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#51 2018-11-07 19:00:44

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

OK, it appears I misread the chart from what you say.  I was indeed looking at cell M25.

So what is the arrival velocity?  Which cell describes that?

And why do you think it is impossible to achieve a smooth reduction in speed when Musk clearly does think it's possible, as per his graph?






kbd512 wrote:

Louis,

To begin with, Dr. Zubrin is still correct when he says 6 months is the proper transit duration.  Next, I'll presume you understand that you're not actually traveling in a straight line when traveling along an interplanetary transfer orbit.  However, straight line velocity approaching the asymptote of the hyperbola (of a non-Hohmann transfer orbit) can be expressed using the term "vinf" since it's so close to actual straight line velocity that it's an acceptably accurate substitute.  A Hyperbola is effectively what you're traveling along when you approach the target of your transfer orbit and the target is a planet of substantial size, like Mars.  The straight line velocity that BFS must reduce to zero to actually land on Mars, whether by firing the engines and/or aerobraking into the Martian atmosphere upon arrival at Mars, can be roughly derived and termed "vinf".  This is how fast you're going at the interface between your orbit and the upper atmosphere of Mars.  If BFS doesn't kill enough velocity to land, then it becomes a man-made crater, gets ejected into interplanetary space if remaining velocity exceeds escape velocity, or enters into an orbit around Mars that it can't get out of without using propellant it doesn't have.

Here's a link to Hop David's blog explaining the concept:

What the heck is Vinf?

Further explanation of the concept from Hop David:

Do transfer orbits toward the central star necessarily result in a higher velocity on arrival due to the star's gravity?

From the linked article above:

"Vinf is a hyperbolic orbit's speed at infinity. For practical purposes a Hohmann transfer becomes hyperbolic when entering a planet's sphere of influence and Vinf refers to the difference in speed between heliocentric orbits."

Could you please tell us what number you're looking at on the "NonHohmannEarthToMars.xlsx" spreadsheet?

Are you looking at cell M25?

If so, that's the dV increment to achieve TEI and has nothing to do with his calculated straight line arrival velocity at Mars.  I guess Hop David thought that SpaceX might want their BFS back after it lands on Mars.


Bob,

Since you or someone with your screen name appears to have read Hop David's blog post from 21 March 2013, could you explain this concept to Louis in a way he understands?

Maybe I'm just terrible at math today, but could you tell me how you derived a 4% structural mass fraction from the total mass?

1,100t (propellant) + 85t (vehicle) = 1,185t (total mass with no payload)

85 / 1185 = .07172 (structural mass fraction of total mass with no payload)

1,100t (propellant) + 150t (payload) + 85t (vehicle) = 1,335t (total mass with max payload)

85 / 1,335 = .06367 (structural mass fraction of total mass with max payload)

I believe .06367 (6.367%) is as low as it will ever be, given the structural mass and maximum propellant mass figures provided by SpaceX.  I've no clue how they arrived at 85t without building their vehicle, so it must be an estimate of some kind.  If you short load propellant or take away payload, then the structural mass fraction only goes up, not down.  The only way that structural mass fraction can go down is if you can cram more propellant mass into the tanks without increasing structural mass or, of course, if structural mass goes down.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#52 2018-11-07 19:07:20

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

I'd like to explain my scepticism about the "150 days minimum" here. It's not based on detailed knowledge clearly. But it reminds me of back in the day when on this site and elsewhere I would get told time and time again "you can't have retropulsive landings on Mars -  it's just not possible because of the thin atmosphere".  Despite my complete lack of scientific understanding of retropulsion, I never believed those claims and I was proved right. Now everyone accepts we can use retropulsion to land on Mars.

So, excuse my scepticism. If Space X think they have found a way to get to Mars in under 150 days, I'm backing them not the naysayers. I suspect it comes down to a non-Hohmann trajectory, powerful retropulsion, new types of ablative shields and possibly lighter payloads.

Last edited by louis (2018-11-08 05:47:11)


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#53 2018-11-07 19:29:05

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 25,731

Re: Journey time to Mars...

https://www.scienceabc.com/nature/unive … space.html

http://curious.astro.cornell.edu/about- … termediate

https://space.stackexchange.com/questio … tead-of-tr

A stright line can only be done with constant acceleration such as what you get from ION drives.

A standard rocket engine pushes and then shuts off and then is gravity curved by the sun as it drifts along the path to the destination.

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#54 2018-11-07 19:32:54

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

My understanding of the non-Hohmann trajectory referenced by Hop David is that it is inbetween Hohmann and straight line.


SpaceNut wrote:

https://www.scienceabc.com/nature/unive … space.html

http://curious.astro.cornell.edu/about- … termediate

https://space.stackexchange.com/questio … tead-of-tr

A stright line can only be done with constant acceleration such as what you get from ION drives.

A standard rocket engine pushes and then shuts off and then is gravity curved by the sun as it drifts along the path to the destination.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#55 2018-11-07 19:49:10

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 25,731

Re: Journey time to Mars...

So the arc cord distance is used to calculate velocity and distance for how much drive we need still constant but a lower levels.

Longer than a straight line for distance and time but shorter than a hohmann curve for distance and time. I will need to lookup arc curvature equations to solve....

as I have long ago forgotten this stuff...

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#56 2018-11-07 21:08:14

kbd512
Administrator
Registered: 2015-01-02
Posts: 5,730

Re: Journey time to Mars...

