How Close Are We To Building A Practical Skyhook?

The problem with space elevators is artificial satellites crashing into them. You could not have a depot at low earth orbit unless you could lift a booster with the payload, which is also why an air launch is impractical for an orbital vehicle. For example, the Falcon 9 (the lower stage) gets to Mach 8 at 40 miles in altitude. We cannot go that fast (let alone Mach 25, which is orbital velocity which takes an upper stage to reach - which is how fast a sky hook to LEO would go. Could an air drop ship - with SRBs attached to it - be able to get to Mach 8 and carry something as big as Falcon upper? There is very little Oxygen at 40 miles up - and 2 miles up is hard for humans to breath without O2. The only way a ship dropper would work is if it did not have to carry oxygen for its whole journey and not use SRBs. Until a broken washing machine anti-grav engine is built, we are stuck with something like Falcon.

A broken washing machine would contain a spinning weight that wants to be perpendicular to its axis of motion but is prevented from getting there with no friction (such as with magnetic repulsion - and with the field not collapsing). Then the spinning mass must have greater force than the mass of the rest of the ship (or maybe including the throw weights). To not spin out of control, you would have to have counter-rotating masses - so two turbines from one or two reactors - and now things get heavy - however the need to create a heavy enough throw weight may be the necessity for an improvement in reactor technology. You can see posts with these systems laid out at spaceconsortium.blogspot.com - with the anti-grav posted in the earliest set in 2004. A post from that time on space exploration was given to a friend who worked at Goddard in 1995, which was used as a resource for both ULA and the current civil space paradigm - although there was also internal Goddard management initiative that predated this resource. 

The going to Mars post, as with all the earliest posts, actually went up in 2002 on Geocities until the platform was taken down - where John Wayne Smith of 1000 Planets - who is known for wanting to use shuttle fuel tanks as space hab modules - hired me as director of colony planning. We discussed launch options and he dropped the fuel tank option, while I dropped air drop. We settled on trying to contact Elon to see if Falcon heavy or the Starship booster might be able to lift modules. The modules we were thinking of were bigger than ship - having 3 levels - but ships may be workable for smaller modules for an orbiting station, as well as a cradle ship to get modules down to the surface of Mars. If they are fully tricked out that would have to be damn heavy cradle to get itself to orbit and then decelerate from orbital velocity with such a load and travel to the service safely. Of course, an anti-grav ship would make that task easier. Modules would be tricked out in LOE or on the Moon. From LOE would like require lunar materiel, as it takes less delta v to get from the moon to LOE than from the earth's gravity well. The fuel to get from LOE to the Moon, if that were the assembly point, would still have to come from the moon unless a catapult or cannon could launch hydrogen and oxygen (or jet fuel) - or more likely ice - but encased in metal - as well as smaller equipment. Any catapult or gun would have to have a small booster and be able to get payload (with booster) to 40 miles and Mach 8.

Dragon upper stage would need landing legs and SRBs for a first trip to the Moon - then fuel up on material from there (or maybe a set of upper stages) to get materiel from the moon as well as being fueled up enough to get payloads to translunar injection orbits. Or they could ferry ship modules (with landing legs of their own or could have a system that land horizontally with engines at both ends). The question is whether to integrate Ships into space station, lunar habitat or Martian habitat is easier in LOE or on the Moon - or maybe both. It depends, ultimately on the welfare of the workforce - which means that some type of artificial gravity for LOE work (and stations) is practical from the point of view of human anatomy. 

A better way to go to Mars is with large space stations that serve as JPL in orbit - with families and support crew to keep the station running - both technically and as a social system. The size of such a crew for any exploration station - or construction station with family - could be planned out in a "table top" exercise with NASA staff from JPL and Goddard doing a conceptual design - with a facilitator who knows something about Colony Planning and Systems Engineering. I know a guy. I see him in the bathroom mirror a few times a day.