The Space Thread

The problem with assembling it in LEO and then flying it to cislunar space is that it would need 1) engines beefy enough to do that, rather than just stationkeeping, and 2) enough structural strength to handle the kind of acceleration that that would entail. Both make things a lot more complicated.

Really from Jupiter, but not an alien.

Uhm.. There is no difference. Well, okay, you have to counter the extremely minute amount of drag you get from the occasional air molecule you encounter in LEO, but the total delta v to deposit a given assemblage in lunar orbit is the same regardless if you do it in one go or in pieces. Actually, no, I lie. As a practical matter, you will need less, because you will bloody well inevitably run into extra mass overhead having separate stagings for pushing separate bits to the moon.

Note that you can use a very small engine to move quite a large station from earth to lunar orbit - you just keep burning at periapsis every orbit until apoapsis reaches lunar orbit, then do a (lunar) gravity assisted insertion burn when things line up. The only thing a bigger or more engines get you is a worse mass ratio, and fewer burns. (Fewer burns does mean you get there faster. But there is no need for the station to be manned for any of this, so... who cares?)

Engine "Beefyness" does not mean very much unless you need to do one-pass gravity assists, or land in gravity wells. Which you do not for moving around in the earth lunar system.

This post brought to you by kerbal space program.

edited 27th Sep '17 1:55:49 PM by Izeinsummer

Actually there is a difference. Even ignoring possible drag from the upper fringes of the atmosphere the fact is that LEO assembly is going to be harder to launch because it's going to put stress on the joints between individual modules and the ones running perpendicular to the direction of thrust are going to experience a lot of load and angular momentum (especially the further away they are from the thrust vector).

These means that you would either risk the connections between modules snapping or multiple engines to spread the thrust more evenly across the structure. And unlike Kerbal Space Program you can't just slave those engines together to a single throttle. They have to be controlled independently to compensate for any slight variations in thrust that might crop up. Which might be fine in low orbit but the higher they get the longer the command and control loop becomes.

Assembly in Lunar orbit basically means that you're only dealing with a single engine and attached manoeuvring thrusters at one time and you don't have to worry about torque on the connections between modules.

Right. The problem isn't the delta v required, it's the structural integrity of the station, which will never be required again after the maiden flight. It's a waste of money. Use transports that can be reused for another mission, much more efficient.

edited 27th Sep '17 6:22:49 PM by DeMarquis

"We learn from history that we do not learn from history."

... What structural integrity ? Again, there is no need to trust hard enough to put a significant load on anything. We have put satellites into moon orbit using ion drives. So one small rocket will do fine. Heck, the lazy way to do this is to just load the station down with fuel and use whatever station-keeping thrusters it by necessity has to have anyway, as long as those have decent isp.

Not sure whether you'd get there in a reasonable amount of time with low thrust. t=(l2/a)^1/2 still applies.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman

Station keeping thrusters are generally designed for relatively short busts. Using them to go from LEO to Lunar orbit would probably burn them out even if they did have enough fuel.

And if you're using something like ion drives, then it might be possible but using chemical rockets to launch individual modules and assembling them in lunar orbit would probably be faster.

as a practical matter, "The flight from LEO to the moon ends up taking half a year" is a completely worth while sacrifice to make to not have to try to assemble a space station in lunar orbit..

..... You are just quibbling to quibble now. Of course you would need somewhat better thrusters than normal for this. The point is that moving large objects in space is not, in fact, all that difficult.

Of course, you are going to need to do some rendevousing at the moon, because, well, you cant really have a crew while you slowly raise orbit in this manner. They would literally eat your mass budget.

edited 28th Sep '17 1:54:10 AM by Izeinsummer

High thrust, low duration burns are more efficient in terms of propellant used, which is why they're SOP for most things. Ion drives are an exception to that rule, but ion drives also have pathetically low thrust — trying to move a human-habitable space station with ion drives would be a losing proposition.

Besides, half the point of building a station in lunar orbit is to practice building things outside of LEO, so we have that experience to draw on when we're building things in Martian or Venusian orbit.

Really from Jupiter, but not an alien.

Higher trust is more efficient when taking of or landing in gravity wells - because it lets you spend less of your total delta v on "not falling to the ground" and when using Obert effects during single passes - that is, when you are flying past a planet going way above escape velocity, and using it to change course or velocity - because when you are doing this, more trust lets you do more of your burn at the closest approach. None of this applies to a station being moved from earth to moon orbit. For this operation, you can keep making the orbit more and more elliptical by relatively doing short burns only at its lowest point, which is where you exploit Obert optimally

While doing this, essentially the only aspect of the engine that matters at all is isp. And as a distant second, how much it weighs.

Trust is only a consideration in as far as your assemblage needs enough to do an injection burn when it reaches lunar orbit.. but that is a very modest delta v change, which you can carry out over a shockingly long burn - the moon just is not going to be moving very quickly relative to an intersecting elliptical orbit. So, your station really, really does not need structural reinforcement to handle that - you would notice it happening if you were aboard, as things would very gently drift to walls or floors, but it is not going to strain any assembly which you would trust to hold in use.

None of this is difficult, as space operations go. You know what is difficult ? Assembling a space station. You know what would be nigh-bloody impossible? Doing it in moon orbit. Because now you need to supply the people doing the work rather bloody far away.

So you build the thing in LEO where we have assembled space stations before. Then you attach a small engine and a bunch of.. .. not so small fuel tanks and move the whole thing.

edited 28th Sep '17 8:37:48 AM by Izeinsummer

I know I'm late, but the flat Earth idiots, if they refuse to acknowledge that the Earth is a sphere, something humans have KNOWN for a FACT since the ancient Greeks, then what exactly do they think the Earth looks like? Do they think it's a flat disc carried through space on the backs of four elephants standing on the back of a turtle?