SpaceNut,

1. You're not traveling in a straight line to Mars, even with ion drives.  Ain't happenin', Cap'n.
2. We don't have a rocket with the thrust and dV capability required to do 1 and we never have.
3. Thankfully, 1 and 2 doesn't matter in the slightest because it's completely unnecessary and actually counter-productive.
4. There's no question that we COULD put enough velocity behind a spacecraft to intercept Mars in 90 days using the rocket technology we already have, whether BFS ever exists or not.  That said, you have to get rid of all that velocity once you get there, assuming you actually want to land on Mars.  Both methods, propellant or heat shield, have a lot of mass attached to them.  Pick yer poison.
5. Why are people so obsessed with doing 4 when it so clearly detracts from so many other aspects of the mission (this was and is Dr. Zubrin's point about the best way to do it)?

Louis,

Look at cells D18 and D22 on the spreadsheet.  Those cells affect how much propellant you need to get to Mars in 3 months, for whatever magic that will do, and how much velocity you'll have to get rid of if you want to land on Mars when you get there.

As far as the question of "Why do we want to get there in 6 months?" is concerned, well, let's see...

1. Humans have a tolerable G-force and vibration limit
2. Radiative heat flux scales with the 8th power of shock velocity (that's kinda important, even if you're willing to kill the crew from excessive G or vibration, just to see BFS land on Mars)
3. More payload increases survivability far more than a 3 month decrease in transit time ever would (a standard ISS tour is about 6 months and then you do one of those reentries and land)
4. Making things stronger generally makes them heavier and excess mass is the mortal enemy of this vehicle
5. If you get there faster than (EDIT: 6 months, not 3 months), there's no possibility of survival if you can't land when you get there (hole in your heat shield?  landing gear won't deploy? too bad, that's all she wrote for you)

Good enough for a start?

I never said that retro-propulsion couldn't work on Mars, nor that the atmosphere would have any effect on the ability to use it.  What I did say is that a system that doesn't require retro-propulsion would be highly desirable if the total mass of the alternative EDL system was lower than the total mass of the retro-propulsion system.  If you could land with a parachute, then why bother with the complications of a rocket?  That was my point.  Eventually, after enough reading and calculating, I came to the conclusion that GW was right and there was no practical way to do this.  WHEREUPON, I GAVE UP ON AN IDEA THAT LACKED MERIT BECAUSE IT WAS IMPRACTICAL TO ACTUALLY DO.

It is POSSIBLE to get to Mars in 3 months.  The real question is, why in the effin hell would you do that when you sacrifice so many other things that matter far more than a 3 month transit?

One question, which I would like answered:

What are you trying to accomplish by doing this?

Last edited by kbd512 (2018-11-07 21:13:48)

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#57 2018-11-07 22:23:19

Oldfart1939
Member
Registered: 2016-11-26
Posts: 2,300

Re: Journey time to Mars...

I've heard many iterations of Dr. Zubrin's presentation of Mars Direct, Mars Semi Direct, and the major argument he always states is the free return trajectory is 180 days. That is making a small sacrifice in payload to get there that fast, but all other faster transits are much more inefficient for a colonization mission. To get there faster requires a higher delta V, and uses a LOT more fuel. The there's the issue of atmospheric entry at higher velocities. There is an optimal mission architecture, and there are penalties incurred by trying other profiles. This topic has been beaten to death. To my mind Bob Zubrin is the ultimate guru in this field.

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#58 2018-11-08 02:23:27

RGClark
Member
From: Philadelphia, PA
Registered: 2006-07-05
Posts: 550
Website

Re: Journey time to Mars...

kbd512 wrote:

Bob,

Since you or someone with your screen name appears to have read Hop David's blog post from 21 March 2013, could you explain this concept to Louis in a way he understands?

Maybe I'm just terrible at math today, but could you tell me how you derived a 4% structural mass fraction from the total mass?

1,100t (propellant) + 85t (vehicle) = 1,185t (total mass with no payload)

85 / 1185 = .07172 (structural mass fraction of total mass with no payload)

1,100t (propellant) + 150t (payload) + 85t (vehicle) = 1,335t (total mass with max payload)

85 / 1,335 = .06367 (structural mass fraction of total mass with max payload)

I believe .06367 (6.367%) is as low as it will ever be, given the structural mass and maximum propellant mass figures provided by SpaceX.  I've no clue how they arrived at 85t without building their vehicle, so it must be an estimate of some kind.  If you short load propellant or take away payload, then the structural mass fraction only goes up, not down.  The only way that structural mass fraction can go down is if you can cram more propellant mass into the tanks without increasing structural mass or, of course, if structural mass goes down.

You’re right, the structural fraction should be 6% of the total gross mass. I may have been looking at the booster section numbers when I got a 4% figure.

But even keeping the same structure weight you can increase speed for a faster trip by going to a smaller mission size, i. e., payload size.

  Bob Clark

Last edited by RGClark (2018-11-08 11:37:59)


Nanotechnology now can produce the space elevator and private orbital launchers. It now also makes possible the long desired 'flying cars'. This crowdfunding campaign is to prove it:
Nanotech: from air to space.
https://www.indiegogo.com/projects/nano … 13319568#/

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#59 2018-11-08 02:48:54

RGClark
Member
From: Philadelphia, PA
Registered: 2006-07-05
Posts: 550
Website

Re: Journey time to Mars...

I think SpaceX wants a shorter mission time because they make no attempt at artificial gravity unlike Zubrin’s mission designs.  Six months or more degrades muscle, bone,  vision, and the immune system.

BTW, NASA has known about this problem of long time exposure to zero gravity for the 30 years or so they have been planning Mars mission architectures. Yet they still have not tested a centrifugal section for artificial gravity in space yet. This leads me to think making one for safe use in space is not as easy as believed.

  Bob Clark


Nanotechnology now can produce the space elevator and private orbital launchers. It now also makes possible the long desired 'flying cars'. This crowdfunding campaign is to prove it:
Nanotech: from air to space.
https://www.indiegogo.com/projects/nano … 13319568#/

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#60 2018-11-08 05:45:06

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

I agree.  I think that's the motivation.  If they can get down from 8 months to 5 months, it's a significant differential in terms of bone and muscle loss.

