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#1 2013-12-27 13:59:57

Quaoar
Member
Registered: 2013-12-13
Posts: 591

Water/CO2 NERVA for Mars

I was very impressed in reading GW Johnson's post in the VASIMR topic about a 500 s Ips water NERVA spaceship that can go everywhere in the soilar system, melt the ice, refuel and came back.

If we project this kind of ship as a bipropellant rocket with one tank for water and one tank for liquid CO2, we can have a ship that depart from LEO using water, land on Mars and get liquid CO2 from atmosphere: LCO2 has very low Isp, but it can be used to expolore Mars, moving from site to site via suborbital hopping. At the end of the mission, the ship will hop to one of the polar cap, refuel of water and return to Earth.

It may be very interesting, but to perform aerobraking and land the ship has to be compact (I imagine some kind of Dragon like 8-12 meter diameter capsule with the rocket in an openable compartment in the centre of the thermal shield, but I'm not an engenier) and I have no idea how to protect crew from radiation douring orbital transfers, when the water propellant tank is empty (the drinkable water tank may be enough to give such a shielding?), or after landing when the crew enter and exit.

It is possible to project some sort of extesible truss/boom to keep the habitat far from the engine douring travel (it may also be used for artificial gravity) that may be oriented orizontally after landing?

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#2 2013-12-28 14:23:02

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,335
Website

Re: Water/CO2 NERVA for Mars

The CO2 would have to be stored as a compressed liquid,  at several hundred psi (several 10's of atm).  That's a very heavy tank structure.  Water can be stored at a hair above 0 C at a hair above 6 mbar pressure.  That's a very light structure,  whose strength will depend more of liquid depth under acceleration than anything to do with storage pressure requirements.  Plus higher molecular weight lowers Isp in a NERVA-like engine.  Water is 18,  CO2 is 44. 

Those things being true,  I think I would look for a place on Mars to land where there is massive buried ice,  preferable not salt-contaminated.  Just mine or drill for water (which requires steam extraction up the well head pipe),  I'm not sure which extraction method is easier.  About all you need do is filter out the dirt and keep it from freezing.  Not every place on Mars will have buried glaciers,  but any base is going to need lots of fresh water.  So why not just land on one,  and put the base there?

That does mean we need some ground truth to find such buried glaciers "for sure" before we start landing components for a base,  now,  doesn't it?

I'm not sure there is a way to get effective shielding in a nuclear lander,  because it needs to be short and squat for landing stability on rough ground.  If it were a long slender shape,  the propellant and tankage structure actually maker a pretty good shield.  Trouble is,  things like that tip over way too easily upon landing. 

If you really want a nuclear lander,  you have to limit your inherently-unavoidable radiation dose by limiting the time you are aboard.  That means you have to pitch your surface habitat remote from your ship.  There's just no way around that dilemma with solid core nuke rockets.  Might be fixed with open-cycle gas core,  but we don't have those things yet.  But it does buy a very large payload fraction for a ferry design. 

GW

Last edited by GW Johnson (2013-12-28 14:29:10)


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#3 2013-12-29 04:15:48

Quaoar
Member
Registered: 2013-12-13
Posts: 591

Re: Water/CO2 NERVA for Mars

GW Johnson wrote:

I'm not sure there is a way to get effective shielding in a nuclear lander,  because it needs to be short and squat for landing stability on rough ground.  If it were a long slender shape,  the propellant and tankage structure actually maker a pretty good shield.  Trouble is,  things like that tip over way too easily upon landing. 

If you really want a nuclear lander,  you have to limit your inherently-unavoidable radiation dose by limiting the time you are aboard.  That means you have to pitch your surface habitat remote from your ship.  There's just no way around that dilemma with solid core nuke rockets.  Might be fixed with open-cycle gas core,  but we don't have those things yet.  But it does buy a very large payload fraction for a ferry design. 

GW

Douring the transfer the habitat may be detatched from the engine via a long cable for artificial gravity. After landing we can imagine a wheeled habitat that move far from from rocket platform. Leaving Mars, the habitat may return on the platform before take-off to separate again via cable after orbital inserction. So crew will be exposed only douring landing and take-off.

Last edited by Quaoar (2013-12-29 04:16:11)

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#4 2014-01-03 09:15:46

Quaoar
Member
Registered: 2013-12-13
Posts: 591

Re: Water/CO2 NERVA for Mars

GW Johnson wrote:

I'm not sure there is a way to get effective shielding in a nuclear lander,  because it needs to be short and squat for landing stability on rough ground.  If it were a long slender shape,  the propellant and tankage structure actually maker a pretty good shield.  Trouble is,  things like that tip over way too easily upon landing.   

GW

Is it possible to directly project the lander as a long slender body, with the habitat at one extremity; the ISRU and the rover bay on the other; and the water tank and the NTR in the middle, perpendicular to the long axis?

Such a vehicle will land on the thermal protected belly and take off in the same way, with the central NTR, balanced by ballast water tanks and chemical rockets on the two extremity.
Douring the transfet, the slender body will tumble for artificial gravity.

Last edited by Quaoar (2014-01-03 09:16:39)

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#5 2014-01-03 10:19:57

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,335
Website

Re: Water/CO2 NERVA for Mars

Sounds like you are talking about using the lander as the manned transit vehicle.  From a dead-head return propellant standpoint,  Apollo showed it is better not to land the return vehicle (hardware and/or propellant).  Just take down the minimum landing vehicle and surface stay supplies and equipment.  I think the habitat required for a surface stay of weeks is different from the transit habitat for years in space. 

From a radiation shielding standpoint,  there is merit to your sideways cylinder geometry with the nuke in the middle.  Depends upon the numbers whether the surface crew could stay in the vehicle cabin while on the surface.  In the vertical geometry I looked at,  they could not.  How big a surface habitat is needed depends upon both crew size and length of stay.  Different mission plans have different surface stay requirements of the landers.  Cannot generalize. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#6 2014-01-03 11:35:37

Quaoar
Member
Registered: 2013-12-13
Posts: 591

Re: Water/CO2 NERVA for Mars

Thanks GW

GW Johnson wrote:

Sounds like you are talking about using the lander as the manned transit vehicle.  From a dead-head return propellant standpoint,  Apollo showed it is better not to land the return vehicle (hardware and/or propellant).  Just take down the minimum landing vehicle and surface stay supplies and equipment.  I think the habitat required for a surface stay of weeks is different from the transit habitat for years in space.

 

My vehicle will get all the water propellant for the return trip melting a surface ice pack on Mars, so I have imagined to land on Mars the whole spaceship, but I'm only an SF writer not an engenier like you, so I dont know if it's a good idea.

GW Johnson wrote:

From a radiation shielding standpoint,  there is merit to your sideways cylinder geometry with the nuke in the middle.  Depends upon the numbers whether the surface crew could stay in the vehicle cabin while on the surface.  In the vertical geometry I looked at,  they could not.  How big a surface habitat is needed depends upon both crew size and length of stay.  Different mission plans have different surface stay requirements of the landers.  Cannot generalize. 

GW

I have no idea how much the ship has to be long for radiation shielding.

Last edited by Quaoar (2014-01-03 11:36:31)

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