LogFAQs > #979253653

ParanoidObsessive posted...

Yeah, but fuel consumption is exponential. Which means it's far more efficient to shuttle everything up into orbit in multiple smaller trips than it would be to load everything into a single ship that has to both reach orbit and then begin a longer trip outbound. The vast majority of its fuel would be consumed on launch. You'd use less fuel with smaller shuttles or rockets launching supplies into orbit.

Well... no, you wouldn't.

I'm not sure what you're referring to with "exponential", because that could be referring to multiple different things. I'm assuming you're referring to the fact that rocket efficiency decreases with mass (since each unit of fuel needs to propel not just the rocket, but all remaining fuel as well)... except, that doesn't really apply here, because your "mass" is static. If our hypothetical Mars ship is going to weigh, say, 10,000 tonnes, then all of that mass has to get up into space; whether you do it in one trip or several, the amount of mass you have to move is going to be the same.

Now, that doesn't guarantee that doing it all in a single trip is the most effective solution either. If you do it that way, you avoid having to ferry up tools and eventual waste materiel, but you deal with less efficient tools. If you ferry up the parts piecemeal, you get more efficient fuel, but you introduce additional inefficiencies (whatever vehicle you used to get up there has to either have the capacity to land and be re-used or be disposable, both of which introduce additional costs and materiel demands). Ultimately, a lot depends on the design of the Mars ship in question - it's not a simple statement to say, "It would make more sense to build it in space," since that's not guaranteed.

ParanoidObsessive posted...

It's the same reason why the shuttles used to have booster rockets and external fuel tanks. It takes a major effort to get up there. Once you're up there, though, it takes much less effort to head outward.

As a counterpoint, though, if you're launching from Earth you already have significant velocity which, depending on your escape vector, you can then simply redirect into heading for Mars. If you're building the ship in a dock in orbit, you'll need to burn some considerable fuel to get yourself out of orbit and headed towards Mars. Not as much as a launch from Earth, granted, but it is more fuel (that will need to be carried up to space).

ParanoidObsessive posted...

It's why most proposed scenarios for long-term space programs usually involve some sort of space-based drydock or other shipyard to build and maintain ships in space without having to constantly bring them through atmosphere.

Sure, for long-term design. I was assuming we were talking about a one-off ship. If you're planning on making a full program out of this and doing it more then once, then absolutely, a stardock of some sort quickly becomes much more sensible.

ParanoidObsessive posted...

Larger structures will have more significant stress points and material fatigue risk.

So, I'm a materials engineer and I deal with fatigue on a daily basis. I can tell you quite confidently, fatigue is not going to be a dominant risk factor on these ships - not unless you're planning on using them for multiple trips (and even then, you'd presumably do maintenance on them in-between, which is what we've been doing for years with other reusable spaceships).

First off, fatigue is primarily associated with metals, as well as some ceramics. I can't claim extensive knowledge of the sorts of materials NASA is building their stuff out of, but given its material properties (lightweight, ductile, very high strength-to-weight ratio), I would not anticipate them being at elevated susceptibility to fatigue damage (many of the historical composites that NASA has helped pioneer are known for their excellent fatigue resistance, far above and beyond what we see out of metals). Now, anything can fatigue if subjected to cyclic load long enough, but then we get to our next issue: spaceflight generally doesn't induce cyclic loading; that's something we more typically see in aircraft. Helicopter rotors and airplane wings are constantly in motion, constantly being subjected to loading and unloading as they flex during flight. But in space, you're not dealing with air or gravity, so stress-cycling tends to be minimal and restricted mostly to when you're near/in a planetary atmosphere. Once you're actually out in space and moving, your stresses are relatively static and you don't have the stressors that tend to cause fatigue damage in aerial vehicles (flexing wings/rotors, gusts of air, etc.).

Not to mention, there's all sorts of things you can do from an engineering standpoint to mitigate fatigue risk, from design (e.g. avoid hard edges) to construction (molecularly bonding material components generally reduces fatigue risk).

Honestly, the biggest materials concerns I would have with space shuttle components are fretting (which has historically been an issue - the Galileo spacecraft's issue with the high-gain antenna was a result of fretting bonding part of the antenna to an abutting component), erosion (because of the sheer temperature involved, ceramics are really the only thing you can build a lot of the external portions of a spaceship out of, and they tend to have poor tribological properties; at the speeds spacecraft are travelling, the air itself can act as an erosive media, which has been a longstanding challenge for spacecraft), and delamination of composites.

ParanoidObsessive posted...

Imagine driving your car down the freeway. Now imagine your check engine light comes on. Even with the most robust built-in diagnostic system to tell you exactly what the fault is, it would be somewhat difficult for you to crawl out of the sunroof, crawl down to the hood, then open the hood and change your alternator while the car is still driving.

This isn't really an accurate comparison.

In-situ repairs of spacecraft are quite viable and we know this because we do it constantly with the International Space Station. Unlike with a car, where you have to deal with gravity, wind, and the fact that you'd presumably want someone driving the car for you, with a spacecraft - even one moving to Mars at considerable speed - you can quite happily go strolling out the airlock onto the exterior of the ship (as long as you're tethered to something), because you're moving at the same speed the ship is and there's no air resistance to slow you down. Fixing the ship mid-flight isn't generally going to be any more difficult than fixing it (or building it) in orbit around Earth, so long as you have supplies on hand. On that note:

ParanoidObsessive posted...

And that's not even getting into external damage - a few micrometeors start punching holes through vital parts and you're screwed.

Micrometeors in the space between planets are rare (they tend to cluster around things with mass, like planets). More to the point, planning for repairs is, once again, a standard part of spacecraft design and has been for a long time. To pick one example, the final flight of the Space Shuttle Columbia was supposed to last two weeks (January 16 - February 1, 2003), but it was equipped with four months of additional supplies, in case something went wrong that rendered the ship unsafe to land.