By the way, here is an paper on lunar orbit insertion targeting that will totally fail to give you a simple answer to your question but does make for interesting reading and gives a flavor for the various considerations that go into plotting a lunar trajectory. When they talk about “worst case plane change” they’re talking to the ecliptic of the Lunar equator, not the Earth’s so it isn’t the same thing but you can see even making plane changes for different Lunar azimuths is not insignificant.
Why was this? Just curious about this detail. Was it because we wanted a certain minimum payload for the moon EVA experience (two humans, a rover, etc.), and we needed to make the ascent stage as light as possible while accommodating this … meaning, the weight difference between descent and ascent stages would be proportionally big.
In other words, once we can afford to have a larger payload land on the moon, will that automatically make the descent stage/ascent stage weight ratio smaller, and therefore “take care of” the problem you noted?
I’m not quite sure if I understand your question, but the reason that the Lunar Module (LM) had separate descent and ascent stages was to optimize payload mass for the lightest possible amount. A single stage descent/ascent vehicle would have massed much more because of the additional amount of propellant necessary to carry the mass of the descent engines and structure back to lunar orbit and rendezvous with the Apollo Command/Service Module (CSM). Understand that most of the propellant mass used on descent was used to slow the propellant still being carried, so once all of that was burned off the amount of impulse required on ascent (and therefore both thrust of the engine and propellant needed) was much less.
The original plan for a manned lunar landing was what was called a direct ascent profile; that is, instead of having a separate CSM, and LM with descent and ascent stages, the spacecraft would be all one system which would descent to the lunar surface, take off, and inject directly into Earth return orbit. This would have necessitated a much larger rocket (the Nova C8) which was estimated to mass almost twice what the Saturn V ended up weighing (and likely would have weighed more at the end of development). Although it would have offered a simpler mission profile (no repeated separation and docking operations) developing a rocket this this additional capability (and all of the facilities to handle it) would have posed a much greater overall risk.
A unified Earth-orbit-to-Lunar-surface-and-back spacecraft is possible, but not very optimal. For a permanent presence on the Moon, each element of the transportation architecture should be seperate as depicted in 2001: A Space Odyssey (although the film did depict a combined shuttle and lunar landing craft). The only way that would change is for an advance in propulsion with much higher propellant mass efficiency.
Stranger: Thank you. After I hit [Post] I realized the issue is not merely the ~5 degrees between Earth orbit and Moon orbit planes. It’s also the ~23 degrees between the Earth’s orbital plane & equatorial plane. As you pointed out in your first sentence. That’s a big angle to manage.
The pdf was informative if a bit over my head. Reading between the lines it’s plain how very hard they’re having to work to minimize impulse/delta V for at least that phase of the mission. Pretty clearly the whole thing is hanging by a thread performance-wise.
Thanks, Stranger. Informative post, but I’m still not sure what you meant by “leaving 80% behind on the lunar surface.” This is that piece consisting of the four landing legs plus, more importantly, the now-empty descent tank, as you described. You explained well why this method was chosen, and I now understand that the necessary improvement to avoid this would a much larger vehicle lifting off from Earth.
I guess the only part I’m still not sure about is why that (a single descent-ascent stage) would be inherently better, as you implied. You mentioned “simpler” – no more mid-course docking maneuvers – but is there something else that would make this a preferable method? Perhaps you were just making a passing comment on the (minor, I think) waste of leaving behind that hardware. Thanks.
For the sake of argument, how often do you check up on Mars probes? What are they doing right now? What have they learned?
Sending people to other planets and moons is gripping stuff, and an astronaut can “learn” more in a couple hours of tooling around in a buggy than a probe can in years, slowly creeping along at a centimeter per day or whatever it is.