RGClark wrote:

I think SpaceX wants a shorter mission time because they make no attempt at artificial gravity unlike Zubrin’s mission designs.  Six months or more degrades muscle, bone,  vision, and the immune system.

BTW, NASA has known about this problem of long time exposure to zero gravity for the 30 years or so they have been planning Mars mission architectures. Yet they still have not tested a centrifugal section for artificial gravity in space yet. This leads me to think making one for safe use in space is not as easy as believed.

  Bob Clark


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#61 2018-11-08 09:14:57

Oldfart1939
Member
Registered: 2016-11-26
Posts: 2,300

Re: Journey time to Mars...

I would agree that shortening the transit time would result in less degradation of bone and muscle tissues, so I am hoping that someone with a grain of sense addresses this issue by other means than reducing the time spent in microgravity. Some form of artificial gravity should at least be explored.

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#62 2018-11-08 12:21:48

GW Johnson
Member
From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,833
Website

Re: Journey time to Mars...

What Spacex has said about the PICA-X heat shield they intend for the BFS vehicle is that max Martian entry speed is 7.5 km/sec.  What you have to remember is that this is a slow ablative,  not a true reusable.  After it survives Martian entry,  it must also survive an Earth entry.  That will be somewhere between 12-13 and at most 17 km/s,  depending upon the eccentric orbits of the planets,  and how fast a return trajectory you use.  I have not seen a figure for max speed at Earth entry from Spacex.

The faster the transfer trajectory,  the higher the speed at entry interface.  This is true at both ends:  Mars and Earth return.  Zubrin advocates a slightly-higher-than-min-energy trip as a 6 month trip,  primarily for free-return safety if propulsion fails.  That transfer orbit is an exactly 2 year period orbit,  with perihelion at Earth's orbit. 

The safety aspect is real,  but the price you pay is 2 full years of life support mass to ride the free return orbit.  That unfortunate aspect is rarely mentioned,  usually because we have no reason to believe a crew can be afflicted by 2 years' weightlessness and still survive a very high gee emergency free return.  Likely above 11 gees.   For such a safety orbit to be feasible as "a way out",  artificial spin gravity simply MUST be supplied. 

Otherwise,  there is simply no point at all trying to fly that fast to Mars.  If you have redundant propulsion,  you don't need the free-return orbit (it being "a way out" for an otherwise unsafe vehicle design).  8-9 months at min energy.  Or if you reduce payload,  you can fly faster with existing technology.  But what would be the point,  if you cannot take the payload? 

That is where Kbd512's advocacy of SEP makes great sense.  Combining it with high thrust propulsion gives you a way to quickly and efficiently (and safely from a radiation-exposure standpoint) leave Earth,  and still make a faster transit.  Too bad that technology is not yet flying in the high-power high-thrust form that we need.  It soon could be,  but is not yet on that development path.

Hohmann transfer min-energy orbits demand the least delta-vee,  but do not offer the free-return safety orbit aspect.  You must have redundant propulsion in some way to use this with men.  To do otherwise is unethical in the extreme.  These transit times vary with the planetary eccentricities from just under 8 months to just over 9 months for the one-way trip. 

Remember,  when dinking with the delta-vees and the rocket equation,  the mass ratio is (inert+payload+propellant-to-be-used+propellant-not-used)/(inert+payload+propellant-not-used).  Propellant-not-used is what you must still have on hand for the next burn,  such as the landing.  Inert + payload+ all-of-the-propellant MUST sum to the initial ignition mass.  No ifs,  ands,  or buts. 

If you are talking about fixed-geometry tankage in an actual design,  you cannot arbitrarily change propellant mass except to lower it by leaving some unloaded. That only reduces delta-vee.  For a given design at full propellant load,  you can only increase delta-vee by not loading as much payload,  and even then it's a fairly small effect because the payload fraction itself is typically rather small at a single-digit percentage.

Most of the stuff you find at various blog sites was calculated by amateurs,  if calculated at all.  Take it with a rather large grain of salt.  Maybe a full salt-lick block.  Stuff calculated by people who have actually done rocket work is far more reliable. 

GW

Last edited by GW Johnson (2018-11-08 12:37:15)


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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#63 2018-11-08 15:58:07

Oldfart1939
Member
Registered: 2016-11-26
Posts: 2,300

Re: Journey time to Mars...

GW Johnson wrote:

Remember,  when dinking with the delta-vees and the rocket equation,  the mass ratio is (inert+payload+propellant-to-be-used+propellant-not-used)/(inert+payload+propellant-not-used).  Propellant-not-used is what you must still have on hand for the next burn,  such as the landing.  Inert + payload+ all-of-the-propellant MUST sum to the initial ignition mass.  No ifs,  ands,  or buts. 

If you are talking about fixed-geometry tankage in an actual design,  you cannot arbitrarily change propellant mass except to lower it by leaving some unloaded. That only reduces delta-vee.  For a given design at full propellant load,  you can only increase delta-vee by not loading as much payload,  and even then it's a fairly small effect because the payload fraction itself is typically rather small at a single-digit percentage.

Most of the stuff you find at various blog sites was calculated by amateurs,  if calculated at all.  Take it with a rather large grain of salt.  Maybe a full salt-lick block.  Stuff calculated by people who have actually done rocket work is far more reliable. 

GW

One way of dealing with the issue about the residual fuel and overall mass is simply do the calculations in pieces and a backwards order of events. For a one-way trip, assuming the Zubrin procedure for manufacture of fuel and LOX, calculate the delta V for the landing maneuver with the dry mass of the vehicle with fuel required. That total mass is then assumed to be the "dry mass" for trans-mars departure, and then calculate fuel mass required for incorporation in the Tsilkovsy equation.

