BLEO capabilities for commercial LEO vehicles

[Hop_David]

Question for you all?

Wouldnt a Bigelow (double sundancer/ single BA330) with a docking/engine structure to tie them together be a better BLEO vehicle? In effect make your space ship a small station. Sitting in any capsule for days on end (or month for a Mars mission) seems very hard on the body to me.

I don't think that anyone is seriously talking about using an enhanced Dragon or other commercial vehicle for longer-duration flights than flights to EML or LLO.  Beyond that, some manner of hab module is needed, not simply for habitation space but also for extra consumables storage.

I see on Wikipedia Dragon will have TPS for re-entry return from the moon or even Mars. Does Elon plan radiation shielding sufficient for passage through the Van Allen belts? Any provision for EVAs?

I had thought Motorola sats and delivery of cargo to I.S.S. was the extent of SpaceX near term goals. And anything beyond LEO was Power Point. It's exciting that they seem to be already bending metal for beyond LEO.

[Robotbeat]

Radiation shielding from acute space radiation (i.e. SPEs) is pretty simple, to be honest. If more is needed, just add more mass (and not that much more, actually). No reason you couldn't install it inside the capsule on a later date. It'd be quite simple. (The Apollo command module was thick enough, so the astronauts would've just retreated there in case of a SPE... Dragon, once fitted for crewed service, might also be thick enough...)

For long-duration missions where chronic radiation exposure might be a concern, you're going to be spending almost all of your time in the mission module, so it doesn't matter as far as Dragon (or other commercial capsules) is concerned.

[docmordrid]

Speaking as someone with a radiation protection background.  Someone please chime in if my memory of structural details are amiss.

My understanding of the Apollo structure is that it had two concentric honeycombed hulls; the outer of thin steel, and the inner one of aluminum alloy.  The thinnest section was .25" in the aluminum hull and the thickest 2.5" in the steel section, most of both being empty space between 2 layers of thin metal.

In both cases most all of the protection from radiation coming in perpendicular to a given surface comes from the honeycombs inner and outer layers.  Non-perpendicular "incoming" would be attenuated a bit more, depending on the angle.

While these sheets, two each of aluminum and steel, would stop cold alpha, weak beta (electrons) and seriously attenuate lower energy x-rays/gamma, they are near useless vs. stronger x-rays, high energy electrons, protons or fast neutrons.  Not much would be effective against them without adding excessive mass.  Just stopping the weakest protons in the belts would require >.6mm of lead sheeting.

If Dragons pressure hull is also honeycombed aluminum alloy in structure and >2.5" thick (it appears to be at least that thick based on the pic of the COTS-2 hull) with thicker skins, then it could already be equal within practical limits to Apollo vs. perpendicular radiation.  If it's solid machined aluminum alloy and that thick, it's probably better than Apollo.

All else being equal the best protection is always reducing the exposure time, meaning in the case of spacecraft - speed. This would be most useful for going through the more energetic outer belt. The inner belt is where  ISS and LEO live, starting at 60 miles/100 km.

[Jim]

There is "spalling" effects and structure activation effects too.

[Ben the Space Brit]

I see on Wikipedia Dragon will have TPS for re-entry return from the moon or even Mars.

I'm assuming that this is referring only to a re-entry directly from return orbit.  The Dragon would be the CRV and maybe the command centre in such a mission but not the crew habitat.

[docmordrid]

There is "spalling" effects and structure activation effects too.

Aluminum gamma emissions from neutron activation run between .8347 and 1.368 mev. These will penetrate most hulls of the metals we're talking about, so unless you line the hull interior with lead, which alone could be toxic....

For Lithium + neutrons you mostly get stable Helium and Tritium, which decays to Helium 3 and a relatively weak electron, or Berillium-8 which decays into two alphas, none of which are going anywhere unless the reactions started in an inner walls surface.

For Compton scatter (spalling) effects from mid-energy gamma you get progressively weaker x-rays until the reaction becomes photoelectric, and at the levels >1mev it takes significant metal to attenuate it - much more than in Apollo.  Significantly higher energies add some electron-positron pair production.  The positrons decay when they hit matter with a mid-gamma emission and the electron is your basic beta particle.

Etc...

Bottom line is that several millimeters of aluminum, perhaps with some polyethylene, gives you about as much shielding as is practical for a capsule without driving the mass to impractical levels.  If you want better, attach a Bigelow hab.

[Jim]

There is "spalling" effects and structure activation effects too.

Aluminum gamma emissions from neutron activation run between .8347 and 1.368 mev. These will penetrate most hulls of the metals we're talking about, so unless you line the hull interior with lead, which alone could be toxic....

For Lithium + neutrons you mostly get stable Helium and Tritium, which decays to Helium 3 and a relatively weak electron, or Berillium-8 which decays into two alphas, none of which are going anywhere unless the reactions started in an inner walls surface.

