How many more billions down the Mars rathole? It is, like LEO, a complete dead end. The snake oil salesmen calling it the next home of humankind are promoting the absurd. Any naked mole rat/humans living in tunnels under Mars would dream of one thing; going back to Earth. Except living for years in a low gravity environment likely guarantees going back to Earth would be a miserable return due to permanent damage from debilitation.
This is why Gerard K. O’Neill’s people ruled out any natural bodies other than Earth for colonization from the start: we evolved in 1G and will only thrive in that amount of gravity. Ironically, it is the very low gravity bodies like Ceres that would allow centrifuge type “sleeper trains” to be constructed as a remedy for very small populations of workers or scientists on multi-year tours. I would guess that both Mars and the Moon have too much gravity to make the sleeper train concept practical. In the case of the Moon, periodic rehabilitation by going up to Low Lunar Orbit 1G space stations would likely be the best solution. Because of the much deeper Martian gravity well the rehab station does not seem to be an option. The more gravity the more difficult and impractical subsurface centrifuge constructs become. See how that works?
At 220,300 pounds of thrust Prometheus has the same problem as other recent rocket engines like the Raptor and BE-4: it is about 1/8 the size appropriate for Human Space Flight Beyond Earth Orbit (HSF-BEO) applications. The benchmark engine being the 1.8 million pound thrust F-1B proposed in 2012.
For reuse the most efficient configuration is probably a single turbopump supplying two approximately 2 million pound thrust bells, with two such engines (4 bells) providing approximately 8 million pounds of thrust and a center engine with variable thrust appropriate to landing back the stage.
This configuration would be much like a Saturn V first stage able to land back. Unfortunately the Saturn V was actually only just powerful enough to accomplish the Moon mission using Lunar Orbit Rendezvous (LOR). The preceding Nova rocket was much more powerful and in hindsight it would have been better to use this original design. We went cheap.
The best path is an upper stage double hull wet workshop enabling a cosmic ray water shield (filled with lunar water) and tether generated artificial gravity system as a basic crew compartment. This “fat workshop” would necessarily be large and possibly double the 33 feet of the Saturn V lower stages. A reusable launch vehicle powerful enough to lift such a mass would be the ideal workhorse Super Heavy Lift Vehicle enabling a cislunar infrastructure. The problem with the SHLV concept in a reusable form has always been the great mass and volume of tankage attached to the engines that has to be recovered. Tankage need not necessarily be reused.
A first stage with 3 turbopumps and 5 bells discussed previously is likely the most efficient arrangement but as noted the largest bell now readily available is the proposed 1.8 million pound thrust F-1B. To send a 60 foot diameter fat workshop to the Moon with this first stage 3 turbopump/5 bell engine configuration triple or more the amount of thrust per bell will likely be needed.
I would speculate on an optimum design by considering the Space Shuttle. The best feature of the Space Transportation System was the basic concept of expending a tank instead of various stages and engines. The rest of the STS, of course, was not so well conceived. Taking away the singular lesson it may be that for the rest of this century the way to get the greatest amount off-world is to accept that sacrificing tankage to the rocket equation is the only practical path.
The way this would work is the first stage would land back an engine module to a ship instead of the entire stage. The tank would separate and parachute into the ocean. Whether parts or the entire tank is salvaged depends, and the same for the second stage. The point being engine modules with feeder tanks and landing gear allowing recovery, even the third stage engine separating from the fat workshop and doing a free return all the way around the Moon and back (it’s free), seems practical. The Rocky Jones space ranger Shiny Starship in my view is not practical at all.
“Lunar Pathfinder is planned to adopt a highly-stable ‘frozen’ elliptical orbit, focused on the lunar south pole – a leading target for future expeditions.”
Unstable lunar orbits, the lack of water, and reluctance to spend money on space, were the 3 main reasons the 1st space age ended in 1972 with Apollo 17.
Frozen lunar orbits, polar ice, and Space Solar Power enabled by lunar resources now make the Moon the place to go to solve the climate crisis. Trillions of dollars now headed to the DOD for legacy cold war toys could be redirected at a “Green New Space Deal” and start the second space age. The main obstacle to this bright future is NewSpace; the worst thing that has ever happened to space exploration.
A half a dozen Trumpist/Musk worshipers squat on this and dozens on other space forums and while they are not trumpeting his imminent return they believe in his fascist agenda to overthrow the government and are unapologetic. They also almost all believe in dismantling the Space Agency, and generally all government, and letting the private sector “do it better.” This Ayn-Rand-in-Space libertarian ideology has been the worst thing that has ever happened to space exploration. It has set space exploration back at least a decade and the damage is accumulating. Not to mention the corrosive effect Neoliberalism has had on the entire human race since the Reagan Revolution.
You go ahead and “move on” buddy….but don’t tell me to let the sociopaths worshiping money as the god of this world burn it all down. I am not in that club of the damned. Your fundamental mistake is using the term “we.” They represent a worldview blind to any collective or utilitarian activity. Obscene greed and Orwellian manipulation drives their behavior. The only given is they lie like the rest of “us” breathe. Obviously.
The only practical propulsion system for Human Space Flight Beyond Earth and Lunar Orbit (HSF-BELO) is Nuclear Pulse. Bombs. It has been that way since Ted Taylor and Freeman Dyson validated the concept in the late 1950’s. Probably going to stay that way for many decades to come. The requirement for a massive cosmic ray water shield, at a bare minimum well over a thousand tons for a small crew, makes Nuclear Pulse Propulsion the only path. Truth.
With it’s first successful flight in 1957, variations of the R-7 have been flying for over 60 years. If we had continued launching Saturn V’s to the Moon instead of going for the cold war toy money machine we would have launched approximately (at 6 a year since 1972) one hundred and eighty lunar missions. If 150 had been one way cargo landings around 4000 tons of construction equipment and supplies for a lunar base could have enabled a permanent Moonbase and it would be operating right now. If lunar lava tubes were found and utilized then workers in underground habitats several times the size of sports arenas might be establishing an industrial infrastructure…right now.
And those Saturn V’s would of course have been modified for greater reusability at some point and continually refined. What could have been, and what could be. With a Green New Space Deal utilizing lunar resources to enable Space Solar Power as the solution to climate change trillions of dollars would be pointed at expanding humankind into the solar system.
The only-for-profit NewSpace scam is the biggest obstacle right now.
Weaponizing Earth orbit and smallsat constellations are a very bad combination. The way to avoid this escalation and proliferation is a space version of a Strategic Arms Limitation Treaty that outlaws anti-satellite weapons and severely limits the number of satellites allowed in Earth orbit.
The best path is large human-crewed GEO platforms that serve much the same purpose as foreign embassies. Humans hovering 22,236 miles above their national borders would be a stabilizing factor as action against them would be far less likely.
The problem with humans performing long duration missions Beyond Low Earth Orbit (BLEO) is radiation of course. Dosing and debilitation are the elephant in the room nobody will address. The massive cosmic ray water shield and structure required for tether generated artificial gravity systems make lunar water and wet workshops the only practical options. It is verboten to discuss largely because NewSpace has damned any voice over the last decade stating humans working in space require a Near Sea Level Radiation 1 Gravity environment (NSLR1G). The reason being that NewSpace above all other things screams cheap as their first dogmatic precept.
Unfortunately, there is no cheap.