The article goes into this in some detail. In particular:
* You have to get the propellant into space. This is going to take a large number of flights (~15) at a pace that has not been done before for a vehicle of that size (a launch every six days)
* You need to launch at pace because otherwise the propellant will boil off, which is another issue - you need to shade or insulate the propellant for a much longer period of time in much harsher conditions
* There is no gravity: whereas on earth the propellant separates relatively cleanly into liquid and gas this isn't the case in space
can you use a plunger, instead of a pump? more like a syringe?
Instead what they do is use thrust to accelerate the whole vehicle a little, which presses all the liquid into one end of its tank where it can be pumped out. Instead of carrying special settling thrusters, they originally planned to use ullage gas for this but it's not clear that can work.
deeper discussion with math: https://forum.nasaspaceflight.com/index.php?topic=60124.60
If you go for a narrower range of temperatures (ie. not structurally stable above 0C), it would need to be manufactured, transported, stored, tested and installed at seriously low temps which probably negates the possible advantage with the added technical complexity.
> You have to get the propellant into space. This is going to take a large number of flights (~15) at a pace that has not been done before for a vehicle of that size (a launch every six days)
SpaceX has done 2 Falcon 9 launches in 1 day, and they would have done 3 if the third one had not have been scrubbed [1]. I really don't think that launching Starship is going to be any different, especially as it was specifically designed for reuse, unlike Falcon 9.
> You need to launch at pace because otherwise the propellant will boil off, which is another issue - you need to shade or insulate the propellant for a much longer period of time in much harsher conditions
First part is same argument as above. Second part (shading) - again, I don't see why it is harder than other hard things. Just add more insulation. Possibly do some passive or active cooling.
> There is no gravity: whereas on earth the propellant separates relatively cleanly into liquid and gas this isn't the case in space
Very similar problem to how you feed liquid propellant into a rocket engine when it relights in zero gravity. You use a small ullage thruster for this.
[1] https://news.satnews.com/2024/03/31/spacex-enjoys-two-out-of...
> Like a lot of space technology, orbital refueling sounds simple, has never been attempted, and can’t be adequately simulated on Earth.[18] The crux of the problem is that liquid and gas phases in microgravity jumble up into a three-dimensional mess, so that even measuring the quantity of propellant in a tank becomes difficult.
And for cryogenic propellents specifically:
> Getting this plan to work requires solving a second engineering problem, how to keep cryogenic propellants cold in space. Low earth orbit is a toasty place, and without special measures, the cryogenic propellants Starship uses will quickly vent off into space.
Fuel transfer and storage in orbit is problematic in many respects.
Who's even left? Northrop? Lockmart? Adds an extra 10 years to the timeline at the most optimistic.
And what for if I may ask?
And please don't say "technological development" or "colonizing space".
ad Development): Most of the tech that needs to be developed for this, is what is commonly called space plumbing: Figuring out ways to make human bodily functions not immediately fail in space. Next to none of these technologies benefit humanity at large in any way. Also: We keep coming up with amazing new tech all the time, without the extra cost of strapping it to a human and shooting that package into orbit.
ad Colonization): There is nothing in our solar system to colonize. Period. Everything other than Earth is less hospitable than Earth would be after a thermonuclear war, by a huge margin. Terraforming another planet is practically impossible fora species that still has to count the kilos for every launch.
And as for the one goal that makes sense, which is exploration: We have a perfectly reliable form of space exploration: Robots. And they are much better at it than we are, for one simple reason: They don't require space plumbing.
There is exactly ONE reason why Apollo was manned by people instead of robots: Because computers, electronics and robotics in the 60s were not up to the task. If todays tech existed back then, I would bet the Apollo rocket would have had exactly one passenger, and that would have been the Lunar Roving vehicle.
As for why to do that, I like to think of Earth as a very cozy cave that humanity’s caveman would serve itself well to venture beyond, if only to increase the number of possibilities for the species. In a universe where there are large human civilizations not just throughout the solar system but also scattered amongst other star systems, there are numerous paths that each branch will take that Earth’s branch in its lonesome may never have trodden.