X2, X3 Makes a fair point though, if we're just writing off assembling in lunar orbit as impossible how the hell are we going to assemble in Martian orbit?

A lunar orbit assembly could be made easier by doing the main chunk with a team on site, but not as part of the station for safety reasons. So you send the initial 4-5 modules and a crewed lunar orbiter all at once (well you probably stagger their launch into earth orbit, but you move then from earth orbit to lunar orbit together), the lunar orbiter crew can then in real time manage the assaembly, and if anything goes wrong they're safe in their orbiter.

"And the Bunny nails it!" ~ Gabrael "If the UN can get through a day without everyone strangling everyone else so can we." ~ Cyran

We are not going to assemble things in martian orbit, is how. At most one might send ahead, well, tankage. Fuel, water, beans. Nothing that would involve doing anything worth the name "construction" should be done beyond LEO. You build the craft near earth because that way you have actual recourse when things go wrong, instead of "And then the astronauts all died, while umpteen millions of voters watched".

edited 28th Sep '17 1:28:32 PM by Izeinsummer

Not to mention the fact that realistically, nothing permanently manned is ever going to be in martian or venusian orbit because it won't do anything towards increasing any politician's chances of reelection, ergo they'll never fund it.

... Honestly, its not even that. Even an extremely expansive space presence is unlikely to faf around with building things at that remove. Earth (and near earth space) is going to be the place to do that more or less up until the point where we disassemble the place to build culture style orbitals. Its where all the infrastructure and all the people are, after all.

—- Also, there is the political control aspect. A lot of people who have read far to much carnography, have this strange fantasy about reenacting the american war of independence in space.

You know who is not going to be down with that? People who live on Earth. And it is very simple to avoid anyone ever following through on that bloody silly idea by, you know, not building any shipyards in the belt. Because why? Its not going to be cheaper than building it in the earth space elevator + ring structure....

Heck, given that the moon is a harsh mistress is a book which everyone has read, regardless of its merit, you know what I predict the outcome of any "penal colony revolt" in space, or similar would be? "And then all the airlocks swung open, because their hardware had zero-levelth exploits built in".

edited 28th Sep '17 3:04:42 PM by Izeinsummer

Well, looking that far ahead is pure speculation. The point of building things outside of LEO is to utilize resources in place, and not to have to deal with Earth's gravity well. If we can build ships in high Jovian orbit, then people who live there will greatly prefer it. Of course, someone has to live there first, so there's that.

Even a slow burn is going to impose types of stresses that simply holding a stable orbit will not. It's not so much that support structures will experience greater stress, it's that different parts will. Off the shelf components call for using inflatable kevlar for some manned modules, thin aluminum plates are used for others, some support structures will be optimized for compression but not torsion, etc. Moving a large heavy object in a particular direction requires that every part be designed to handle the unique stresses of that maneuver. So building something to both maintain a stable orbit above the moon, and also transit from LEO to the moon, is going to involve additional engineering solutions, and that adds expense, not to mention weight.

So the question resolves to "is the cost of the additional engineering requirements less than or greater than the cost of building additional transport craft?" If the transports are reusable, I'm betting on the latter.

"We learn from history that we do not learn from history."

... Transport craft. So now, you want to transport more extra mass to the moon and back repeatedly to avoid the mass requirements of building a space station to a sane degree of solidity in the first place. You do see the logical flaw in this, right?

Space stations cannot be soap bubbles. That is not a viable design. They need to be able to accelerate in arbitrary directions, both for station-keeping, and also so that the entire thing does not shatter like glass because a leak sprung in the wrong wall. This means any space-station which is a viable habitat at all can also have an engine slapped on and be moved.

I really doubt thats true, but Im not an engineer. I do know that nobody throws money away. If "soapbubble" engineering can be shown to be more cost effective over time, then that's what they will use. The logic behind using reusable transports is the same as that of reusable today- they pay themselves off in the long run.

"We learn from history that we do not learn from history."

I don't think the pessimism about the future of space travel by @Izeinsummer is entirely warranted; if we had cheap access to space via any of the various proposed non-rocket launch/launch assist systems, there'd probably already be outposts on the moon, Mars and so on. Such systems would offer significant strategic benefits to whichever polity reached that point first, which is more than enough of a reason for those sorts of space infrastructure projects to be pursued.

If fusion power is ever perfected, the sheer energy costs of space travel become a lot less daunting, and at that point the costs of large scale colonization become small enough hat the relatively modest (in the short term) benefit of redundancy in case of some cataclysm here on Earth is reason enough for such efforts to proceed.

edited 2nd Oct '17 5:31:26 AM by CaptainCapsase

It is not pessimism about space travel. I fully expect a lot of it.

I just also fully expect any part of it which may be called "industrial" to be centered on earth, because of the logistics. I am not saying there wont be a space station in moon or mars orbit - I am saying it got built in near earth orbit.

At first that's probably going to be the case, but after a certain point it becomes more economic to manufacture stuff in situ than to ship it from Earth.

Monster planet found orbiting dwarf star: 'surprised' astronomers

Look with century eyes... With our backs to the arch And the wreck of our kind We will stare straight ahead For the rest of our lives

I was going to ask what the smallest star we know of is, but then I found this article.

Dick Gordon, Command Module Pilot of Apollo 12, died on November 6th.

"Yup. That tasted purple."

Bumping this up, since ʻOumuamua has been recently confirmed to be an extrasolar object - the first object in the solar system confirmed to have come into it from the outside.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman