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#101 2020-07-09 17:25:16

tahanson43206
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Registered: 2018-04-27
Posts: 3,574

Re: Solving Mars mission docking with Phobos

For GW Johnson re #99, and SpaceNut re #100

Thank you both for your thoughtful analysis of the situation which will be faced by a ship's navigator approaching Mars on a Hohmann transfer trajectory.

This post is reserved for notes after (what I hope will be) careful study.

***
In another topic, RobertDyck has undertaken analysis of and creative thinking about a large scale passenger ship for Earth/Mars travel.

It is too early to know if that initiative will go further or if it has reached its natural conclusion, but it it does continue, the navigation of a large vessel in the vicinity of Mars will be of concern to the crew, and to the mission planners who will be providing the equipment and supplies needed to insure a safe journey for up to 1000 people.

I will endeavor to show that Phobos is capable of imparting (on the order of) 2 km/s of velocity change (acceleration) to a vehicle that engages with the mass of the moon on the backswing, and then rides along as Phobos travels around Mars.

That 2 km/s of velocity change will (and does) represent a savings of fuel and oxidizer which would otherwise be needed to enter orbit around Mars.

(th)

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#102 2020-07-11 16:22:18

GW Johnson
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From: McGregor, Texas USA
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Re: Solving Mars mission docking with Phobos

Sending 1000+ people to Mars is way to hell and gone beyond anything currently contemplated,  even at Spacex.  That is most definitely a very large-scale colonization ship effort. 

My advice for that is to use nuclear pulse propulsion in a ship exceeding 10,000 tons,  starting from the backside of the moon,  to shield the Earth from the EMP of its departure "burn".  It will need to use an arrival at Phobos,  in order to shield those on the surface of Mars from the EMP of its arrival "burn".  Such "burns" would likely be thousands of thermonuclear devices,  not 1955-vintage fission devices.

Aerobraking for a ship of that capability will be entirely irrelevant.  You are looking at something about like a WW2 pocket battleship,  around 10,000 to 20,000 tons initial mass,  and over half the ignition mass is dead-head payload.  Because you are looking at 10,000 sec+ Isp at a vehicle acceleration in the 2-4 gee range.  Made from 2-inch thick steel armor plate.  Such a thing can reach Mars in 2-5 months single-stage,  two-ways,   and is big enough to easily spin for artificial gravity between "burns". 

There are a number of missing support technologies necessary to make such a thing possible.  Not the least of which is a nuclear propulsion development-and-test station on the moon,  where there is no air or water to pollute,  or neighbors to annoy,  if you have a problem.  You will need a way to transport that thousand people and all those thousands of tons of supplies to a location beyond the moon,  in order to board a vessel like that.  Plus a way to transport same from beyond Phobos to the surface of Mars.

You WON'T be launching things that large from Earth,  or landing them upon Mars,  because of the collateral-damage the EMP does.  Compared to that,  the radioactive fallout risk is trivial.

GW

Last edited by GW Johnson (2020-07-11 16:24:57)


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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#103 2020-07-11 17:24:33

tahanson43206
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Re: Solving Mars mission docking with Phobos

For GW Johnson re #102

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SearchTerm:NuclearFissionPassenger

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#104 2020-07-11 19:16:14

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 532

Re: Solving Mars mission docking with Phobos

EMP is caused by gamma rays stripping electrons from air molecules in the upper atmosphere.  These are subsequently deflected by the Earth's magnetic field, resulting in a huge induced current lasting for nanoseconds - the so called E1 pulse.

The effects scale with the square root of yield and are more severe in single phase fission devices, because there is no surrounding compressive charge to absorb gamma rays.  Some mitigation options:

1. Smaller devices generate smaller pulse, all else being equal;
2. Gamma rays decline in intensity according to the inverse square law.  A burst at 40,000km will have only 1% of the intensity at ground zero of a burst at 400km, but will effect the entire hemisphere.  Unfortunately, launching at that height would pump the Van Allen belts with charged particles, which may damage satellites.
3. One could shield devices by encasing them in hydrogen rich materials to absorb gamma pulse.  This will increase weight and reduce effective energy density of the pulse units.
4. Fire small devices within some kind of enclosing engine bell, and use the radiated energy to heat hydrogen propellant.  That would do bad things to power to weight ratio and I expect the engine would need to be huge.
5. Launching from the poles would be advantageous, as there is no tangential magnetic field.

I doubt that the pulse rocket would need to launch from behind the moon.  But it may be advisable to launch from above the inner Van Allen belt to avoid damage to geo stationary satellites.

Mars has a thin atmosphere and no magnetic field.  I do not know how a EMP inducing event would play out under those conditions.  It has never been tested.

Any decent performance space drive is a weapon of mass destruction.  There is no way of releasing such huge amounts of energy without collateral damage.  Interestingly, I am not sure that a pure fusion inertial confinement drive would solve these problems.  The explosions are usually envisaged as being smaller, but there is no surrounding charge to absorb gamma rays.  Not an easy problem to solve.  Though there are solutions that would reduce the problem, maybe even making it tolerable.