The way to solve complex problems is by breaking them down in to a series of smaller and less complex ones.

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#64 2018-11-08 18:59:33

kbd512
Administrator
Registered: 2015-01-02
Posts: 5,730

Re: Journey time to Mars...

Louis,

My personal belief is that NASA is not pursuing artificial gravity because their latest "ARED" exercise equipment and routines haven't simply slowed or even stopped bone loss, they've actually reversed it.  Their latest test results indicate that the astronauts adhering to the new regimen have greater calcification of their bones than when they left Earth, based upon before and after testing following a 6 to 8 month stay aboard ISS.  Obviously an injury in orbit would be a problem, but any significant injury is a problem during a long duration space flight.

Alright, I'll go first.  Here's what I'm trying to accomplish by getting to Mars in 6 months.

1. Transport the crew there in ambulatory condition.  That means no brain damage or other debilitating injuries.
2. Ensure that the heat shield of BFS remains reusable for as many flights as possible, minimizing the danger of a catastrophic failure from extreme heating and/or vibration.
3. Ensure that the composite structures are not vibrated or deformed to the point of catastrophic failure.  Although composites can actually dampen vibration, there's always a cost associated with absorbing excessive vibration.  The carbon fiber composites don't fail gradually; they snap like glass rods.  That's the tradeoff for its incredible stiffness.  There's very little elongation (stretch) before failure.  Making a determination of any structural damage is a very time consuming process that requires specialized equipment and there's often no outwardly apparent signs of damage to composites that have been weakened internally.  It's not something to play with, as numerous aerospace engineers have discovered after lives were lost.
4. Ensure that avionics are not subject to vibration levels that break components or electrical connections.
5. Minimize the quantity of propellant required to refuel in orbit and to land on Mars using retro-propulsion.  The point of this mission is not to maximize fuel delivery, but to maximize delivered tonnage.  It's a numbers game and the number that counts is delivered tonnage.  No points are awarded for fuel delivery, which is just a means to an end.
6. Conserve some residual propellant for emergency electrical power using an onboard fuel cell.
7. Maximize the quantity of crew consumables and spare parts available, just in case they have to stay on Mars for two cycles because something catastrophic happens to the propellant plant and we're unable to bring the crew back.  Petrochemical refineries are dangerous, as anyone in the business of Methane manufacture, transport, and storage can attest to.
8. Maximize the radiation shielding provided for the crew, especially GCR shielding.
9. Minimize cost associated with excessive performance requirements from any one system or component of the overall machine.
10. Preserve the 2 year free return trajectory so that if the propellant in the header tanks sublimates, the landing gear won't deploy, or the heat shield has an issue, there's at least a chance of fixing the problem before it becomes a fatal, unrecoverable problem.

To touch on some of GW's points:

1. PICA-X is not an indestructible material.  It's about the same weight as balsa wood.  I'd rather not push the absolute limits of what it can withstand and survive.  It's not an engineering model for longevity or associated cost reduction; heck, basic survival if we're honest.
2. SEP can efficiently reduce arrival velocities to acceptable values when required for orbital insertions, mid-course corrections, and reentries.  Obviously some time is required, but the time period in question is less than 30 days.
3. I'd agree that a redundant propulsion system negates the need to protect the free return option, but this implies SEP is incorporated into the vehicle.
4. Agreed.  There's little point in a fast transit if it eats up your delivered tonnage.
5. I see you're onto what I was trying to express, regarding the use of a Combined Chemical-SEP (CC-SEP) solution.
6. I also believe it's unethical to not have contingency propulsion solutions, but I think of this aspect of CC-SEP as an investment protection plan.
7. If Hop David even thought about the mass of propellant in the header tanks that BFS requires to actually land on Mars or Earth, then I don't see it in his spreadsheet.  Merely showing that the rocket has enough propellant capacity to support the dV required by a specific maneuver is not an accurate representation of what the vehicle can truly do when laden with the additional propellants in the header tanks that are required to land.  Last but not least, BFR/BFS is a paper rocket.  Nobody knows what the thing will weigh, just like Lockheed-Martin can't tell you what the final weight of Orion will be.
8. BFS is clearly fixed geometry tankage.
9. Agreed.  This is amateur level stuff.  It's good for determining whether or not an assertion is even feasible from a basic math standpoint, but that's it.  If it was professional level stuff, then I probably wouldn't know how to check it.

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#65 2018-11-08 19:14:12

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

That's interesting to learn about the ARED experiment, that they have reversed bone loss and achieved bone addition in space. Quite remarkable. It wpuld appear those approaches could also be adapted to the lower gravity on Mars if necessary.

It would be fantastic if we could tick the bone and muscle loss box...that some other physiological issues (eg vision) that I think are manageable plus radiation protection as the big health problems that have to be addressed.



kbd512 wrote:

Louis,

My personal belief is that NASA is not pursuing artificial gravity because their latest "ARED" exercise equipment and routines haven't simply slowed or even stopped bone loss, they've actually reversed it.  Their latest test results indicate that the astronauts adhering to the new regimen have greater calcification of their bones than when they left Earth, based upon before and after testing following a 6 to 8 month stay aboard ISS.  Obviously an injury in orbit would be a problem, but any significant injury is a problem during a long duration space flight.

Alright, I'll go first.  Here's what I'm trying to accomplish by getting to Mars in 6 months.

