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Comment by benmorris | Hacker News Reader

Could the ISS be retrofit with boosters and sent to mars? Honest question... I don't know the physics behind this. Would the structure hold enough fuel to be able to slow down? We have shown people can live there for a year at a time.

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I believe the biggest issue is that ISS is not built to withstand the higher radiation you are exposed to in higher orbits / between planets. Also the thermal loads are different when you are in sun 24/7/365 rather than orbiting the earth and spending a significant portion of time in a shadow - though I don't know if that is better or worse.

I have asked this same question before and the consensus response was that it would be more expensive to try and retro fit the station then to build a new interplanetary vehicle. For example the joints between the compartments are probably not designed to have a rocket strapped to one compartment and fired up.

They already have a rocket strapped to one of the compartments, which they use several times a year to reboost the ISS to higher altitudes. Getting to Mars just requires boosting for longer.

It would make about as much sense as turning a fishing boat into an airplane. You're better off starting from scratch.

I think a more feasible idea would be to build a mars shuttle in parallel orbit to the ISS.

ISS is in the wrong orbit for a spacecraft to launch for Mars. The inclination is too high.

At any inclination, isn't it still moving parallel to the orbital plane halfway between the ascending and descending nodes?

No the ISS never moves parallel to Mars' orbital plane.

Years at a time, yes, but with regular resupplies from the surface...

I'd be more worried about how much force the structure can actually take - is it stiff enough to hold together if you strap a big rocket to the back and turn it on?

It can handle some degree since they do reboosting burns occasionally. But it does remind me of this incident quite a while ago.

http://www.space.com/4432-nasa-weighs-excessive-vibrations-s...

The reboosts are at hundredths of a g.

Reboosting is a capability that ISS supports. If it can handle the G force of acceleration through a reboost, it could presumably handle the same acceleration and deceleration for the longer burn time it takes to get to Mars. But in reality it's probably just more economical to send a new ship to Mars.

https://en.wikipedia.org/wiki/Reboost

ISS reboost is about 0.0185 m/s/s, says https://youtu.be/ZkdfkRC4DbA?t=121 .

Delta-V Earth/Mars is 5,748 m/s, says http://www.projectrho.com/public_html/rocket/appmissiontable... .

That's 3.5 days of boost.

The ISS mass is 419,455 kg. The minimum energy needed is 1/2 m v^2 or 7E12 J.

A Saturn V is about 1E11J, says http://www.ocean.washington.edu/courses/envir215/energynumbe... .

So if you attach a few score Saturn Vs to ISS, you might be able to pull it off.

The ISS engines might be more efficient than the F-1 was, so the mass could be less than 70 Saturn Vs. Still a ridiculously enormous amount of energy. Makes you wonder how we're ever supposed to build anything significant on or around Mars.

It seems to use UDMH and N2O4 or similar, a hypergolic mix that's convenient for maneuvering thrusters but is not particularly efficient.

That is the only option which makes sense. LOX isn't good for things which wait. Solids have low efficiency. Hypergolics are a good choice for something that has to sit idle for a very long time, and the chemistry has been understood for decades. I guess I was hoping the Russians had figured out ion thrusters in the 70s or something.

What about ion engines? I'm extremely not sure I have this math right but NEXT claims 4190 second ISP [0]. Plugging that into the rocket equation I get:

  iss_mass = 419600
  sep_isp = 4190
  delta_v = 5748
  htv_payload = 3310
  falcon_launch_cost = 62000000
  total_mass = iss_mass * math.exp(delta_v / (9.8 * sep_isp))
  fuel_load = total_mass - iss_mass
  dragon_launches = math.floor(fuel_load / htv_payload)
  total_cost = dragon_launches * falcon_launch_cost

I get:

  Total vehicle mass: 482,646.89 kg
  Fuel to add: 63,046.89 kg
  Dragon 2 launches: 19
  Total cost: $1,178,000,000.00

Not what you'd call cheap but at least in the realm of the possible. I'm assuming the ISS' solar panels (~120 kW [1]) is sufficient to power the thruster, that a dragon launch costs no more than a falcon launch (certainly false), and that the ion engine is already on board the station.

  [0] https://en.wikipedia.org/wiki/NEXT_(ion_thruster)
  [1] https://en.wikipedia.org/wiki/Electrical_system_of_the_International_Space_Station

If there's only one engine then that's 236 mN thrust according to your [1]. Lets call it 4.12 N - put a few thrusters on or improve the technology.

The station mass is 419,455 kg. F = ma so the acceleration is 1 µ G.

It will take a very long time to get to Mars that way.

About 18 years assuming I'm using the right formula:

http://www.wolframalpha.com/input/?i=acceleration+formula&ra...

AFAIK the ISS isn't very rigid and isn't designed to withstand high-g accelerations, hence the robotic arm docking, etc.

Spacecraft dock with the ISS without the robotic arm, the arm is used only with the current version of Dragon spacecraft because it's not certified for unilateral docking.

The next version of the Dragon will dock with the station by it self just like the Soyuz does.

Lets land it on the moon, and use it for spare parts when we build a Lunar Base!

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