetic nozzle confines and directs the exhaust, so no physical surface contacts the plasma. Exhaust velocities on the order of 50 km/s are achievable, roughly an order of magnitude above chemical propulsion. The engine's name reflects a further capability: the ratio of thrust to specific impulse can be tuned in flight, trading high thrust for high efficiency as a mission phase requires.
The cost is thrust density. Electric propulsion produces forces measured in newtons rather than meganewtons, making it useless for surface launch and dependent on a substantial electrical power source — a serious constraint, since a high-power VASIMR would require nuclear or very large solar power. Chang Díaz founded Ad Astra Rocket Company, with operations in Texas and Costa Rica, to develop the technology toward in-space cargo transport and eventual crewed interplanetary missions.