It also just seems a bit cruel to be able to see the vastness of the universe and never be able to touch any of it in person. At the risk of being dramatic, only sending rovers and probes while we remain on earth feels a bit like being stuck in a gilded cage piloting around drones and RC cars to explore what lies beyond.
No matter how much of the universe we touch it will always just be a vanishing sliver.
You can live that life if you want, plenty of people up there live off grid and only come into town once a month or so.
-48 is a hell of a thing. The most beautiful place I've ever been.
I am just replying to a single comment, so forgive me for addressing everyone else as well as you here. I think it's very funny that people are making obvious replies to my comment to defend against (the also very obvious) observation that perhaps being born and dying in a tin can on another planet might be an undesirable fate for the vast majority of the human race.
That is not going to happen, without technology that currently only exists in Science Fiction, like artificial gravity, for the simple reason that we require 1g to live, let alone thrive.
> because of immediate access to materials that’d need to be launched into orbit otherwise.
1. How does this "immediate access" benefit the aforementioned "very large, permanently spaceborne crafts", which apparently won't be moored to planetary bodies?
2. There is no "immediate access". Having rocks next to me, and having the sort of highly refined materials that go into building the tech required for spacecraft, are 2 VERY different things. But, I am always happy to be proven wrong: Let's take a very simple task, like ISRU'ing LOX & Methane, and let's do it, at scale, here on Earth, where there is no lack of energy, breathable atmosphere, building materials and labour. Strange, isn't it, that no one seems to be doing that.
> In a universe where there are large human civilizations not just throughout the solar system but also scattered amongst other star systems, there are numerous paths and discoveries that each branch will take that Earth’s branch in its lonesome may never have trodden.
I agree. But given that, what evidence supports the idea that the branch that eventually allows us to leave our solar system requires us to first waste tons of resources on trying to send people to inhospitable, irradiated rocks for no good reason?
Especially since we have a perfectly good alternative to this waste of time: Sending robots.
> It also just seems a bit cruel to be able to see the vastness of the universe and never be able to touch any of it, in person.
Unless we discover a way to do FTL travel, it doesn't matter if that feels cruel or not, it is reality.
And I can pretty much guarantee that the person discovering the means to cheat physics in such a way won't be doing so while constantly worrying about his habitats airlock malfunctioning, or the piss-regeneration system giving out, or the supply ship getting canceled in the next congressional-bickering about the budget.
It will happen here on Earth, likely by someone who never visited even LEO, someone who works and lives in a stable environment with books, people to talk to, air to breathe and delicious non-freeze dried food to eat, who never has to worry whether there will be enough recycled piss to make his next cup of coffee.
Artificial gravity is easily generated via rotation or thrust.
> 1. How does this "immediate access" benefit the aforementioned "very large, permanently spaceborne crafts", which apparently won't be moored to planetary bodies?
It will be far easier to get materials into space from the moon than from the much deeper gravity well of earth.
> I agree. But given that, what evidence supports the idea that the branch that eventually allows us to leave our solar system requires us to first waste tons of resources on trying to send people to inhospitable, irradiated rocks for no good reason?
How do you see us developing the technology for humans to leave the solar system if we never develop the technology to visit the moon?
Technology is generally driven forward by increments, and having smaller goals leading to the larger one is pretty normal. Also, you don't need to "cheat physics" to explore space.
https://space.stackexchange.com/questions/1308/why-are-there...
Sure, "easily".
> It will be far easier to get materials into space from the moon than from the much deeper gravity well of earth.
No it won't, for a very, very simple reason:
Every single kilogram of stuff you launch from the moon, has to be launched FIRST from exactly that "deeper gravity well" here on Earth. Because the Moon is shockingly devoid of any steelworks, factories, fuel refineries, Astronaut training facilities, food processing plants or any of the other myriad sources of stuff required in space.
So yeah, launching something from 1/6th of Earths gravity is easier. However, all this does, is add another launch to the equation.
> How do you see us developing the technology for humans to leave the solar system if we never develop the technology to visit the moon?
For the same reason why we developed radio transmission, without first inventing super-sonic carrier pidgeons.
Technology does not only advance incrementially. Ever so often, a radically new technology emerges, that is leaps and bounds better than existing systems, and often wasn't developed from these systems either.
