Comment by bfe | Hacker News Reader

Comment by bfe | Hacker News Reader

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At this point, sending humans to Mars is also just an engineering problem, not a science problem.

The trip time using conventional chemical rockets is limited to modified Hohmann transfer orbits of around six to eight months each way, and the duration on the surface would be about eighteen months before the next orbital launch window between Earth and Mars. With basic well-understood radiation mitigation techniques, total radiation exposure for each astronaut over the duration of the mission is around 50 rem, less than the amount needed to increase by 1% the chances of a 35-year old adult of developing cancer at some point later in life, a lower risk than by spending the same amount of time smoking cigarettes on Earth.

Getting the astronauts back isn't prohibitively more difficult than getting them there in the first place: the energy expenditure for escaping the surface is 17% of the Earth's, and using the Mars Direct mission architecture, you can send a return vehicle ahead of time with seed hydrogen to manufacture liquid methane propellant and liquid oxygen oxidizer from the carbon dioxide air, through the Sabatier reaction for both and also with electrical dissociation of the carbon dioxide for more oxygen. You can also test it in flight after producing its fuel before deciding to launch the humans to Mars in the first place.

We still need a heavier-lift heavy lift vehicle of at least 100 to 140 tonnes to LEO to start being able to send major assets to the Mars surface in one shot, but SpaceX is becoming increasingly credible as a possible creator of such a vehicle. Putting humans on Mars within twenty years would require at least a few billion dollars a year (a small fraction of NASA's budget) and ironing out some engineering details, but is entirely doable. The biggest obstacle is just having leadership with a sustained focus on that goal over the course of the couple of decades required, and that hasn't been forthcoming from within government, but perhaps could be supplied from a private entity. SpaceX has been laying down a solid record of presenting visionary plans for spacefaring capabilities and then making them happen.

I second the first two paragraphs but I'm not sure about the third one.

I want to see a 140t to LEO rocket, too and I'm sure SpaceX is already building it.* However, is this really necessary? I think the orignal, soviet moon flight plan would have been to put several stages into LEO and combine them there. The plan was given up, but I think it should still be possible if no larger rocket is available soon. Consider, SpaceX plans to lunch 10 Flacon heavy (53t to LEO) per year at a combined cost of 800-1250 million.*

And of course, the Saturn V did already put 119t to LEO in 1967. Hence, their knowledge in building space stations like MIR. How much was bailing out those rust belt dinosaurs again?

Besides the added complexity of assembling components together in LEO, placing any mass into low Earth orbit is really inconsistent with putting it into a trans Mars trajectory and adds a giant extra delta-V penalty. It's a much different orbital inclination, plus there's the added energy loss of circularizing a trajectory into low-Earth orbit and later boosting out into a hyperbolic escape trajectory. As far as I understand it, it would be faster and cheaper to develop a true heavy lift launcher.

True, a better Saturn V would be the best. I was just trying to think about, if for whatever reasons, a better rocket would not get ready in time. But maybe what I wrote was just plain bullshit. ;-) Of course it would make more sense to send all the small rockets towards mars at the same time and 'somehow' combine them but that doesn't sound easy either.

Why bother putting it into a circular orbit? I don't see the point.

As I understand it the big drawback to EOR from an engineering standpoint is that the added complexity introduces a bunch of extra failure modes.

If you boost something up to orbital velocity and LEO altitude but don't circularize, its orbit still intersects the atmosphere, if not the earth. The delta-v required to keep from falling back down would be better used to push into whatever transfer orbit you want.

Why do you say the inclination is different? You can go into LEO at any inclination.

I also don't see why you need more delta-v to go from launch orbit to circular orbit to transfer orbit compared to the transfer orbit directly. My orbital mechanics are rusty, but it seems to me that since you need to lift the perigee from Earth's surface to Mars orbit, you'll pass the circular orbit on the way.

If you go into low Earth orbit from the Earth's surface, you need to go into an orbital inclination equal to the latitude of your launch site - which is extremely unhelpful if you want to then take that mass anywhere else in the solar system. You also need to circularize your orbit, then later boost that mass from circular orbit to hyperbolic escape orbit - also extremely wasteful.

Ok, good point about the inclination.

I still don't see why circularizing the orbit would be wasteful. Circularizing an elliptical orbit at apogee still requires positive delta-v, which will help you on the way to the hyperbolic orbit. Only if you want to circularize at perigee would you need to slow down, which would indeed be extremely wasteful. It's true that you get more energy boost for the delta-v if you do it early rather than late, but I don't know if the ellipticity of the launch orbit is high enough for that difference to qualify as "extremely" wasteful. But maybe it is.

Perhaps Don't assemble them - launch them separately and they arrive separately.

To some extend thats of course possible. Wouldn't that be funny if in the end, all crew member travel separately because its cheaper? :-D Now thats isolation! At least they would have short latency when talking to each other.

Of course, once you've build a big enough rocket, its usually cheaper than multiple small ones. But there is a market for medium size rockets like the Falcon Heavy already. Not so, for bigger rockets - yet.

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