If chemical propulsion can be used to ship people, propellant and freight to low earth orbit, then a large interplanetary transport ship using gas core nuclear thermal propulsion would be one way of avoiding EMP problems.

Last edited by Calliban (2020-07-11 19:31:31)


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#105 2020-07-12 12:32:34

GW Johnson
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From: McGregor, Texas USA
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Re: Solving Mars mission docking with Phobos

Sending 1000 people to Mars at time has nothing to do with exploration,  and everything to do with colonization or large base-building.  I personally consider that day a very long ways off,  especially for any government-led efforts. 

But if it is a long ways off,  then there is time to develop both gas core nuclear thermal and nuclear pulse propulsion,  and there is time to build a safe place to do that development work,  which in turn is a safe place from which to launch them. That safe place is the moon.  Which should be obvious to the casual observer. 

What you are looking at with gas core is either "nuclear light bulb" concepts (where the uranium is kept separated from the hydrogen) or open-cycle gas core,  where an intimate contact flow scheme must be devised that effectively sets the uranium throughflow at 1/1000 of the hydrogen throughflow. 

None of this has undergone any development at all,  but the early exploratory design work says the light bulb concepts might have Isp ~ 1300 sec max,  at engine thrust/weight greater than 1,  maybe a lot greater than 1. They were looking at double-walled quartz window separation,  cooled in between,  so that thermal radiation transmitted through from the core fireball to the hydrogen propellant.

The open cycle gas core operates at much higher gas core temperatures and power levels,  limited primarily by hydrogen transparency to thermal radiation when it gets hot enough.  There was also a limit where regenerative cooling of the chamber and nozzle bell was no longer feasible,  because the demand for coolant exceeded the supply (the engine throughflow of hydrogen). 

Open cycle gas core was thought possible with regenerative cooling up to ~ 2000 maybe 2500 sec Isp,  with engine thrust/weight exceeding 10,  maybe far above 10. That could actually be used to launch from the surface of the Earth,  if you can accept the radiation in the exhaust stream.

Such exhaust stream radiation is minimized but not eliminated,  if you get to the target 1000:1 flow ratio.  The fission products are there in the exhaust,  at best.  At worst,  there are products plus unused uranium,  all in very hot gas form,  and moving really,  really fast.

Operated at higher core power,  with powered active cooling and a gigantic 3500 F waste heat radiator,  open-cycle gas core could operate up to the transparency limit,  thought to be no more than about 10,000 sec Isp.  But nobody knew "for sure". Or how to actually build that radiator.  Or what from.

The first design target for development set this at 6000 sec Isp, in order to stay well away from the transparency limit.  This design necessarily includes the radiator and powered cooling circuit as part of the engine equipment,  with an effective engine thrust/weight well under 0.1.  Only suitable for in-space propulsion.

The other main attraction of open-cycle gas core over nuclear light bulb,  or any of the solid core concepts,  was safety after shutdown,  which is abortability.  Stop the flows,  and you have an empty steel can,  no reactor core at all.  It "cools" quickly in a radiation sense to levels that can be dealt with,  fairly easily. Nothing that retains a core,  solid or gaseous,  can do that.

Now as I said,  this was all early design speculation,  no real development testing was ever done with any of this.  But the potential really is there.

As for pulse propulsion,  it has not received any development at all,  since it was shut down in 1965.  The design was an armored hull with shock absorbers and a massive steel blast plate,  using small kiloton-range shaped-charge fission devices based on our 1955 technology.  It appeared mostly feasible at the time,  excepting the lack of feasible fractional-kiloton device technology in 1955,  needed to launch from the surface in the atmosphere. We have that now.

Many folks erroneously question the blast plate,  but it was essentially a massive heat sink with radiational cooling,  and pretty much survived in the same way as other massive steel items survived pretty much intact at Hiroshima,  Nagasaki,  and in the surface tests in Nevada.

This stuff actually works more efficiently in larger sizes.  It was barely attractive at a ship initial mass of 5000 tons.  But for ships in the 10,000 to 20,000 ton range,  you are looking at effective Isp in the 10,000 to 20,000 sec range,  and it is hard to hold the ship acceleration levels to the 2 to 4 gee range. In other words you are building vessels of steel armor plate that are the size of WW2 battleships.  And totally unlike electric propulsion,  the thrust levels are very high indeed. Even the Isp is higher than electric.

For really,  really large vessels transporting 1000-to-10,000 people at a time,  there is no other known choice but pulse propulsion.  Nothing else we know scales up that big.  It will scale up even larger.  Pulse propulsion just won't scale down effectively,  so it needs to be used only in such large applications,  not the smaller ones. It just needs an update to 2010/2020 nuclear technology.