For Compton scatter (spalling) effects from mid-energy gamma you get progressively weaker x-rays until the reaction becomes photoelectric, and at the levels >1mev it takes significant metal to attenuate it - much more than in Apollo.  Significantly higher energies add some electron-positron pair production.  The positrons decay when they hit matter with a mid-gamma emission and the electron is your basic beta particle.

Etc...

Bottom line is that several millimeters of aluminum, perhaps with some polyethylene, gives you about as much shielding as is practical for a capsule without driving the mass to impractical levels.  If you want better, attach a Bigelow hab.

Do some searches on "Shuttle activation monitor", it might interest you

[docmordrid]

I read Haskins, et al's paper about SAM on NIH's Pubmed long ago. 

It basically confirmed what radiation safety people have been taught for decades - more metallic shielding often results in a high flux of more absorbable "scattered" radiation. That's why we measure doses in terms of absorbed dose, not raw flux.  Radiographers are also familiar with this effect as it can impact image quality.

IMO this is exhitlbit "A" against those who claim that Dragon "needs" substantially more shielding to go on short to medium duration BLEO missions. 

No, what's needed is a small Bigelow type hab (thicker polymer walls are good at absorbing scatter) with a shelter area for solar events.  IIRC Bigelow has a patent on just such a shelter.

[Patchouli]

I read Haskins, et al's paper about SAM on NIH's Pubmed long ago. 

It basically confirmed what radiation safety people have been taught for decades - more metallic shielding often results in a high flux of more absorbable "scattered" radiation. That's why we measure doses in terms of absorbed dose, not raw flux.  Radiographers are also familiar with this effect as it can impact image quality.

IMO this is exhitlbit "A" against those who claim that Dragon "needs" substantially more shielding to go on short to medium duration BLEO missions. 

No, what's needed is a small Bigelow type hab (thicker polymer walls are good at absorbing scatter) with a shelter area for solar events.  IIRC Bigelow has a patent on just such a shelter.

Apollo did not really have much in the way of dedicated radiation shielding.
The CSM mostly depended on it's normal structure and the propellant in the SM for shielding.
The LEM had some panels coated with a special radio-luminescent paint and some thin acrylic sheets for shielding soft X-rays.
http://www.informantnews.org/modules.php?name=Content&pa=showpage&pid=10

Dragon and DreamChaser in this aspect might be better off then Apollo was.
Dragon has several large propellant tanks at the base a lunar variant likely also would have propellant tanks in the trunk.
Dream Chaser is even better equipped because of it's composite pressure hull and the placement of the hybrid propulsion system.

The CST-100 would likely just do what Apollo did place the SM between it and the sun during a solar storm.

It's close enough to Apollo it might even be possible to use some of the old Apollo data.

[Jim]

I read Haskins, et al's paper about SAM on NIH's Pubmed long ago. 

I was their launch site support while in the USAF

[moose103]

Good talk by docmordrid and Jim.

Since excellent protection (an ocean of molecules) is much more expensive than just "good" protection, the best way to go is bunk beds in the propellant tanks.

[alexw]

Good talk by docmordrid and Jim.
Since excellent protection (an ocean of molecules) is much more expensive than just "good" protection, the best way to go is bunk beds in the propellant tanks.

   Or polyethylene, or other hydrogen-rich material. Paraffin. Hey, add some LOX, and people could travel safely in giant wax candles in space! (Why does this remind me of Ares I?)

   More seriously, does this suggest a lighter capsule to hightail it to EML2 where the hab is waiting? To what extent are (rapid) T-EML2-I trajectories' elapsed time sensitive to delta V, and are there additional requirements on the insertion burn at L2?
    -Alex

[ChefPat]

I read Haskins, et al's paper about SAM on NIH's Pubmed long ago. 

I was their launch site support while in the USAF

Damn it Jim, your an Engineer not a Doctor!!!

[Patchouli]

Good talk by docmordrid and Jim.

Since excellent protection (an ocean of molecules) is much more expensive than just "good" protection, the best way to go is bunk beds in the propellant tanks.

If they ever build a BLEO capable version of Dream chaser one safest places to be in a solar storm might be in the back of the vehicle.

N2O and HTBP is not the best radiation shield but you'd have a few tons of it.

Maybe put the water and O2 tanks in the ceiling and floor and you'd have a fairly complete shelter for nearly free cost in mass.

A smart move if you're docked to a Bigelow module would be to keep the crew vehicle between the module and the sun during a solar storm.

A hydrogen propellant depot could make one of the best storm shelter's possible.

[Robotbeat]

Actually, a methane depot, for the same volume, would be a lot better than the hydrogen depot. (EDIT: As far as radiation shielding is concerned... Same probably goes for other hydrocarbons.)

[docmordrid]

Probably so, just as long as you can keep it >91°K.