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Mars trips may involve less radiation exposure than previously thought:
There's no question that the first human mission to Mars will be extremely dangerous. Some studies have suggested that the radiation levels would exceed the maximum career dose for a given astronaut, greatly increasing the risk of cancer and other illnesses. It might not be quite so bad as it sounds, though. Newly presented ESA ExoMars orbiter data indicates that astronauts would receive "at least" 60 percent of their maximum recommended career radiation exposure on a round trip to Mars that takes six months both ways. That's still several times what ISS crew members receive, but it's relatively gentle compared to what some had feared.
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Mars Trips May Involve Less Radiation Exposure Than Previously Thought
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your next problem.
(Score: 2) by fyngyrz on Thursday September 20 2018, @04:13PM (4 children)
Try looking at it the other way around: do it like military tankers do for fighter planes. The tanker matches velocity with the fighter. Likewise, the space tanker would match velocity with the mars-bound craft.
What this does is eliminates the need for the mars-bound craft to carry and accelerate extra fuel; that's done by the tanker, a special-purpose craft that doesn't have to carry much of anything except fuel. In turn, that means the mars-bound craft can carry more stuff that is relevant to its task - passengers, supplies, etc.
Another thing: fuel for takeoff and fuel for landing can be supplied at the trip ends, rather than being carried to the ultimate orbital source and destination by the craft making the whole journey.
Another: tug/pusher roles. Ships dedicated to basically nothing but engines could temporarily attach themselves to the whole-trip craft and help it accelerate and decelerate for extended periods, all near their fuel sources. This is (essentially) what multiples stages on to-orbit rockets do. No reason it can't be used in space, with the extra bonus that recovery would be a lot easier.
There are lots of good reasons to set up such a system, and most (perhaps all) will make the craft that make the actual trips do their jobs better.
Yet another thing: some acceleration could be provided at the starting point in space by (relatively) fixed position acceleration devices, basically something that could add a few gees for a limited amount of time. As such a thing would be essentially stationary (in orbit), refueling it wouldn't pose the same degree of challenge.
Might even be possible to implement the reverse at the destination orbital terminus: a "catcher" that can apply several gees of deceleration for a short period of time. Every little bit helps. Again, such systems would be (relatively) fixed in orbit, and could be refueled without much ado.
This is all predicated on the idea that there will be ample fuel, but reducing the costs of accelerating it for the whole-trip craft is still a good idea.
(Score: 2) by Immerman on Friday September 21 2018, @12:10AM (3 children)
Midflight fuel tankers still have the problem that you're accelerating all that fuel up to speed regardless - so why not just include it with the original ship and avoid accelerating the additional mass of a second ship up to the same speed? (not to mention presumably then stopping the tanker and returning to the depot). If you're looking to make the trip dramatically faster than a Hohmann transfer orbit, then you're probably only going to have lost a small percentage of speed by the time you pass the fuel depot, so the deltaV advantage will be minimal.
Not to mention you have to get that fuel out to the depot in the first place, from its origins at Earth or Mars (or at least their moons). The asteroid belt is even further away, so you can't make it in place unless you first move an entire asteroid into place, which is even more fuel intensive. Though I suppose that's not terribly relevant if speed on a particular flight is your primary concern. You'd need a whole solar-orbit ring of such depots though, unless you're willing to severely limit your launch windows to the times when all three destinations are in alignment
Certainly you want orbital fuel depots at either end - in fact you don't necessarily want to land in the same ship that you fly between planets in - the design criteria for the two tasks are rather different. Planetary shuttles for loading/unloading, and then more space-optimized ships between planets (nuclear powered ion drives would be *far* faster than chemical rockets between planets, but useless, dangerous mass during landing and takeoff.)
"Pusher ships" near the ends make sense as well, especially for conventional rockets - as you say, every little bit helps, and the rockets can return to service the next one easily enough.
Some sort of "stationary" launch platform makes sense as well. A giant "spinning cable" launcher in orbit would likely be the simplest solution - you could spin it up to speed over a long period (ion drives?) then dock with the center and slide out along the cable for a substantial boost. Heck, launch the "tugboats" as well for even more boost - At that point even they would likely be going considerably faster than needed make the whole journey as well, albeit more slowly than the "second stage" ship they gave an initial boost to. So you might be better off letting them make the journey and do the same job at the other end, rather than burning all that course-reversing fuel for a quicker turnaround.