And btw. Rocket Engines are just one such technology as it happens. Before them, the strongest way to propel something through the air, were propellers, a technology which we since improved by alot, but is still incapable (and never will be capable to) put things into space.
So no, doing what we have done before is not a reqirement for finding a much better way to do it.
> Also, you don't need to "cheat physics" to explore space.
Where exactly did I assume that? But you do need to cheat our current understanding of physics for FTL travel.
> Sure, "easily".
The top post of the link is talking about building a ship with a diameter of 200m. In reality you would just need a tether and counterweight. So yes, as far as new space technology goes, "easily."
> No it won't, for a very, very simple reason:
> Every single kilogram of stuff you launch from the moon, has to be launched FIRST... etc
That is the entire point of building out the moon. Sure the investment is difficult, but the longterm return makes it worthwhile. Your argument seems similar to saying "why would we build a steel foundry, when we will need steel to build it in the first place."
> How do you see us developing the technology for humans to leave the solar system if we never develop the technology to visit the moon? etc..
The technological difficulty with going to the moon is way more than just rocketry. There's life support systems, shielding, navigation, long term space habitation etc... There are literally hundred if not thousands of technologies that will need to be refined over time, and manned moon missions will go a long way to advancing them.
> But you do need to cheat our current understanding of physics for FTL travel.
My point was that you do not need ftl to travel through space.
And a ship that still maintains its course, and can still be steered when bound to such a contraption. Oh, and a tether material that can actually hold against that strain under conditions found in space reliably. The temperature differential between in- and out-of-sun would destroy most materials under such a stress. And a way to deploy the whole thing, start its rotation, and keep it stable over time.
So no, as far as any technology is concerned, this is not done "easily".
And all this effort STILL doesn't get you gravity. It gets radial acceleration over a short distance. Just imagine, for a moment, the difference in "gravity" experienced between the feet and the head of a person in such a contraption, and what that will do to their brains, skeleton, muscles, circulatory system, etc.
Oh, and: While the whole "rotating thingamabob" idea works theoretically in space, there is no practical way to use in on the surface of a low-gravity planetary body. So, what's the plan for keeping people alive against 1/6th gravity on a permanent Moon Base?
> That is the entire point of building out the moon
Building out what exactly, foundries and factories? On the moon? You know, the place where the dust alone is enough to kill almost any machinery exposed to it?
Let me ask you a question: If oil is found in antarctica, where would we build the refinery? I think we both know the answer to this one. And building machinery as comparatively simple as an oil refinery in antarctica is a cakewalk compared to building even a simple ore-smelter on the Moon.
Build a city of 100K on the northern-most habitable tip of Antarctica and have it (physically, socially, and economically) last 10 years, and I'll be convinced that we are ready to at least attempt Mars.
We have the technology as a species to be able to inhabit Antarctica; there's just no compelling reason to do so at present, so we don't.
We don't really know how much we need. I think we'd probably do just fine in 0.9g for instance, and maybe even substantially lower than that. Humans thriving in Lunar gravity isn't out of the question, we don't have data that rules out such a possibility.
But a manned outpost beyond earth would make the logistics for large scale space exploration (even with robots) much more feasible, no?
How would it do so exactly? Please give me a technical reason for this assumption.
Because, I predict it would do the exact opposite: Keeping humans alive away from Earth eats up an enormeous amount of resources all on its own. Resources that could instead go into building better robots, building more robots, building more rockets.
And this counters my argument...how exactly?
Even forgetting the fact that scientific progress isn't measured in "kilometers driven" (just count the number of experiments that Perseverance carries, and compare the amounts of data produced(, there is no technical reason a robot cannot drive as far as a vehicle carrying humans.
In fact it's the opposite: One of the most important restrictions regarding the LRVs driving distance wasn't technological in nature, it was due to the the fact it had to carry humans:
https://en.wikipedia.org/wiki/Lunar_Roving_Vehicle#Usage
An operational constraint on the use of the LRV was that the astronauts must be able to walk back to the LM if the LRV were to fail at any time during the EVA (called the "Walkback Limit"). Thus, the traverses were limited in the distance they could go at the start and at any time later in the EVA.
