Mayflies are a class of spacecraft described on p.94 of GURPS Terradyne. They may plausibly be encountered in a Transhuman Space or many other space-based scenarios.
Mayflies are one-use orbit-to-surface cargo transports. More material is sent from orbit than is brought up from Earth (or Mars) so a considerable proportion is sent down by mayfly. A mayfly is a simple glider that is built in orbit. It has a low-cost computer guidance system and some means of thrust to nudge it out of orbit. Mayflies may be catapulted from orbiting constructions but they still require some means of independent thrust and manoeuvre.
A small mayfly has a 5 yard wingspan, occupies 5 cubic yards and has a loaded mass of 3 tons. The largest have a 23 yard wingspan, occupy 100 cubic yards and mass 65 tons. About 80% of a mayfly’s mass is cargo so the largest carry 50 tons/ 100,000 pounds. GURPS Terradyne prices a large mayfly at $7,500.
Mayflies are constructed in orbit using materials and techniques that are readily available. They are unlikely to use exotic materials or anything that is more expensive than it needs to be. Likewise, the thrust and manoeuvre system is likely to be something simple and cost-effective such as a chemical rocket with the minimum of fuel reserve.
Mayflies are cargo vessels and not designed to carry passengers. Resourceful players may find ways around this or locate NPCs that can help them do so, for a price.
Acquiring a mayfly from a non-reputable source comes with its own potential problems. A badly built mayfly could break up on re-entry, destroying its cargo. More usefully from a narrative point of view is for a malfunction in the guidance system or control surfaces to cause the mayfly to land many hundreds of miles from its intended destination. Perhaps the system has been hacked or deliberately sabotaged. A player may be placed in hostile territory, or the opportunity for a salvage operation created.
A rogue mayfly might be directed to crash into a planetside target. Mayflies that stray too far from their registered flight path may be shot down!
Being relatively simple, most mayflies lack sophisticated security systems so some smugglers see them as a good place to hide contraband. Some smugglers use “parasite pods” which detach or eject from the mayfly some distance from its intended landing spot.
We are given no information on where mayflies land. Wilderness areas such as deserts or large bodies of water seem likely. Perhaps a problem with locals stealing cargos may arise. Or a salvage operation needs deep diving expertise and equipment.
Landed mayflies are broken up and their components repurposed for various planetside uses. In some parts of Earth or Mars buildings made from mayfly parts are a common sight.