The Lunar Module consisted of two separate stages; the descent stage, which contains not only tankage but the higher performance and highly throttleable TRW Lunar Module Descent Engine, landing gear and structure, batteries for the extended surface duration with a total wet mass of ~11.6 tons plus the Lunar Roving Vehicle (LRV) and Scientific Instrument Module (SIM) for the later J-class missions, and various consumables; and an ascent stage with a wet mass of around 4.7 tons. The dry mass of the ascent module is about 2.2 tons, including the habitat, the minimal consumables needed for ascent and docking, the lower performance but restartable and highly reliable Bell Lunar Module Ascent Engine which was qualified for multiple restarts.
I got my “~80%” figure from comparing dry masses of the ascent stage and total vehicle, but it should actually include the mass of the propellant used on initial ascent as well. Technically, we could real compare the separate and combined dry mass of the two modules, which are about equal, so the descent/ascent split is closer at the point of initial ascent to 40%/60%, but regardless, it is still a significant amount of mass to carry back up to no purpose as the ascent stage is jettisoned en route back to Earth and burns up in the atmosphere.
This is only true if you are comparing a probe to a person working in a shirtsleeve environment. However, in an extraterrestrial environment an enormous amount of effort has to be put to just getting an astronaut to the surface and keeping him alive, with the penalty of having to operate through bulky gloves and a clunky environment suit, with the range that an astronaut can travel limited by needing to return to a habitat every few hours and ultimately having to be returned back to Earth. Human astronauts also require often awkward human machine interfaces to equipment that aren’t required with instrumentation mounted on an probe. [POST=14496683]Here[/POST] is a comparison I made a few years ago between the capabilities and liabilities of a human explorer versus a rover which delineates the various considerations.
Sending people may be initially “gripping”, but note that by the third lunar mission it took a nearly catastrophic failure to even get more than cursory news coverage and any wide public interest. Most people could give fuck all about space exploration unless it gives them some direct benefit, and even then they tend to dismiss it as “unnecessary”, as if GPS services could somehow be provided without satellites.
Working in a spacesuit for a couple hours might be strenuous, but that’s what they get paid the big bucks for. It’s a good thing robotic probes are a lot cheaper, or we wouldn’t bother with even that.
Astronauts augmented with robots is the way to go, it seems to me. I’m all for sending robots first, but the goal is sending people. The coolest thing about Mars Discovery was getting there and OMG that landing! After that NASA has had to resort to sending back robot selfies with the robot pretending to be a person. “Hi, this is me on Mars opening my solar panels!” What we want, is Chris Hadfield on Mars sending back selfies. “Hi, this is me moving 5 billion years of rocks and sticking my shovel down there.”
That only happened twice, on episode 9, where they just practiced docking maneuvers in Earth orbit, and on the Tom Hanks mission, where they needed its air and juice for the return. Most of the time, the LM ascent module was more of a mass liability than they wanted to haul back down, so it ended up in the lunar junkyard.
Robots may be cheaper. But what happens when they quit working and their is nobody around to repair them? What happens if they fall over a cliff or in some other way get stuck? For example, I hear the current Mars rover has 2-3 wheels not working. If there was a human around those could be easily repaired and the mission could be extended.
What I see as the best solution is a kind of mixed human-robot mission. Where you have 2-3 humans going to Mars, but bring along 10 or more robots or even flying probes which could then cover way more surface area and the humans would just be there for repair, troubleshoot, or to check out points of interest the robots find.
Well, it’s pretty amazing we can keep our robots on Mars operating as long as they do. No way we could keep humans alive on Mars for years. Not yet anyway. With regard to the OP, I’m not sure I agree Mars will be the last planet humans ever set foot on. We may never put humans on Mars, alive at least. But if we do, Mercury or Ceres don’t seem out of the question at that point. It’ll take a lot to land humans on Mars and keep them alive. It’s a lot more than a quick sprint to the moon and back.
“Viking Grand Tour?” Did you mean “Voyager Grand Tour?” And when you say it couldn’t be done today, I’m guessing you mean that a crewed Grand Tour couldn’t be done (plus, don’t know how often the alignments make a Grand Tour feasible).