(Score: 2) by fyngyrz on Friday September 21 2018, @07:21AM (2 children)
Not midflight. Early stage leaving, and late stage arrival. As to why, because if you try to carry it on the whole-trip vessel, then you also have to accelerate, and decelerate, the extra tankage, piping, etc. The tanker can do that on a short leg, and at both ends. The whole-trip vessel has more important stuff to carry than fuel, and the more of it you can dedicate to that stuff, the better. Passengers, supplies, etc. Remember: the postulated circumstance means plenty of fuel is readily available, so the trick is to figure out how to use it most efficiently for the whole-trip vessel, not in the general case.
No. None of those. Those are terrible sources. The best source is cometary bodies.
No. You get fuel moving, slowly and efficiently, from the source (which is WAY past the asteroid belt) and once you have a stream of source material coming in, that's when you get to use it efficiently.
(Score: 2) by Immerman on Friday September 21 2018, @02:00PM (1 child)
Either way you have to accelerate the fuel to the same speed as the main rocket anyway - so why accelerate it separately? What's the advantage? Unlike on Earth, there's no cost associated with distance traveled - it doesn't matter if the fuel travels a thousand miles or ten million - it takes exactly the same amount of fuel to reach speed X.
I'm picturing your scenario: tanker accelerates away from from planetary neighborhood slowly, ship accelerates away more quickly, eventually overtaking tanker as they match speed, then fuel is transferred. Is that about right?
The problem is, in that scenario the tanker has spent no less fuel getting the "transfer fuel" up to speed than the original rocket would have - and it actually spent *more*, because it accelerated the fuel, plus a whole second rocket, plus enough extra fuel to get the second rocket back home (or stopped at the far end). You would have spent less fuel getting to the same scenario by simply giving the original ship much larger tanks - the square-cube law means one bigger tank weighs less than two smaller tanks with the same capacity, and thus gets more total acceleration from the same amount of fuel.
Booster rockets might make sense because they're extra engines and thrust getting the main ship up to speed, which then depart so that the main ship doesn't have to continue to accelerating the extra engines. I'm not even certain of that though - usually boosters are used in scenarios where thrust is important or you don't want to build a bigger ship - once you're outside a gravity well thrust no longer matters much - only absolute delta-V. Doesn't make much difference whether it takes you two hours or two days to get up to interplanetary coasting speed, unless you're going so fast that an extra few days is really worth shaving off the travel time.
Of course that all changes if you're trying to get on and off a planet with the same rocket you use to travel between them - but if you're talking about having all this in-space infrastructure, then why would you do that? You don't need a rocket that can provide (and survive!) massive thrust for interplanetary travels - the large engines and strong skeleton are just wasted mass you have to accelerate. You want one that can burn its fuel as efficiently as possible, wringing every last erg of potential thrust out of it to minimize the losses to the rocket equation
>No. You get fuel moving, slowly and efficiently, from the source (which is WAY past the asteroid belt) and once you have a stream of source material coming in, that's when you get to use it efficiently.
Unfortunately there is no "slowly and efficiently" in space - Hohmann transfer orbits are as good as it gets unless you can use gravitational slingshots. But for that you need to swing past a planet with a significant speed difference. If you're trying to bring a trans-Neptunian object close enough to Neptune for a slingshot you have to accelerate it (slow it down) enough to fall all the way in to reach Neptune first. If you swing past just right, with twice Neptune's speed, you can dump almost all your orbital speed and fall almost straight toward the sun - but then you need to slow down with just as much delta-V as it would take to push the thing out to Neptune's orbit in the first place. You might be able to dump most of that speed with another slingshot around Jupiter or Mars, but the planets have to be in the right alignment to make that possible - which means you can't do it frequently - once every few years at best, possibly only once every few decades.
(Score: 2) by fyngyrz on Friday September 21 2018, @06:45PM
Yes.
No. That's a not a problem at all. Fuel is not scarce. The resource to be conserved is the amount of mass carried in the whole-trip vessel.
Think about fighter jets. Why don't they carry all the fuel required at takeoff to go as far as one might want and fight as long as one might want?
Answer: Because they need to be able to accelerate, and decelerate, well beyond what they could if they were carrying all that mass.
So: tankers. Problem a considerable way towards being solved.
Long-haul spacecraft: same issue. The more mass on the vessel, the more it takes to accelerate it and decelerate it. This matters at both ends, because it means that speeding it up is harder, and so is slowing it down.
It's not the tanker we're concerned about. That can make a traversal at the long-trip transit rate and service multiple craft in the process, thus meaning that the long-trip vessel isn't carrying anything it doesn't absolutely have to carry. Less mass means less cost to accelerate and decelerate what actually needs to get to the destination.
Sure there is. Why do you think comets fall into the inner solar system? All it takes is a nudge. If you want something to end up at a specific point x, it'll need to be a very careful nudge, but it'll get there. Eventually. And eventually is fine.