1. Transport the crew there in ambulatory condition.  That means no brain damage or other debilitating injuries.
2. Ensure that the heat shield of BFS remains reusable for as many flights as possible, minimizing the danger of a catastrophic failure from extreme heating and/or vibration.
3. Ensure that the composite structures are not vibrated or deformed to the point of catastrophic failure.  Although composites can actually dampen vibration, there's always a cost associated with absorbing excessive vibration.  The carbon fiber composites don't fail gradually; they snap like glass rods.  That's the tradeoff for its incredible stiffness.  There's very little elongation (stretch) before failure.  Making a determination of any structural damage is a very time consuming process that requires specialized equipment and there's often no outwardly apparent signs of damage to composites that have been weakened internally.  It's not something to play with, as numerous aerospace engineers have discovered after lives were lost.
4. Ensure that avionics are not subject to vibration levels that break components or electrical connections.
5. Minimize the quantity of propellant required to refuel in orbit and to land on Mars using retro-propulsion.  The point of this mission is not to maximize fuel delivery, but to maximize delivered tonnage.  It's a numbers game and the number that counts is delivered tonnage.  No points are awarded for fuel delivery, which is just a means to an end.
6. Conserve some residual propellant for emergency electrical power using an onboard fuel cell.
7. Maximize the quantity of crew consumables and spare parts available, just in case they have to stay on Mars for two cycles because something catastrophic happens to the propellant plant and we're unable to bring the crew back.  Petrochemical refineries are dangerous, as anyone in the business of Methane manufacture, transport, and storage can attest to.
8. Maximize the radiation shielding provided for the crew, especially GCR shielding.
9. Minimize cost associated with excessive performance requirements from any one system or component of the overall machine.
10. Preserve the 2 year free return trajectory so that if the propellant in the header tanks sublimates, the landing gear won't deploy, or the heat shield has an issue, there's at least a chance of fixing the problem before it becomes a fatal, unrecoverable problem.

To touch on some of GW's points:

1. PICA-X is not an indestructible material.  It's about the same weight as balsa wood.  I'd rather not push the absolute limits of what it can withstand and survive.  It's not an engineering model for longevity or associated cost reduction; heck, basic survival if we're honest.
2. SEP can efficiently reduce arrival velocities to acceptable values when required for orbital insertions, mid-course corrections, and reentries.  Obviously some time is required, but the time period in question is less than 30 days.
3. I'd agree that a redundant propulsion system negates the need to protect the free return option, but this implies SEP is incorporated into the vehicle.
4. Agreed.  There's little point in a fast transit if it eats up your delivered tonnage.
5. I see you're onto what I was trying to express, regarding the use of a Combined Chemical-SEP (CC-SEP) solution.
6. I also believe it's unethical to not have contingency propulsion solutions, but I think of this aspect of CC-SEP as an investment protection plan.
7. If Hop David even thought about the mass of propellant in the header tanks that BFS requires to actually land on Mars or Earth, then I don't see it in his spreadsheet.  Merely showing that the rocket has enough propellant capacity to support the dV required by a specific maneuver is not an accurate representation of what the vehicle can truly do when laden with the additional propellants in the header tanks that are required to land.  Last but not least, BFR/BFS is a paper rocket.  Nobody knows what the thing will weigh, just like Lockheed-Martin can't tell you what the final weight of Orion will be.
8. BFS is clearly fixed geometry tankage.
9. Agreed.  This is amateur level stuff.  It's good for determining whether or not an assertion is even feasible from a basic math standpoint, but that's it.  If it was professional level stuff, then I probably wouldn't know how to check it.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#66 2018-11-08 19:47:24

kbd512
Administrator
Registered: 2015-01-02
Posts: 5,730

Re: Journey time to Mars...

Louis,

The vision changes and cardiac muscle atrophy remain intractable problems.  Nothing NASA has tried has solved those problems, but they've greatly slowed the cardiac muscle atrophy.  NASA thinks another revision or two to the ARED apparatus can resolve the remaining thigh muscle and cardiac muscle atrophy issues, but they haven't tested those latest changes in space as of yet, thus have no way of knowing if they can completely solve the bone and muscle loss issues.  Once the improve apparatus goes to ISS, we'll know in another few mission cycles.  If they can do that, then there are technological solutions to the vision change problems (adaptive contact lenses).  I've no clue where they are on the immune system issues because I don't follow that research, but my guess is that they're still spinning their wheels.

Radiation protection needs to be addressed using a mitigation strategy involving the use of BNNT (the stuff looks like tufts of cotton, even though it's an entirely different material) fabrics or composites that elastically absorb ion bombardment from GCR's without producing secondary and potentially more damaging particle showers, as Aluminum alloys do.  No reasonable amount of material can stop heavy ion bombardment, but thankfully light ions make up the lion's share of the overall problem.  Current PE materials are still too heavy, but PE foams are a step in the right direction.  That said, BNNT fabric is the star material here, performing better than LH2 and without LH2's considerable storage issues.  NASA wants to make it a structural composite, but I don't think it's the right material for that application.  A simple fabric liner and multi-cell PE water tank or soft bags (for user configurable CME/SPE shielding) inside a suitable carbon fiber composite structure is the right way to block the CME's/SPE's and GCR's.

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#67 2018-11-09 04:38:19

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

This is why I would argue for an extended stay lunar pre-Mars mission (maybe up to a year if time allows), to assess how well bone and muscle loss can be contained in the nearest we have to a Mars like environment. With weighted suits (not an issue for the Apollo missions) , careful selection of crew (because b&m loss varies hugely between individuals) and use of ARED-like exercise machines I think the problem can be kept to an acceptable minimum.

My theory on the immune system is that people's immune systems are getting lazy in the low pathogen environment of a space mission. Some hypothesise about microgravity effects on molecular structures. I don't think the immune issue will be any worse than an extended stay on the ISS.

