The departure corridor

Why a departure is different

Aperture, light time, predictability.

A receiver in a known orbit can be served indefinitely and a receiver that maneuvers unpredictably cannot be served at all. That rule kills general vehicle service. It does not kill the launch phase.

Aperture. A vehicle cannot carry a three-kilometer receiver and at short range does not need one. A 100-meter receiver masses about four tons, a rounding error on a cargo tug, and a five-kilometer array fully illuminates it out to about 1.3 AU. That is not a limitation. It is exactly the launch phase.

Light time. At 0.5 AU the round trip is eight minutes rather than the 86 minutes to Jupiter. Lead-angle prediction over four minutes for a vehicle on a filed low-thrust profile is an ordinary navigation problem.

Predictability. A cargo tug executing a planned continuous burn is arguably the most predictable object in the solar system: constant known thrust, a published profile, and every incentive to stay on it. The vehicles beamed power cannot serve are the ones that maneuver unpredictably, and a beam-fed tug has no reason to.

What genuinely fails is fault response, and that is treated below as a failure category rather than pretended away.

What it delivers

Three weeks of holding, and 55 km/s.

A 1,000-ton tug taking one gigawatt into a 100-meter receiver, converting at 50 percent into a 50-kilometer-per-second thruster, produces 20,000 newtons. That is 0.02 meters per second squared, and at a mass ratio of three it gains 55 kilometers per second of velocity in 19 days over about 0.2 AU. Both numbers sit comfortably inside the service volume.

The beam does not have to follow the ship anywhere. It has to hold it for three weeks.

Underneath the arithmetic is the finding that matters most. A beam-fed vehicle carries no power plant. The outer system’s wall is a statement about watts per kilogram of a reactor the ship has to bring, and a ship that brings none is not on that curve at all for the powered arc.

The laser-thermal variant

The one propulsion architecture with no radiator.

Heating hydrogen directly with the beam, rather than running it through electricity and an ion thruster, inverts the trade: about 147,000 newtons per gigawatt at roughly 9.5 kilometers per second exhaust — 26 times the thrust at a quarter the specific impulse.

Its real interest is thermal. There is no radiator, because the propellant carries the waste heat out of the vehicle. The sorting rule says the flux advantage dies wherever heat has to be rejected; a laser-thermal rocket is the one propulsion architecture that never rejects any. It is the sorting rule’s cleanest positive case.

Three consequences

Braking, bootstrapping, and mass ratio.

The beam brakes as well as it pushes. A light sail can only be pushed, because the momentum comes from the photons. A beamed-power vehicle is under no such constraint: the beam delivers energy and the thrust vector is the ship’s own choice. So the array owns both ends of every leg that begins or ends inside 1.3 AU.

Mercury’s first customer is its own supply line. Orbit insertion at Mercury costs 7.50 kilometers per second and is the whole reason nobody has gone. The array is at the destination. An inbound cargo vehicle spends its final weeks well inside the service volume and can be braked on power it does not carry, so the barrier that kept Mercury under-explored is retired by the thing Mercury exists to build. Every phase after the first tug is cheaper than the one before it, and that is the only compounding mechanism in the Mercury case that does not depend on manufacturing throughput.

Round trips stop being four burns. A flyby needs one velocity change, an orbiter two, and a round trip four, and the exponent is where programs die. A vehicle whose departure and return burns are both beam-supplied carries propellant for two. The mass ratio falls from the fourth power to the second, which is the largest structural improvement available to any architecture in the inner system.

A port, not a power station

One aperture, many beams.

A phased array forms its beam by controlling element phase, and the same aperture can synthesize several beams at once in different directions, dividing the available power among them. A ship needs a gigawatt, not 100, so a 100-gigawatt array can hold on the order of 100 vehicles simultaneously, each on its own diffraction-limited spot.

That also fixes the market. A continuous power contract is indivisible; departure slots are divisible, schedulable and short, which is what makes them tradeable. The interplanetary markets acquire their first genuinely liquid instrument, and it is capacity rather than a commodity.

What the boost costs

Velocity bought on the way out, handed back at arrival.

30 kilometers per second added at 1 AU crosses to Jupiter’s orbit in about five months rather than 2.7 years, and arrives with 33.5 kilometers per second of excess, where capture is not merely expensive but impossible. Inside 1.3 AU the array handles both ends of a leg; outside it, the array buys the outbound leg only.

One-way cargo to a destination with an atmosphere takes the full benefit, because aerocapture absorbs the arrival — which raises the stakes considerably on a technique that has never been flown anywhere. Anything that must stop propulsively takes a partial benefit: 5 to 15 kilometers per second shortens a Jupiter crossing by roughly a third while leaving the arrival inside what a multi-flyby tour can absorb, and that is the realistic operating point.

Beamed departure does not open the outer system. It compresses the inner system into something that behaves like a single logistics theater, and it makes freight to a body with an atmosphere routine.

The interstellar sail is not on this list and should not be. A sail array wants different wavelength agility, a different duty cycle, an acceleration tolerance five orders of magnitude away, and optics matched to a four-meter target across a 22 percent Doppler band. What the two programs share is the collector field, the film industry, the metrology and the operating institution; the beam line is dedicated. See the array and the beam architecture.

The recommendation

Build for departures, sell idle time to Callisto.

Set the two uses side by side. Pointed at Callisto, the beam attacks the power wall — and the power wall is the one fission can eventually brute-force. 2.5 million tons of reactor plant is an absurd number, but it is a number, and a civilization with a lunar mass driver and a 50-year horizon can eventually pay it. Pointed at departing vehicles, the beam attacks the transit wall, and nothing else in the architecture attacks it at all.

It also survives the political objection better. A Callisto power contract makes the outer system dependent on whoever holds Mercury. A departure beam serves ships leaving the inner system on the leg where everyone’s interests already align — though it still hands the array’s owner a throttle on outbound traffic, which is a smaller lever than a power monopoly and not a trivial one.

Build the array for departures and sell the idle time to Callisto. The two uses are not in tension, because a departure burn is 19 days and a Jovian power contract is continuous only if somebody wants it to be. The scheduling market falls straight out of that, and it prices something no market has ever priced: thrust and power as the same commodity, distinguished only by where the array is pointing.