The Lunar Landing Research Vehicle (LLRV) and its successor, the Lunar Landing Training Vehicle (LLTV) were built for NASA by Bell Aerosystems to explore the dynamics of vertical rocket landing in the Moon’s gravity and train astronauts to perform landings. A down-pointing, gimballed jet engine canceled 5/6 of the vehicle’s weight, allowing hydrogen peroxide monopropellant thrusters to control the vehicle’s attitude, translation, and descent. These contraptions were famously difficult to fly, and of a total of 2 LLRVs and 3 LLTVs built, three were destroyed in crashes with, in each case, the pilot ejecting and safely landing by parachute. Here is Neil Armstrong ejecting from an LLRV instants before it crashed on 1968-05-06.
What was the, then, primary barrier to a combination of horizon detection and inertial guidance to at least semi-automate vertical landing?
The landing process in the actual Lunar Module was highly automated. The details are explained in “A User’s Guide to the LUMINARY1A Lunar Landing Programs” [PDF]. The braking phase was managed by P63 which was engaged at 50,000 feet altitude at 5575 ft/sec velocity relative to the surface reduced the velocity to around 500 ft/sec at 7800 feet altitude. This was completely automatic, although astronauts could intervene in case of emergency. At this point P64 took over and continued the descent automatically.
The final phase of landing could be performed by three alternative programs: P65, which was fully automatic, P66 which allowed steering by the astronauts but controlled the descent rate throttling automatically, and P67 which was fully manual. In practice, P66 was used for actual landings on the Moon, since it allowed visual selection of the landing site to avoid obstacles and sloping terrain.
One of the main reasons the LLRV/LLTV was used in training is that the relationship between tilt of the thrust vector and lateral movement depends upon gravity, and astronauts flying the lunar module needed to develop the instinct for maneuvering in that environment.
What do you believe was the primary technical barrier to SpaceX style reusable launch vehicles starting in the late 1960s?
I don’t know. I believe SpaceX uses GPS to navigate the boosters to the landing site, and that wouldn’t be available in full-time global coverage until December 1993. But there are probably other navigational systems such as TACAN (which the Space Shuttle used before switching to GPS) which were available in the 1960s.
As far as throttleable rocket engines and control systems, the DC-X used modified RL-10s and demonstrated vertical landing in 1994, but the RL-10 had been around since 1963, and I don’t think any of the modifications made for the DC-X required technologies unavailable in the late 60s.
I think it was more a lack of imagination or perceived need. Launch cadences were so slow that the break even point for reusability would be sufficiently distant that it would be hard to justify the development cost. Also, the culture of the time probably couldn’t handle the number of photogenic failures it took SpaceX to get it right. After the fourth or fifth kaboom, everybody involved would be hauled before Congress and have their budgets zeroed out. In fact, I wonder if SpaceX could have managed it if they had been a U.S. publicly traded company.
Back when NASA envisioned aggressive follow-ons to Apollo, there were a number of proposals to reuse the first stage of the Saturn V, including the wacky scheme to catch the stage in mid-air with a giant helicopter.
I think it was more that JFK lacked the wisdom to see he was being conned into running a commie space program by the commies. The history of prize awards for incentives had kept the US in the lead not only in aviation (Kelly Act and Orteig) but ground transport (Guggenheim).
As one would expect of a commie space program.
However, would Congress be all that eager to reign in private entrepreneurs like Hughes, who had crashed in his own plane, or any of a variety of others likely to be chomping at the bit to grab a series of prize awards for various milestones? Nowadays, yes the School Marms and Biddies would be all over it and probably in cahoots with military contractors used to cost-plus… of course. But in a perceived cold war contest between capitalism and communism while the WW II veterans were taking charge of everything?
Not on your life!