Solid Rocket Exhaust

The launch loop launches vehicles into elliptical Hohmann transfer orbits. If the destination is a circular orbit at high altitude ( like GEO = 42164 km radius ), additional delta V is needed to circularize the orbit. If the destination "anomaly" (angle around the orbit) is properly timed, the trip will take 5.25 hours to GEO. Cryogenic propellants can survive that long. However, on occasion the trip will require many additional orbits( at 10.5 hours each to GEO) to arrive at the destination. The propellant will either need to be some storable liquid combination like N₂O₄/N₂H₄, or solid fuel such as ATK-Thiokol TP-H-3340 . Solid rockets are usually easier to operate, but their ISP is lower than cryogenic fuels, and as they approach burnout they can release macroscopic chunks of unburned material as space debris.

Fortunately, the exhaust from a circularization rocket burn is thrown retrograde to the transfer orbit, and most of those atoms will be in a trajectory that falls to intercept the Earth.

The chemicals and the exhaust

Thiokol TP-H-3340 is 71% Ammonium Perchlorate (NH₄ClO₄), 18% Aluminum, and 11% HTPB, Hydroxyl-terminated polybutadiene ( polymerized C₄H₆ ). By weight, the propellant is:

I'm not exactly sure what it burns to, but it seems to be somewhat deficient in oxygen, fuel rich. One possible guess of exhaust constituents:

ATK-Thiokol specifies an ISP of 285.3 seconds, corresponding to an exhaust velocity of 2798 meters per second. The expanded exhaust temperature is not stated, but may be around 2000 Kelvin. That results in an RMS thermal velocity component in the exhaust velocity direction of about 1000 m/s for the ammonia and water, and less than 400 m/s for the alumina and carbon soot, which will probably form nanoparticles.

For a vehicle headed for the M288 orbit (12789 km radius, 5583 m/s) , the transfer orbit from a launch loop at 80km altitude will have an apogee velocity of 4554 m/s, for a delta V of 1029 m/s. At the start of the circularization, the solid rocket exhaust will be moving prograde at 1756 m/s (4554-2798) and at the end of the circularization it will be moving prograde at 2785 m/s. These velocities are both less than the transfer orbit velocity, so their trajectories will have a lower perigee than the loop, and will intercept the earth.

For a vehicle headed for GEO orbit (42164 km radius, 3075 m/s) , the transfer orbit from a launch loop at 80km altitude will have an apogee velocity of 1576 m/s, for a delta V of 1499 m/s. At the start of the circularization, the solid rocket exhaust will be moving retrograde at 1222 m/s (1576-2798) and at the end of the circularization it will be moving prograde at 277 m/s. These will result in trajectories that intercept the earth. However, the exhaust velocity will vary in thermal velocity, so some of the ammonia and water from the start of the burn may be moving retrograde at 2200 m/s, which puts it in a retrograde orbit with a perigee well above the earth. While these molecules will eventually collide with high altitude particles and gasses and re-enter, they will stay in orbit for a very long time, perhaps slowly eroding the front surface of satellites.

In all cases, solid rocket motors typically do not shut down cleanly. When the guidance system detects that enough delta-V has been added, the motors are shut off by firing a pyrotechnic charge, which ruptures the pressure vessel and stops the burn. Chunks of the unburned fuel may be scattered, in orbits approximating the destination orbit.

Instead, tethers

The solid rocket motors are expensive, imperfectly reliable, and reduce the payload fraction of vehicles launched by the launch loop. For "popular" destinations like M288 and GEO, well outside the crowded LEO orbits, rotating and elevator tethers may be used instead.

Random Weblinks

• Google htpb ap toxicity exhaust

  • https://en.wikipedia.org/wiki/Hydroxyl-terminated_polybutadiene

  • Environmentally safe (chlorine-free): new green propellant ...