The radiation problem is something we have solutions for, at least to reduce the risk to an acceptable level.

kbd512 wrote:

Louis,

The vision changes and cardiac muscle atrophy remain intractable problems.  Nothing NASA has tried has solved those problems, but they've greatly slowed the cardiac muscle atrophy.  NASA thinks another revision or two to the ARED apparatus can resolve the remaining thigh muscle and cardiac muscle atrophy issues, but they haven't tested those latest changes in space as of yet, thus have no way of knowing if they can completely solve the bone and muscle loss issues.  Once the improve apparatus goes to ISS, we'll know in another few mission cycles.  If they can do that, then there are technological solutions to the vision change problems (adaptive contact lenses).  I've no clue where they are on the immune system issues because I don't follow that research, but my guess is that they're still spinning their wheels.

Radiation protection needs to be addressed using a mitigation strategy involving the use of BNNT (the stuff looks like tufts of cotton, even though it's an entirely different material) fabrics or composites that elastically absorb ion bombardment from GCR's without producing secondary and potentially more damaging particle showers, as Aluminum alloys do.  No reasonable amount of material can stop heavy ion bombardment, but thankfully light ions make up the lion's share of the overall problem.  Current PE materials are still too heavy, but PE foams are a step in the right direction.  That said, BNNT fabric is the star material here, performing better than LH2 and without LH2's considerable storage issues.  NASA wants to make it a structural composite, but I don't think it's the right material for that application.  A simple fabric liner and multi-cell PE water tank or soft bags (for user configurable CME/SPE shielding) inside a suitable carbon fiber composite structure is the right way to block the CME's/SPE's and GCR's.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#68 2018-11-09 10:47:58

Oldfart1939
Member
Registered: 2016-11-26
Posts: 2,300

Re: Journey time to Mars...

I am highly skeptical about the bone loss experiment referred to above. That said, the bone loss CAN be mitigated against through various hormonal drug treatments using a combination of Calcitonin and Amylin to stimulate Osteoblast formation and Osteoclast suppression, respectively. But I don't like the idea of medicating astronauts where a simple solution using spin gravity is 100 % effective. I don't know whether or not NASA has conducted these hormonal manipulation tests or not, but this was the basis of a research proposal I had started writing back in 2009 before I retired.

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#69 2018-11-09 15:10:35

kbd512
Administrator
Registered: 2015-01-02
Posts: 5,730

Re: Journey time to Mars...

Louis,

I still think the right way to do this is a split-mission using a small fleet of BFS variants.  The primary reasons for this are habitable volume limitations, the radiation problem, and all these new dust particle clouds we've very recently discovered around Earth near the LaGrange Points.  There's no telling where other dust clouds are located since we only discovered those clouds by chance, which means we don't have a good method for locating debris fields that could impact a spacecraft in interplanetary space.  The walls of the Bigelow Aerospace inflatables are well over a meter thick, far superior to what's reasonably achievable with BFS, for absorbing impacts from debris.

Anyway, here's what I was thinking:

ITV - Interplanetary Transfer Vehicle - The combination of a radiation protected BA-2100 inflatable habitation module that stays in orbit and contains consumables for interplanetary transits, docked to a specialist space tug variant of BFS

BFS and special equipment features denoted with asterisks

Vehicle Geometry (VG):
STD - standard, which means standard overall vehicle length and bow canards; suitable for prepared surface landings
SHT - short, which means shorter overall vehicle length and smaller bow canards; suitable for rough field landings
TUG - space tug vehicle without a heat shield or aerodynamic control structures for reentries; Combined Chemical / SEP (CC-SEP) propulsion using Raptor and Aerojet-Rocketdyne X-3 hall thrusters

Vehicle Accommodations (VA):
H - habitable
R - robotic, therefore not habitable

Heat Shield (HS):
NR - No Reentry (not capable of reentry)
SR - Single Reentry (not durable)
MR - Multiple Reentries (durable)

Avionics (A):
R - radiation hardened
N - not radiation hardened

BFS-OC - orbital cargo delivery of satellites and spacecraft components
* VG - STD; VA - R; HS - MR; A-N
* robotic arms for cargo deployment and recovery

BFS-OT - orbital cryogenic propellant delivery; to be tested for satisfactory operation with BFS-OC
* VG - SHT; VA - R; HS - MR; A - N
* propellant tanks slightly longer than standard BFS propellant tanks, since the payload is propellants, to save weight
* specialized pumping and refueling probe equipment installed to avoid docking maneuvers

BFS-ST - orbital transfer space tug that provides propulsion for crew and cargo deliveries to interplanetary destinations
* VG - TUG; VA - R; HS - NR; A - R
* LOX/LCH4 propulsion for orbital transfer
* Argon-based electric propulsion for cruise, course correction, and spiraling into orbit at the destination
* multi-megawatt class deployable thin film solar arrays for primary electrical power
* robotic arms for cargo mating
* chemical propellants replenished using BFS-OT (in LEO; no docking required) or BFS-MP (in LMO; requires docking)

BFS-MC (robotic) - contains the LOX/LCH4 propellant plant
* VG - SHT; VA - R; HS - SR; A - R
* deployable thin film solar arrays for primary power to avoid complications associated with construction of permanent surface structures
* onboard CH4 fuel cell for startup / shutdown / emergency power
* specialized propellant tank insulation for long duration propellant storage
* specialized pumping and propellant transfer equipment
* provides a source of spare engines or engine components for the BFS-MP, if required
* stays on Mars and never returns to Earth

BFS-MP (crewed) - contains the consumables and surface habitation inflatables for living on the surface of Mars
* VG - SHT; VA - H; HS - MR; A - R
* designed only for crew transfer in Mars orbit and subsequent landing on Mars, rather than long duration habitation in deep space
* crew radiation protection provided by cargo and consumables, rather than liners, to save weight
* more pressurized volume allocated to cargo than crew accommodations
* specialized cargo transfer equipment for deployment of surface habitation structures and transfer of consumables
* can also be repositioned using suborbital hops, as required
* stays on Mars and never returns to Earth

This family of vehicles will work with each other to make the Mars missions possible

