“-two short SPS burns, called Descent Orbit Insertions (DOIs), necessary to lower the orbit of the docked spacecraft so that Challenger could carry more landed payload than otherwise possible.”

The AJ-10 hypergolic engine first flew in 1958 and has been used on Apollo, the Shuttle, and now Orion (de-rated to about a third of the power of the Apollo version). It is hypergolic and pressure-fed but unlike the Lunar Lander engines it features regenerative cooling and at over 20,000 pounds of thrust was about twice as powerful as the Lunar Lander descent engine.

Again, these hypergolic engines do not require troublesome cryogenic propellant storage or turbopumps which make them an order of magnitude simpler and of course more reliable. This allowed for the multiple firings without worrying about spinning up turbopumps or maintaining cryogenic propellants. When considering the type of engine for a future lander there is a list going from the simplest, which would be the Apollo ascent engine to the most complex, an example of which would be the BE-3 hydrogen and oxygen (cryogenic propellants), turbopump-fed regeneratively cooled design.

One compromise between the two poles of engine design might be methane propellant as it is easier to store and transfer than hydrogen. Lunar ice will likely have volatiles trapped in it that allow for methane production. Modifying the BE-3 to burn methane may or may not require extreme modification of the design. Another design feature worth considering might be a pair of smaller engines mounted separate from the landing arrangement and main engine allowing for a cargo payload and most of the lander to be jettisoned in an emergency- somewhat like the Apollo lunar lander.

It is great to hear the accounts of the astronauts, as well as the technicians, from those memorable times. We could all use insight from the lessons learned.

As a former helicopter aircrewman I was also interested in the autorotation training and this does relate to the whole subject of lunar landers which is discussed regularly here.

It is one thing to specify techniques like “propulsive landing” and landing back rocket stages but another thing entirely to make them a standard practice. When humans are not involved it is simply a matter of money but if it is going to be “human-rated” then going cheap is not an option.

The commercial airline industry was used as the model for the space shuttle and this was…a disaster. What is so misunderstood about airliners is their extremely conservative design and how every part of the aircraft is subject to strict quality controls and frequent maintenance, inspections, and checks. A rotary wing is a different animal and has never been easy to make money transporting people with because everything that goes for airplanes goes double or triple for helicopters. Making a profit becomes problematic when even the smaller models with half a dozen seats cost well over a thousand dollars an hour to operate.

The Apollo lunar lander could, during landing, jettison the lower stage at any time and ignite the entirely separate ascent stage as an emergency abort option. As related by Jack, there was not much that could go wrong with the pressure-fed hypergolic ascent engine. The descent stage engine was also a pressure-fed hypergolic with no regenerative cooling but being variable thrust there was more to it.

When singing the praises of the liquid hydrogen/oxygen “reusable” lunar lander that will seemingly make landing on the Moon no big deal it must be understood such a machine would be a far, far, different animal than the Apollo lander. There is no cheap.