a launch vehicle. Solid-propellant motors burn a hydrocarbon fuel with ammonium perchlorate as the oxidizer. The resulting rocket emissions at ground level include other combustion products (e.g., CO, CO2, N2, H2, and H2O) as well as HCl, but the Air Force has considered HCl to be the most hazardous. Liquid-propellant motors often use Aerozine-50 (50:50 blend of hydrazine and unsymmetrical dimethyl hydrazine) as the fuel and N2O4 as the oxidizer. For normal launches, nitrogen oxide emissions at ground level are negligible. However, in the case of an accident during fuel transfer or a catastrophic abort near the ground, large quantities of N2O4 can be released. In the atmosphere, some of the N2O4 is rapidly converted to NO2. The Air Force speculates that NO2 rapidly converts further to HNO3 (T. Clapp, U.S. Air Force Space Command, personal commun., February 25,1997).

Aerozine-50 also can be released from rockets using liquid propellants in catastrophic aborts (or during transfer operations); however, the Air Force did not ask the subcommittee to review the toxicity of hydrazines for this report. COT recently reviewed the toxicity of hydrazine for the National Aeronautics and Space Administration (NASA) (NRC 1996).

The Air Force has measured and estimated rocket-emission exposure concentrations and durations in the event of normal and catastrophic launches. Projected exposure concentrations and durations during a normal Titan IV launch and a catastrophic abort at Vandenberg are listed below (D. Dargitz, U.S. Air Force 30th Space Wing; L. Philipson, ACTA Inc.; and T. Clapp, U.S. Air Force Space Command; personal commun., March 14, 1997):

A. Normal launch

Peak HCl concentration reaches ≈30 parts per million (ppm) out to 3 kilometers (km) downwind

Range of passage times

≥10 ppm, ≈2 min (7 km)

≥5 ppm, ≈3 min (15 km)

≥2 ppm, ≈4 min (15 km)

B. Catastrophic launch abort

1. Peak HCl concentration reaches ≈90 ppm out to 4 km for an abort 20 sec after takeoff

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