When you think about it, a moon shot could be a practical possibility today. We could launch a Lunar Explorer craft and dock it with the ISS, then launch a modular fuel package that the moonship would rendezvous with after leaving ISS. After the moon landing, the moonship would return to ISS, where the crew (of 2) would wait for a ride back to earth.
The mass of such a setup would be much less than with Apollo, because the moonship would not have to handle atmospheric re-entry, and the smaller crew would afford more room for payload. If the fuel package included the landing platform (legs), the moonship itself might be able to make more than one trip, remaining docked to ISS between them.
I stand corrected. You are right; the ascent stages were separated before trans-Earth injection and commanded to impact the Moon.
What happens if a human astronaut gets injured, or becomes ill or fatigued, or suffers a failure of life support? And having a human available to maintain a rover like the MSL would mean that they, too, would have to have some means of a portable habitat. The typical uncrewed rover or interplanetary probe mission end-to-end cost is ~2-3 billion US dollars. (MSL cost ~US$2.6B.) The estimated cost for a minimalistic, high risk (~90% chance of success) crewed mission to Mars is widely estimated at ~US$200B, a cost of which could easily bear 70-100 robotic missions (more, actually, becomes of the economies of scale that would come of missions). A more realistic cost for a crewed mission of six is around US$500B; for that cost, we could pepper the surface of Mars with enough probes to explore a vastly greater area than any single crewed mission could probably cover, and in more depth and greater duration, and most importantly, without having to recover the rovers in the end.
In fact, a comparison of ultimate cost/benefit analysis of prior robotic versus crewed missions on duration alone a [POST=14031846]gives a ratio of between 50:1 and 100:1[/POST], as I detail in the linked thread. This is even absent of a comparison between the science value, on which robotic missions have provided vastly more than crewed missions where the bulk of the effort is in keeping the crew alive in transit and in-between brief work periods. More robust capable robotic systems are being developed which will be able to work with greater autonomy and perform even more scientific tasks with less direct instruction and oversight.
Yes, you are correct. The planetary alignment needed to perform a Grand Tour-type mission is about a once in two centuries event. Nor do we have the capability today to put on a mission of that scope for what it cost in 1977. In fact, the future robotic interplanetary missions that are not already in work are being deferred or scrapped with alarming frequency, which means we’ll lose both the technical and human capability to put on such missions in the future. Given that we could potentially launch many interplanetary missions using large smallsats (6U or 12U CubeSats) at a cost of less than a single F-35, this is nothing short of shameful.
I’m sorry, but none of this is correct. The ISS is in a terrible inclination (51.6° to the Earth’s equator) for a translunar injection trajectory such that there are only a few brief periods in a year when their may be a favorable conjunction. The ISS has no provisions to store or handle a significant mass of volatile propellants or perform fueling operations, and anything but cryogenic fuels (LOX and LH[SUB]2[/SUB] would pose a contamination risk to instrumentation on the ISS).
Upon return, the Apollo capsule relied upon aerodynamic braking in order to slow it down without having to carry extra propellants. At the point at which it begins interacting with the atmosphere (called “Entry Interface”) it is actually moving at a speed in excess of 3 km/s with respect to Earth, which is very fast (about three times as fast as a .308 Winchester rifle bullet), but also less than half the orbital speed of the ISS (~7.7 km/s). Even if the capsule could be accelerated to the speed of the ISS, the discussion above regarding the limited number of suitable orbital conjunctions applies equally here, save that the required precision to achieve an intercept with the ISS would be a couple of orders of magnitude higher than an arbitrary lunar orbit.
If such a scheme were plausible it would already have been done, and in fact there are a number of conceptual studies for using the ISS or an ISS-like platform as part of a translunar transportation infrastructure, all of which pretty much conclude that there is minimal advantage to this and a lot of negatives. In short, this isn’t a workable proposal for a “cheap” Lunar mission capability.