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SpaceX, the upstart start-up rocket company founded by famous techwealth kingpin Elon Musk, is to build and test-fly a "Grasshopper" hover rocket based on the massive first-stage fuel tank of the company's Falcon 9 vehicle, capable of carrying ten tonnes of cargo or seven people into orbit.

As yet SpaceX is not discussing the Grasshopper publicly, but we learn some interesting details of the new craft from an environmental impact statement covering planned test flights filed with the Federal Aviation Administration (65-page PDF/1.4MB).

The Grasshopper RLV consists of a Falcon 9 Stage 1 tank, a Merlin-1D engine, four steel landing legs, and a steel support structure. Carbon overwrapped pressure vessels (COPVs), which are filled with either nitrogen or helium, are attached to the support structure. The Merlin-1D engine has a maximum thrust of 122,000 pounds.

The propellants used in the Grasshopper RLV include a highly refined kerosene fuel, called RP-1, and liquid oxygen (LOX) as the oxidizer. The Grasshopper RLV has a maximum operational propellant load of approximately 6,900 gallons; however, the propellant loads for any one test would often be lower than the maximum propellant load. Even when the maximum propellant load is used, the majority of the propellant would remain unburned and would serve as ballast to keep the thrust-to-weight ratio low.

A normal Falcon 9 first stage, of the sort which has already sent test payloads including a large cheese into orbit and back, has nine Merlins in a grid at its base, not just one. Furthermore SpaceX specifies that it doesn't plan to fly the Grasshopper above 11,500 feet, so it's clear that the main purpose of the project is to prove the concept of vertical takeoff and vertical landing (VTVL) rocket operations.

Long ago it was assumed by science-fiction authors and futurists that reusable VTVL rockets would be the normal means of flight into and out of space. But in fact modern-day rockets don't have the performance to operate this way: even if they burn all their fuel up in the ascent stage, they must be stacked to deliver useful payloads to orbit.

Burning all fuel in ascent means that at least the first stage must fall to Earth and be destroyed, removing any possibility of using it again, which is one reason why space launch is so expensive. Various ideas have been advanced for getting around this, often involving lower stages gliding or parachuting back down to be reused. Such plans have yet to be proved practical, however.

An alternative scheme would be to have an upper stage separate from a lower while the lower still had a fair bit of fuel left. The lower stage might then descend to hover down on its exhaust to a vertical landing in the fashion of a Moon lander, ready to be refuelled and hurl another upper-stage/payload package into the sky.

Naturally, retaining fuel in this manner would rob the whole stack of lifting performance, but being able to re-use the hardware might make such a plan pay nonetheless. It would seem that SpaceX's Grasshopper plans are intended to discover just how feasible such a scheme might be, in particular the vertical pad landings.

SpaceX is not alone in investigating such plans. Blue Origin, the secretive space start-up founded by Amazon supremo Jeff Bezos, is also known to be working on reusable, vertical-landing rocket boosters. Bezos' "New Shepard" test vehicle, like the Grasshopper, isn't expected to achieve orbital performance, but it is meant to lead one day to reusable VTVL space rocketry. Blue Origin has carried out two tests so far, the most recent of which ended in failure when the Shepard departed from planned flight, had its thrust cut off by safety systems and subsequently crashed.

Grasshopper has yet to appear, but the Falcon technology on which it is based has now made four successful flights following early failures. ®

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