Alternative launch concepts

Rockets are expensive, so alternatives are proposed constantly. Most do not work, and they mostly fail for the same reason. This page states the constraint first and then assesses the concepts against it.

The constraint

Reaching orbit requires two things that are often conflated: altitude and velocity. Altitude is cheap. Lifting a kilogram to 100 km costs about 1 MJ of potential energy. Orbital velocity at that altitude is 7.85 km/s, worth about 31 MJ per kilogram of kinetic energy.

Roughly 97% of the energy of reaching orbit is velocity, not height. Any concept that helps mainly with altitude is addressing 3% of the problem.

The second constraint is where that velocity is imparted. A ground-based accelerator delivers its energy at sea level, where the atmosphere is at full density. An object leaving the ground at several kilometres per second faces dynamic pressure and heating comparable to re-entry, but on the way up and through the thickest air. A rocket avoids this by being slow when it is low and fast when it is high, which is exactly what a ground accelerator cannot do.

Almost every concept below fails on one of those two points.

Air launch

Carry the rocket aloft under an aircraft and release it at altitude.

The genuine advantage is not performance but flexibility: launch azimuth and inclination are set by where the aircraft flies, not by the geography of a launch site, and weather can be flown around.

The costs are structural. The rocket is limited to what an aircraft can carry, which caps payload in the hundreds of kilograms, and horizontal carriage imposes bending loads that a vertically launched vehicle never sees, so the airframe is heavier. A dedicated carrier aircraft is itself a major capital and operating expense.

Pegasus flew successfully for decades at about 443 kg to LEO. LauncherOne reached orbit and was cancelled in 2023. The record suggests air launch works technically and struggles commercially, because the aircraft’s fixed cost is spread over very little payload.

Rotary and centrifugal launch

Spin a projectile in a vacuum chamber to several kilometres per second and release it through a hatch.

The energy input is efficient and uses ground power rather than propellant. The problems are severe and mostly unaddressed by the concept itself.

Acceleration

Sustaining kilometres per second on a tether of tens of metres implies accelerations in the thousands to tens of thousands of g. Very few payloads survive that, and none that contain optics, deployables or people.

Atmospheric exit

The projectile leaves at sea level at hypersonic speed, into the densest air. Peak heating and dynamic pressure are extreme, and the aeroshell mass to survive them eats the payload.

It does not reach orbit

A release speed of about 2 km/s is a fraction of the 9.4 km/s required. An onboard rocket stage still provides most of the delta-v, and the accelerator has replaced only the first stage.

Circularisation

Any ballistic launch from the ground returns to the ground. An apogee burn is mandatory, so a rocket stage is required regardless.

The concept is best understood as replacing a first stage, not as replacing rockets.

Electromagnetic mass drivers

Railguns and coilguns face the same two problems as centrifugal launch: survivable acceleration and atmospheric exit. Adding electromagnetic complexity, rail erosion and pulsed power at gigawatt levels does not remove either.

On the Moon, however, the analysis reverses completely, and this is worth stating clearly because it is the one place where a non-rocket launcher is genuinely the right answer.

A lunar mass driver launching processed regolith, oxygen or metals into orbit for use elsewhere has no atmospheric problem, needs a seventh of the velocity, and carries cargo that does not care about acceleration. This was the core of the 1970s space manufacturing studies and remains the most credible non-rocket launch concept anywhere.

Space elevators

A tether from the surface to beyond geostationary altitude, held in tension by the centrifugal force on the counterweight, with climbers ascending it.

The concept fails on materials, and the failure is not marginal. The relevant figure of merit is specific strength, tensile strength divided by density. Because a tether must support its own weight along its length, the required cross-section grows exponentially with the ratio of the characteristic length to the specific strength, so a shortfall does not mean a heavier tether, it means an absurd one.

Individual carbon nanotubes approach the required strength. Macroscopic materials made from them do not, by a wide margin, because defects and inter-tube load transfer dominate at any useful length. There is no bulk material within an order of magnitude of what an Earth elevator needs, and none in prospect.

On the Moon and Mars the requirement falls dramatically because gravity is lower and the rotation is slower. A lunar elevator anchored through Earth-Moon $L_1$ is buildable with existing commercial fibres such as Zylon. That is a real engineering proposition rather than a materials fantasy, and it is limited by cost and demand rather than by physics.

Tethers and momentum exchange

A rotating tether in orbit, a rotovator, catches a payload at the bottom of its rotation and releases it at the top with added velocity. Momentum comes from the tether’s own orbit, which decays and must be replenished, but the replenishment can use efficient electric propulsion or an electrodynamic tether pushing against the geomagnetic field.

The physics is sound and the concept survives scrutiny better than most on this page. What blocks it is systems engineering: capture is a rendezvous at kilometres per second of relative velocity with a timing tolerance of milliseconds, the tether is a very large debris target, and the entire capital cost must be committed before any payload is moved. Nothing beyond small demonstrations has flown.

Beamed energy

Direct a laser or microwave beam at a vehicle from the ground, and use it to heat propellant. This decouples energy from the vehicle in the same way electric propulsion does, allowing specific impulses well above chemical limits without carrying a power source.

Small lightcraft demonstrations have flown to modest altitudes. Scaling to orbital launch requires ground power in the hundreds of megawatts to gigawatts, optics of a scale not yet built, and tracking accurate enough to hold a beam on an accelerating vehicle through atmospheric turbulence. The physics does not forbid it; the infrastructure cost is comparable to a power station and the technology is not ready.

Summary

The pattern is consistent. On Earth, the atmosphere and the 11.2 km/s escape velocity defeat nearly everything that is not a rocket. On the Moon, with no atmosphere and a fifth of the velocity requirement, several of these concepts become the obvious choice. The right conclusion is not that alternative launch is a dead end, but that it is a technology for somewhere else.

References

• Sarigul-Klijn, M. and Sarigul-Klijn, N. “Air Launching Earth-to-Orbit: Effects of Launch Conditions and Vehicle Aerodynamics”, Journal of Spacecraft and Rockets, 42:569, 2005.
• O’Neill, G. K. The High Frontier: Human Colonies in Space, William Morrow, 1976. The origin of the lunar mass driver concept.
• Pearson, J. “The orbital tower: a spacecraft launcher using the Earth’s rotational energy”, Acta Astronautica, 2:785, 1975.
• Edwards, B. C. and Westling, E. A. The Space Elevator, BC Edwards, 2003.
• Penoyre, Z. and Sandford, E. “The Spaceline: a practical space elevator alternative achievable with current technology”, Acta Astronautica, 198:139, 2022. On the lunar case.
• Hoyt, R. P. and Uphoff, C. “Cislunar Tether Transport System”, Journal of Spacecraft and Rockets, 37:177, 2000.