Sprint 1
1. BFS-MC is delivered to orbit
2. Multiple BFS-OT flights transfer fuel to BFS-MC until the tanks have been filled
3. BFS-MC performs the TMI burn
4. BFS-MC arrives at Mars and performs EDL burns
5. The fuel cell is activated to provide startup power, the solar arrays are deployed, and propellant production tests begin
6. If sufficient propellant production rate is achieved, then the next sprint begins

Sprint 2
1. BFS-OC delivers the BFS-MP to ISS for temporary storage
2. BFS-ST delivered to orbit below ISS
3. Multiple BFS-OT flights transfer fuel to BFS-ST and BFS-MP until the tanks have been filled
4. BFS-ST performs the TMI burn
5. BFS-ST arrives at Mars and circularizes its orbit, waiting for the ITV

Sprint 3
1. BFS-OC delivers the ITV to ISS for temporary storage while inflation, checkout, consumables and crew loading takes place
2. BFS-ST delivered to orbit below ISS
3. Multiple BFS-OT flights transfer fuel to BFS-ST until the tanks have been filled
4. BFS-OC delivers the consumables to ISS, for transfer to the ITV
5. Falcon 9 delivers the crew to the ITV using Dragon 2
6. ISS disconnects the crewed ITV from ISS, whereupon the ITV is mated to BFS-ST
7. ITV’s BFS-ST performs the TMI burn
8 ITV arrives at Mars and BFS-ST circularizes its orbit

Sprint 4
1. ITV docks with BFS-MP in Mars orbit for crew transfer to the BFS-MP
2. BFS-MP performs the EDL burns to land on Mars
3. Crew unpacks, inflates, and transfer consumables from BFS-MP to the surface habitat
4. Crew performs surface mission until BFS-MC propellant production is complete
5. Crew transfers propellant from BFS-MC to BFS-MP
6. Crew boards BFS-MP for return to the ITV

Sprint 5
1. BFS-MP ascends to orbit and docks with ITV
2. Crew transfers to ITV in preparation for TEI
3. ITV performs TEI burn
4. BFS-ST previously attached to BFS-MP performs TEI burn
5. ITV returns to Earth and spirals into LEO
6. ITV and BFS-ST docks at ISS
7. ITV crew departs ISS aboard their Dragon 2
8. Dragon 2 performs reentry burn and returns to Earth

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#70 2018-11-09 15:37:22

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

We can perhaps agree: 1. That the problem can be greatly ameliorated by exercise programmes/specialist equipment together with space medicine. 2. Genetics is v. important. Some people lose bone and muscle mass at a much higher rate x2 or x3 compared with others.  So informed selection is key.  3. Commitment/psychology is important. Valeri Polyakov was incredibly committed to demonstrating humans could live in space healthily - and he took the record with over 400 days in space in zero G. 4.  Weighted suits will have some ameliorative effect. 5. With a cargo of 500 tonnes it is not impossible for the Mars Pioneers to take a 1G centrifuge for the Mars surface which could be assembled if needs be - ie in event of a medical emergency. Whether you need to go further is a matter of debate.

Oldfart1939 wrote:

I am highly skeptical about the bone loss experiment referred to above. That said, the bone loss CAN be mitigated against through various hormonal drug treatments using a combination of Calcitonin and Amylin to stimulate Osteoblast formation and Osteoclast suppression, respectively. But I don't like the idea of medicating astronauts where a simple solution using spin gravity is 100 % effective. I don't know whether or not NASA has conducted these hormonal manipulation tests or not, but this was the basis of a research proposal I had started writing back in 2009 before I retired.

Last edited by louis (2018-11-09 15:45:44)


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#71 2018-11-09 18:23:57

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 25,731

Re: Journey time to Mars...

Thanks for the list of vehicle acronyms in your post kbd512 and they have been added to the bfr acronym topic....

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#72 2018-11-09 19:24:36

louis
Member
From: UK
Registered: 2008-03-24
Posts: 7,208

Re: Journey time to Mars...

Space X will be sending cargo BFSs to Mars first, two years before any humans are sent there. If the cargo BFSs make it unscathed , I think that is good enough evidence to suport humans being transported to Mars.  A Mars mission is not without risks. We are probably in the Formula One motor racing risk zone I think. It's dangerous but not absurdly so.

I am sure we could all tinker with the mission architecture but it is what it is unless and until Space X tell us it's something different. No one else is in the game.



kbd512 wrote:

Louis,

I still think the right way to do this is a split-mission using a small fleet of BFS variants.  The primary reasons for this are habitable volume limitations, the radiation problem, and all these new dust particle clouds we've very recently discovered around Earth near the LaGrange Points.  There's no telling where other dust clouds are located since we only discovered those clouds by chance, which means we don't have a good method for locating debris fields that could impact a spacecraft in interplanetary space.  The walls of the Bigelow Aerospace inflatables are well over a meter thick, far superior to what's reasonably achievable with BFS, for absorbing impacts from debris.