For transporting 100-1000 people (or 100-1000 tons) at a time,  open-cycle gas core nuclear orbit-to-orbit ship designs might do well,  with far fewer side effects.  That's still early colonization or large base-building. Nothing to do with exploration.  These would have to be combined with chemically-powered transports of one kind or another,  at each end of the trip,  to get people and cargo between the ship and the surfaces.

On a smaller-still scale (10-100 people or tons),  the chemical and solid-core nuclear thermal schemes look practical.  That addresses both explorations and early small base-building.  These need not be orbit-to-orbit transports,  if one can accept the conflicting design requirements of orbit transports versus landers.  So,  guess why the Spacex Starship/Superheavy design is what it is,  and gets touted for this application. 

Just putting some perspective to this.

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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#106 2020-07-12 14:51:25

Calliban
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From: Northern England, UK
Registered: 2019-08-18
Posts: 532

Re: Solving Mars mission docking with Phobos

Thank you very much for an interesting post GW.

I get the feeling that we won't be colonizing Mars until the world stops being scared of radiation.  Both nuclear pulse and gas core nuclear rockets would bleed fission products into the atmosphere.  If the world stopped caring about that, we could launch colonization ships like the one you described from Antarctica, where magnetic field lines are close to vertical and avoid most of the problems from EM pulse.  We could have regular interplanetary transport.  But the price would be higher background radiation levels for anyone living back on Earth.

What kind of a nation would do that?  What kind of government?  Certainly none that I know today.  Maybe the Chinese are ruthless enough.  But the US and Europeans would need governments that are nothing like the ones they have today.  I just cannot imagine it.

Last edited by Calliban (2020-07-12 14:53:56)


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#107 2020-07-12 15:25:45

SpaceNut
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Re: Solving Mars mission docking with Phobos

Radiation is solved on earth so why can we not do so for trips beyond LEO has to do with cost to lift the materials or sufficient mass which work. That is why we design with the water we must take, the food that is frozen, and with a variety of materials to make it so that less of that fear gets through.

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#108 2020-07-13 08:12:25

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,078
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Re: Solving Mars mission docking with Phobos

For exploration missions,  which might or might not establish a small outpost,  all you need is stuff like we already almost have,  launched from the surface of the Earth or from low Earth orbit.  That is exactly what the refilled/outside-LEO mission proposals for Starship / Superheavy are all about,  and it is exactly what NASA claims SLS / Orion is for,  even though it still cannot do such missions. 

Spacex has serious issues to resolve with docking tail-to-tail,  with transferring cryogenic propellants,  with inert mass fractions in their vehicle designs,  with landing pad bearing pressure on soft surfaces,  and with landing leg pattern lateral dimension vs cg height static stability issues on rough ground.  Resolving those is what the prototypes and testing is all about.  That takes time,  and failures are inherent in that.

NASA has serious issues to resolve,  too.  They have as yet no lander design for the moon,  and nothing at all for Mars.  They have no deep space habitat design ready to go,  and what they have does not address microgravity disease or solar flare radiation protection.  They do not have the throw weight capability in Block 1 SLS to do any of this,  and it is doubtful we will ever see Block 1B or block 2B,  because of the cost issue.  The cost issue is that,  pound for pound,  flying on SLS is at least 10 times as expensive as anything else. Maybe 100 times as expensive,  if one is honest with the book-keeping.  They literally cannot afford to use their own rocket.  And the flight rate is too low to do anything significant with it.

NASA's biggest problem is that it long ago devolved into a corporate welfare scheme for the big old-space contractors,  micromanaged in terms of funded projects by congress.  And that is exactly what congress wants from NASA,  to serve its own porkbarrel politics interests.

So,  you are looking at doing early exploratory-level stuff to the moon and Mars in the next several years.  The transportation that does it will most likely be things like Starship / Superheavy,  Falcon-9 / Falcon-Heavy,  Atlas-5,  Vulcan,  and maybe New Glenn.  I doubt that SLS will ever play much of a real role,  but I do think there will be a few stunt flights.  To this mix you need to add habitats resembling Skylab,  equipped with radiation shelters,  some scheme for at least partial spin gravity,  and,  phasing-in later,  some updated nuclear thermal propulsion,  most likely solid core,  if you want to seriously send any "bigger things" to Mars.

Its going to take many,  many years to build a mature transportation system between Earth and the moon,  routinely shipping mass tonnages and big passenger loads back and forth.  At that time is when we will need the advanced nuclear development station(s) on the moon,  and it is from there that the really big colonization-class vehicles will depart,  using that advanced nuclear propulsion.  None of us will live to see that.  All we can do is point the way for our descendants.

GW

Last edited by GW Johnson (2020-07-13 08:17:29)


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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#109 2020-07-13 08:58:09

tahanson43206
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Registered: 2018-04-27
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Re: Solving Mars mission docking with Phobos

For GW Johnson re #108

SearchTerm:ForecastSpaceDevelopment
SearchTerm:OutlookSpaceDevelopment
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