And even though they relaxed the constraints later on, the fact still remains: As soon as you have a human in the mix, things become more cumbersome, way more expensive, slower, less risks can be taken, and if things go wrong, the results can suddenly involve dead people instead of just trashed equipment.
Also note that even with the limitations, the humans surveyed more ground. Remove the limitation by making the rover a mobile habitat and now the humans can have an even more expansive and productive mission.
Ultimately we're going to colonize space, why take 50x the time to gather the science needed for that goal, when worst-case we can spend 50x the budget and just put humans there to incidentally also gather knowledge on how to live in space.
And yet they have put one on Mars. https://en.wikipedia.org/wiki/Perseverance_(rover)#Instrumen...
Thing is: Building something that can autonomously navigate the many many variables of city traffic without killing people in the process, is a whole different problem space than building something that can stick a scientific instrument into the ground in an empty rock-desert.
> the humans surveyed more ground
Again: Scientific progress is not measured in "kilometers driven". And what "surveying" were they doing exactly? How many experiments did they perform during these runs? How many Terabytes of Data did these excursions produce per kilometer driven?
I don't know the number tbh. but I am willing to bet that the Mars rovers did better. ALOT better.
But okay, if you want to measure distance, lets:
Perseverance (which is still active btw.) covered 25.113 km so far. The Ingenuity drone (which perseverance carried), covered a total of 17.242 km.
So that's a grand total (so far, again, Perseverance is still active) of 42.355 km.
The longest LRV drive was LVR-3 on Apollo 17: 35.89 km. And, let's be clear: That is the total of all its excursions, not a single drive.
So yeah, sorry, but the robots have also out-distanced humans already. Comfortably so.
> Ultimately we're going to colonize space
No, we're not, until such time as we figure out how to leave the solar system and travel to other Earth-like planets.
That seems unfair and unsatisfying, I know, but there is simply no way around the facts: other than Earth, every single place in the solar system that doesn't just outright kill humans the moment they leave the spacecraft (and quite a few would kill people instantly even before that), is less hospitable than Earth would be during an ice age, or after a nuclear war.
The Soviet Union did send a rover. Anyway, the science wasn't worth it and the project was driven by romantics who thought that it was the duty of mankind to explore. Putting men on the Moon was the real point of it.
Right now, with our current science and technology, sending humans on space-exploration missions, simply isn't worth it. It adds a huge pile of problems to an already difficult task, and technically speaking, we get almost nothing out of it; Robots are just better at examining rocks on other planets than we are, for the simple reason that the robot doesn't require a huge support infrastructure just to be kept alive.
And the usual argument that developing such infrastructure would, in itself, confer some future advantage, has to be viewed with a lot of scepticism; fact of the matter is, the development of space-toilets does very little to improve the day-to-day tech we use here on Earth.
Space Exploration is not comparable to any exploratory task in history, based on the sheer amount of resources and time required. These resources are finite. Allocating them correctly may not be super romantic, may not tickle the "human spirit", sure. But when things are this expensive and difficult, such considerations simply take a backseat.
And if they don't, well, then there is the very real possibility of programs running into so many problems, delays and exploding costs, that at some point governments and companies can, or will, no longer support them, meaning decades before any significant development is even tried again.
And as someone who wants space exploration to go forward as quickly and efficiently as possible, that simply doesn't seem like a very desirable outcome to me.
What a weirdly confident statement. I could imagine all kinds of technology coming from that that would benefit life on Earth.
If NASA is going to use the same playbook to be benefit space exploration, I’m not remotely upset.
That makes a lot more sense. It's still sub-optimal but not as bad as it looks at first glance.
Side-goals, fake goals and scope creep are one of the biggest red flags for “projects to avoid”.
If SpaceX and Blue Origin can't. Then Nasa will find someone who can. Cryogenic refueling is the projects real engineering target. Landing on the moon in the twenty twenties just isn't that impressive anymore.
The Artemis program is nominally about going to the moon, but it really isn't. It's about building and living in habitats beyond low orbit, in orbit refueling, building habitats on the surface of another planetary body, and obviously in the future in situ resource extraction and surface refueling.
If the mission was to land on the moon, a carbon copy of the Apollo program would do. But the mission is to prove they can do what it takes to go to and return from Mars.