Anyway, here's what I was thinking:

ITV - Interplanetary Transfer Vehicle - The combination of a radiation protected BA-2100 inflatable habitation module that stays in orbit and contains consumables for interplanetary transits, docked to a specialist space tug variant of BFS

BFS and special equipment features denoted with asterisks

Vehicle Geometry (VG):
STD - standard, which means standard overall vehicle length and bow canards; suitable for prepared surface landings
SHT - short, which means shorter overall vehicle length and smaller bow canards; suitable for rough field landings
TUG - space tug vehicle without a heat shield or aerodynamic control structures for reentries; Combined Chemical / SEP (CC-SEP) propulsion using Raptor and Aerojet-Rocketdyne X-3 hall thrusters

Vehicle Accommodations (VA):
H - habitable
R - robotic, therefore not habitable

Heat Shield (HS):
NR - No Reentry (not capable of reentry)
SR - Single Reentry (not durable)
MR - Multiple Reentries (durable)

Avionics (A):
R - radiation hardened
N - not radiation hardened

BFS-OC - orbital cargo delivery of satellites and spacecraft components
* VG - STD; VA - R; HS - MR; A-N
* robotic arms for cargo deployment and recovery

BFS-OT - orbital cryogenic propellant delivery; to be tested for satisfactory operation with BFS-OC
* VG - SHT; VA - R; HS - MR; A - N
* propellant tanks slightly longer than standard BFS propellant tanks, since the payload is propellants, to save weight
* specialized pumping and refueling probe equipment installed to avoid docking maneuvers

BFS-ST - orbital transfer space tug that provides propulsion for crew and cargo deliveries to interplanetary destinations
* VG - TUG; VA - R; HS - NR; A - R
* LOX/LCH4 propulsion for orbital transfer
* Argon-based electric propulsion for cruise, course correction, and spiraling into orbit at the destination
* multi-megawatt class deployable thin film solar arrays for primary electrical power
* robotic arms for cargo mating
* chemical propellants replenished using BFS-OT (in LEO; no docking required) or BFS-MP (in LMO; requires docking)

BFS-MC (robotic) - contains the LOX/LCH4 propellant plant
* VG - SHT; VA - R; HS - SR; A - R
* deployable thin film solar arrays for primary power to avoid complications associated with construction of permanent surface structures
* onboard CH4 fuel cell for startup / shutdown / emergency power
* specialized propellant tank insulation for long duration propellant storage
* specialized pumping and propellant transfer equipment
* provides a source of spare engines or engine components for the BFS-MP, if required
* stays on Mars and never returns to Earth

BFS-MP (crewed) - contains the consumables and surface habitation inflatables for living on the surface of Mars
* VG - SHT; VA - H; HS - MR; A - R
* designed only for crew transfer in Mars orbit and subsequent landing on Mars, rather than long duration habitation in deep space
* crew radiation protection provided by cargo and consumables, rather than liners, to save weight
* more pressurized volume allocated to cargo than crew accommodations
* specialized cargo transfer equipment for deployment of surface habitation structures and transfer of consumables
* can also be repositioned using suborbital hops, as required
* stays on Mars and never returns to Earth

This family of vehicles will work with each other to make the Mars missions possible

Sprint 1
1. BFS-MC is delivered to orbit
2. Multiple BFS-OT flights transfer fuel to BFS-MC until the tanks have been filled
3. BFS-MC performs the TMI burn
4. BFS-MC arrives at Mars and performs EDL burns
5. The fuel cell is activated to provide startup power, the solar arrays are deployed, and propellant production tests begin
6. If sufficient propellant production rate is achieved, then the next sprint begins

Sprint 2
1. BFS-OC delivers the BFS-MP to ISS for temporary storage
2. BFS-ST delivered to orbit below ISS
3. Multiple BFS-OT flights transfer fuel to BFS-ST and BFS-MP until the tanks have been filled
4. BFS-ST performs the TMI burn
5. BFS-ST arrives at Mars and circularizes its orbit, waiting for the ITV

Sprint 3
1. BFS-OC delivers the ITV to ISS for temporary storage while inflation, checkout, consumables and crew loading takes place
2. BFS-ST delivered to orbit below ISS
3. Multiple BFS-OT flights transfer fuel to BFS-ST until the tanks have been filled
4. BFS-OC delivers the consumables to ISS, for transfer to the ITV
5. Falcon 9 delivers the crew to the ITV using Dragon 2
6. ISS disconnects the crewed ITV from ISS, whereupon the ITV is mated to BFS-ST
7. ITV’s BFS-ST performs the TMI burn
8 ITV arrives at Mars and BFS-ST circularizes its orbit

Sprint 4
1. ITV docks with BFS-MP in Mars orbit for crew transfer to the BFS-MP
2. BFS-MP performs the EDL burns to land on Mars
3. Crew unpacks, inflates, and transfer consumables from BFS-MP to the surface habitat
4. Crew performs surface mission until BFS-MC propellant production is complete
5. Crew transfers propellant from BFS-MC to BFS-MP
6. Crew boards BFS-MP for return to the ITV

Sprint 5
1. BFS-MP ascends to orbit and docks with ITV
2. Crew transfers to ITV in preparation for TEI
3. ITV performs TEI burn
4. BFS-ST previously attached to BFS-MP performs TEI burn
5. ITV returns to Earth and spirals into LEO
6. ITV and BFS-ST docks at ISS
7. ITV crew departs ISS aboard their Dragon 2
8. Dragon 2 performs reentry burn and returns to Earth


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#73 2018-11-09 20:14:17

kbd512
Administrator
Registered: 2015-01-02
Posts: 5,730

Re: Journey time to Mars...

Louis,

By that logic, SpaceX isn't in the game.

There would be no thin film solar arrays without NASA, although there is now.

There is no LOX/LCH4 plant without NASA.

There is no surface habitation solution without NASA.

There is no long duration life support equipment without NASA.

There is no adequate radiation protection without NASA.

There is no low-G / micro-G mitigation strategy without NASA.

SpaceX doesn't have trained astronauts, either, although there are other places to get them from besides NASA.

Did you catch on to the problems, yet?

Nobody has the solutions to all problems.

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#74 2018-11-10 11:14:26

Terraformer
Member
From: Ceres
Registered: 2007-08-27
Posts: 3,528
Website

Re: Journey time to Mars...

To be fair, though, there aren't many of those things *with* NASA.


"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony

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#75 2018-11-10 11:35:05

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 25,731

Re: Journey time to Mars...

Terraformer
Nasa is investigating each as Space x is not and are just making rockets.
Nasa has hardware that works for some of these and space x does not.

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