Destination Orbit Plane Change

A launch loop oriented along the 8 degree south latitude line launches into a plane defined by that vector and the center of the Earth. The launch plane rotates, and orbits launched into one plane will need some north/south delta V to transition to a different plane.

The easiest place to change the orbital plane is at the apogee of a Highly Eccentric Earth Orbit (HEEO) such as a Construction Orbit. The apogee velocity of a one day construction orbit is 1021 m/s prograde; changing that to a retrograde orbit requires a delta V of 2042 m/s, and to a polar orbit requires 1444 m/s .

Less delta V is required for a higher orbit with a lower apogee velocity:

Rocket launch from an island launch site offers more direct access to different orbital planes, and a launch from Vandenberg provides direct access to polar and retrograd orbits, but ... rockets are expensive. This should be used for passengers, not bulk payload that can tolerate multi-week journeys.

Loop launch to a construction station is assumed. At the construction station, vehicles will be assembled (with a bit heat shield!) and fueled for the plane change and subsequent delta V. The sequence:

• 1) A large delta V is applied at construction station perigee, with significant prograde and N/S propellant plume velocity to change the orbit. For bulk payload, this can take place incrementally over many orbits, so a high ISP electric engine can be used. There could be a "traffic jam" at apogee and perigee from all the converging orbits, so those altitudes should be modified as well, establishing "flight levels" for the evolving orbital inclinations, much like different aircraft flight altitude levels are assigned for different flight directions.

• 2) When the orbit plane rotation is complete, retrograde apogee thrust (prograde propellant plume) drops perigee into the upper atmosphere. Drag will drop apogee to the destination orbit apogee. This can be done "all at once" with a deep dive to 50 km altitude and a high gee, high heat slowdown, but a slow, multiple pass slowdown (2a) permits a cheaper heat shield, low acceleration forces, and a perigee somewhat above launch loop altitude. Presume a 100 km altitude (6478 km radius) perigee.

• 3) Another series of destination-apogee thrusts raise perigee to match the destination orbit and time.

Example: 1000 tonne module from a 5 day construction orbit to ISS

Plane change is cheaper for longer period construction orbits. ISS is in a 5550 second orbit, inclined 51.642° with an eccentricity of 0.0001694. Let's ignore eccentricity ( ±1.1 km radius) and J2 effects; the semimajor axis is 6775 km and the orbital velocity is 7670 m/s.

A 5 day construction orbit has an apogee velocity of 337.22 m/s; changing that to a 51.642° orbit requires a delta v of 337.22 * 2 * sin( 51.642° / 2 ) = 294 m/s of delta V to complete plane change (1), added to some fraction of a 40 m/s retrograde thrust (2) to drop perigee into the 6478 km atmosphere.

Multiple atmosphere drag passes (2a) drop apogee to 6775 km radius. The resulting 6478 x 6775 km orbit has an apogee velocity of 7590 m/s; an 80 m/s apogee delta V (3) will rendezvous with ISS. This might be accomplished with a "net catcher" similar to the construction station, so that the ISS-associated catcher station can use a high-ISP electric engine and the cargo vehicle can be passive. Perhaps 320 m/s of total delta V needed for the plane change, most of it from high-ISP electric engines.

Strategic Consequences

An all-azimuth orbit-velocity rocket launch can deliver a nuclear weapon anywhere on Earth in less than an hour. A construction station could hypothetically deliver a weapon, but the delivery time would be days, during which time the weapon (and the construction station) is vulnerable to intercept and destruction. The weapon from the construction station would arrive at 10.5 km/s rather than 7.6 km/s; reentry heating and turbulence would more than double, greatly increasing vehicle mass and potentially disabling damage. This makes no strategic sense as a weapon delivery system; it is more costly and far easier to track and intercept. Assuming the construction station and the entry vehicle design process are monitored, this is vastly less of a strategic threat than a rocket launcher. Launch loops might be a potential threat to countries along the equator, but a "weapon system" that costs more than its target, and is far easier to destroy, is a Really Stupid weapon. It's like pulling the pin from a hand grenade, and throwing the pin rather than the grenade.

Some will still feel threatened, irrationally. The rational response is education and inclusion. If everyone on Earth benefits from a launch loop, insane people will still be a threat, but the sane people around them will have a strong incentive to prevent the insane people from damaging launch loops, and endure inspections and limits to preserve shared benefits.