New Mars Forums

EGADS, what is it with all this really cool spacecraft propulsion tech at my own university I was unwaware of?

First it's M2P2 and now this PMWAC drive that were right under my nose!

This PMWAC really has my interest piqued, do you have any direct comparisons between this tech and VASIMR?  They both seem to be high efficiency, variable Isp drives.  Is there any clear advantage of one over the other?

I'm too lazy to go back to our old LEO to Mars thread and look up my calculations but if one posits a dry spacecraft launch and LEO fuelling, even standard chamical engines can throw something like 60 MT to Mars without too much trouble with a 120 MT liftoff cargo mass. If you start playing with things like ion drives and low energy transfer orbits (for non-perishables) your Mars cargo load starts approaching your LEO capacity.

As for accurate landings, NASA has been consistently improving their landing accuracy, Spirit and Opportunity were both pretty accurate throws.  Also, the Mars 2008 orbiter (MRO) will have an optical camera/UHF transponder system that will greatly improve the ability of incoming spacecraft to figure out their location in space, further improving landing accuracy.  If there's a landing beacon and nothing catastrophic happens in the orbital insertion, I don't see why we can't drop a payload on Mars within a 1 km oval.

For an example, look at Gravity probe B.  It had a 1 second launch windown and ended up hacing a postitional accuracy 6 times what they were expecting.  We've got the technical capacity to make it happen and NASA is already obviously planning on using that capacity to the fullest.

The reason you see such precious materials being used is that some of them have nice properties and so, since weight is at a premium and the costs are so high anyway, you might as well splurge on the good stuff.

quartz - not a precious material, per se - quartz is just ordinary, high purity glass that cooled more slowly and so has a crystalline structure.  Quartz has very nice optical qualities and very predicatable thermal expansion.  It passes all light from fairly deep in the IR down to about 190 nm in the UV which makes it ideal for lenses and such.

diamond - very hard, clear and the best thermal conductor in existence.  It is also one of the best electrical insulators in existence and therefore makes a fine substrate for very expensive computer ships that need to be resistant to radiation damage.

gold - a very good conductor, doesn't oxidize or rust or react chemically with other materials.  Many somsumer electronics have gold on the electrical contacts for this reason.

silicon - not a precious material, basically purified beach sand.  It's used for making computer chips.  In that capacity, it is the single most pure material on Earth by far.  Modern semiconductor tech requires silicon that has impurities in the part per trillion range.

sapphire - actually just aluminum rust.  emeralds and rubies are sapphire that has various impurities in it.  Useful because it is very hard and has some good optical properties.

Sorry, I just moved and so all my stuff is still largely in boxes.  If I find the latest Science and Sci Am, I'll check.  In the meantime, check out this http://einstein.stanford.edu/]link.  Go to the 'what is gp-b' then 'story of gp-b' and check out page 4.  The diagram isn't as good as what I saw but still gives a good rundown.  The whole 'story of gp-b' link is the best resource for getting info on GP-B.  It gets a bit technical at points but is well written.

Incidentally, I found out that Gravity Probe A was launched back in 1976.

Why bother with gasses?  Fill your 'airship' with vacuum.  I know that my low temp freezer at work uses vacuum filled insulation, greatly reducing the wall thickness required.  I'm not too faniliar with aerogel strength but could one simply evacuate the aerogel and cover it with a thin metal film?  You'd greatly increase the lift capacity of an airship if there was no gas inside of it.

Interesting.  I know that back in the 60/70's, in a bit of misguided PR, the US military tried to sell their 100 megaton hydrogen fusion bombs as 'humane' nuclear weapons because of the low fallout.

If it weren't for the fallout from the fission portion of nukes, the old 'plowshare' nuclear programs for building things like canals would have been quite useful.

The primary use of amorphous metals is in high temperature turbine blades.  At the temperatures and speeds they operate at, the blades inevitably start stretching out like taffy and need to be replaced on a regular basis.  Normal metals are composed of small crystallites that tend to stretch differently on the different axes.  The internal stresses and defects that result can result in failure at the grain boundaries. 

