New Mars Forums

Well,  it's starting to "hit" here in the US.  My best guess is that this virus has an oddity or two about it.  One seems to be infectiousness for perhaps a week before any detectable symptoms kick in.  The other seems to be that this is about like a cold or a mild flu for most folks,  but potentially very deadly for oldsters (like myself),  especially those with underlying heart of breathing problems. 

That first oddity is why the field tests taking temperature proved to be so ineffective at containing this outbreak.  I might be wrong because the real data are not yet in,  but that's what the available experiences to date very strongly suggest.

All I can say is stay away from crowds,  Quaoar.  I will do the same. 

Meanwhile,  here is a quote from from AIAA's "Daily Launch" newsletter regarding why Spacex is using stainless steel for "Starship".  It aligns very closely with what I have been saying for some time now. 

It doesn't name the alloy,  but I suspect 316L or maybe 347.  316L is cheaper than 347;  otherwise they have similar properties,  and both are good past 1200 F,  unlike 301.  Or 304L. 

You want the L suffix if you intend to weld.  If you don't select for that,  your welds crack.  Surprise,  surprise!

GW

Quote from "Daily Launch":

SpaceX To Change Starship Stainless Steel Alloy

SPACE (3/12) reports that thus far, SpaceX has constructed its Starship prototypes out of a stainless-steel alloy named 301. But “aerospace engineers have been using that particular metallic blend since the middle of the last century, and it’s time for SpaceX to make a change, Elon Musk said.” Musk said during the Satellite 2020 conference, “We should be able to do better in the 2020s than they did in, like, the ‘50s, you know? So, I think we’ll start switching away from 301 maybe in the next month or two.” SpaceX still plans to use stainless steel for both the Starship and Super Heavy rocket, but the company will “migrate to a different alloy, whose constituents SpaceX will tweak over time, Musk said.” SpaceX is using stainless steel because it is cheaper than carbon fiber and handles heating better than carbon composites, among other reasons.

197.  Damn.  Pandemics are hell,  even at a low effective death rate.  The Black Death 600 years ago was far worse.  But still .....

Radiators on lee surfaces during entry are exposed to low wake-velocity scrubbing speeds,  but the same really high driving temperatures for heat transfer,  and similar plasma radiation,  above 10 km/s.  They are not as effective as you otherwise might think.  But they are better than no such radiator at all.  Once the plasma in the boundary layer goes opaque at really high speeds,  they are totally ineffective. 

GW

This is an academic paper study.  An idea.  Most such do not turn out to be very practical. 

The waverider is likely to lose as much delta-vee in drag,  as gain any from from Venus's orbital speed.  That's because Venus has a denser atmosphere.  So,  I don't see a lot of promise in the mission concept. 

The nosetip for the waverider may have an iridium coating,  and it may be cooled from the backside with heat pipes,  but it is still carbon-carbon,  and that means it's an ablative.  Simple as that.

It changes shape as it ablates.  That shape change affects vehicle aerodynamics.  That effect,  more than simple thickness loss,  is very likely what limits its life.  No different than with Space Shuttle nosetips and leading edges.

In point of fact,  the iridium coating might be a bad idea.  It lowers the thermal emissivity of the carbon-carbon,  being a shiny metal.  That greatly reduces the ability of the carbon-carbon to cool by radiation to the environment. 

You have that,  and you have backside cooling by conduction.  Or maybe transpiration cooling if you add porosity and a sacrificial liquid to the design.  That's about all the ways there are to cool the structure in any practical way.  So why "kill" one of the best that you have with a shiny coating?

The heat pipe cooling is an alternate implementation of the graphite nose tip with imbedded tungsten rods that was proposed for the 1955-vintage X-20 Dynamic-Soaring ("Dyna-Soar") design.  This was cancelled with the first three production articles on the assembly line in 1958. 

Heat conducts down the metal rods,  or a bit more efficiently down the heat pipes,  cooling the graphite or carbon-carbon from the backside;  there is very little actual difference.

For a space vehicle,  you must deal with the heat conducted down the rods or heat pipes.  You either heat sink it in some way,  or find some way to dump it overboard.  It just does not "disappear". 

GW

There's nothing magic or new about waverider technology.  Hype to the contrary is just that:  hype.  A waverider generates lift by the shock-induced pressure rise on a flat bottom,  exactly the same effect as the component of supersonic wing lift that is generated by the bottom of a supersonic wing.  It's just not a wing lower surface doing this.

It's not new at all.  Some would say this dates to the 1965-vintage XB-70 Mach 3 bomber.  About half its lift surface was a flat bottom,  not the wings.  But this was used even earlier in the 1953-vintage F-100 Super Sabre fighter jet.  That aircraft raised its lift/drag ratio during supersonic dash with a flat fuselage bottom.  Dash speed was Mach 1.3. 

As for openings for thruster nozzles on the lateral surfaces during entry conditions,  there is no reason you cannot have them,  as long as you design to preclude through-flow when the thruster is not firing.  The static gas column in the open port is a better insulator than any solid we could install.  But the key word is "static":  if vented,  hot gas intrusion destroys structure.  That is why shuttle Columbia shed a leaky wing with a hole,  and was destroyed.

If by "air augmented rocket" you mean a mixing device that accelerates air with the jet pumping action of an embedded rocket stream,  that works far better at low flight speeds,  essentially subsonic,  certainly not good at all at supersonic,  and certainly not hypersonic/orbital-speed class. 

As for windows during entry,  you can have them on surfaces that are embedded in separated wake zones,  where there is no scrubbing action,  only very low-speed contact with hot plasma.  Sometimes knowing where these locations are can be tricky.  A hypersonic slipstream jet would reattach to the shuttle's nose at angle-of-attack above 40 degrees,  impinging straight into the windscreen,  and guaranteeing its destruction in mere seconds. 

The same was true below 20 degrees.  Thus the shuttle had a very tight angle-of-attack restriction during entry:  20 < AOA < 40 degrees.  Loss of such control would lead to destruction of the vehicle within seconds.  Nose shape details did not matter,  it was true for any of the shapes considered for the shuttle's nose.  This was found in Mach 5 wind tunnel tests during 1973,  before shuttle's design was finalized.  I helped conduct and analyze those tests as a graduate student.

GW

http://www.astronautix.com/p/propellants.html

https://space.stackexchange.com/questio … f-a-rocket

http://www.braeunig.us/space/propel.htm

https://www.mpoweruk.com/rockets.htm
Rocket equations present on the page

It occurs to me that any engine involving a nuclear reactor is going to have to deal with post operation heating due to short half life isotopes continuing to break down after the reactor is shut down. This will probably require a small flow of hydrogen through the core. The thrust when it is dumped can be balanced by splitting the flow into two opposite streams, but there will still be a negative effect on overall Isp. Reactors don't go from high power to no power like chemical reactions.

Not having done the calculations, electric thrusters may be a good praxis for spin-up and spin-down.  Their fuel consumption is very small, and their consistent low thrust is fine (even desirable?) for adaptation to rotation and for minimizing the oscillation of the spaceship-tether-counterweight system.

Hydrogen-impregnated stainless steel foams or simply foams with voids can dramatically lower the mass of shielding and research has shown them to be substantially more mass-efficient (more effective at blocking Gamma and Neutron radiation at the same time with a single material) than the prototypical High-Z (Gamma) / Low-Z (Neutron) material combinations, like DU (Depleted Uranium) or W and LiH, for example.

A pebble bed reactor, perhaps?