The two solutions are to either use single crystal blades where the entire blade is one big metal crystal or to use glassy amorphous metals where the whole thing is disordered and therefore doesn't have any crystalline boundaries in it.  These days I think that amorphous metals are used more commonly because it's cheaper to make.  Basically, you take molten metal, drop it onto a very rapidly spinning disk so that it's splattered into tiny droplets that then plunge into liquid helium.  The cooling rate of millions of degrees a second prevent any crystal formation.  The powder is them cold pressed into the final desired shape.

In contrast, single crystal metals are formed by taking a small seed piece of metal that's attached to the rest of the blade by a small channel that looks kind of like a pig's tail.  The metal starts crystallizing on the seed as they progressvely cool the metal and the curlique channel causes all but one of the crystal grains to eventuall run into a channel wall and die out.  The single crystal then grows out of the channel and into the die where the blade is sitting.  Through very careful control of the temperature, you can ensure that the entire blade is a single crystal of metal - not easy to do.

The thing is that hydrogen is quite easy to spot.  You look for certain spectroscopic absorption lines (Lyman Alpha lines, IIRC) that tell you where and how much hydrogen there is.  There are areas of the galaxy that have high concentrations of gasses but unfortunately none of them are very conveniently located. 

A Bussard ramjet would work but the problem is that you need a magnetic field generator that is far more powerful than anything we currently can make or can even think about making.  It also requiresthat you are already going a significant fraction of the speed of light.  If a bussard ramjet could be made to work it would be the most effective spacecraft drive in existence since it doesn't have to carry fuel along with it.

The Plank scale is something entirely different - that's supposed to be the fundamental quantum unit of length, something like 10^-35 m or something like that.  A Plank length to an atom is equivalent to an atom vs approximately 10% of the entire observable universe.  Very small.

Quantum effects are something one tends to see at under 10 - 20 nm or so.  One sees things like surface plasmon confinement effects on metal particles below about 20 nm in diameter.  True 'quantum dot' effects kick in for semiconductors at 5-10 nm and for metals at 1-2 nm.  Things like electron position uncertainty are only a problem below a couple of nanometers. 

For example, part of the reason that chips run so hot these days is that there's a lot of current leakage from the transistor gates into the circuits.  Presently, the SiO2 insulator seperating the gate from the rest of the transistor is less than 5 nm thick.  The use of high K dielectrics for the gate insulator allows the ates to be made thicker again, lowering current leak but is probably boing to be the eventual show stopper for current microprocessor tech.

The real advance tha a lot of people mistake for nanotech is self assembly.  The ideal way to buidla computer is like how living organisms assemble themselves - through tiny subunits that put themselves together.  Unfortunately, our knowledge of how to do this is still very primitive.

The fusion research community is highly politicized.  The tokomak folks have basically taken over and any alternate designs have been marginalized, relying on shoestring budgets to keep operating.  On one hand, the tokomak design is the most promising but on the other, the tokomak design has some serious issues which really recommend that we keep looking at alternate issues.

If you look for a history of the Fusor design, it's clear that it was internal company politics rather than design flaws that killed mainstream fusor research.  Although ir's quite possible that the fusor design is inherently incapable of breakeven, there's no reason why it should be ignored as much as it is.

Having been offline for a week or so, I don't know if this came up in discussion yet:

http://www.space.com/news/us_china_040428.html]link

It turns out that the Chinese designed their capsules to be able to dock to ISS.  They approached NASA to do joint missions but NASA basically told them they didn't know what they were doing and to take a hike.

As much as I'd love to throw in behind that theory, molecular cladistics (tracing evolutionary lineage by looking at DNA sequences) shows that D. radiodurans is a pretty ordinary bacterium. 

For a while, it was thought that its really wierd quadruple nucleoid DNA structure and incredible double strand DNA break repair ability was the source of its radiation resistance but recently, evidence has come out that calls that into question.

Unfortunately, whatever gives radiodurans its rad resistance, it's probably quite compatible and not something that would work with human DNA.