Methane backup not nuclear vs solar

tahanson43206 · 2019-10-08 12:42:43

For GW Johnson ... Re #75 and finding things in forum database in general...

Thanks again for adding the text strings for your series on EducationDoneRight.

That is (more or less) what I had in mind, and your implementation was spot on.

Every forum package has strengths and weaknesses. The product chosen by "someone" back in the very early days of this forum is "FluxBB".

RobertDyck has passed along several tips about how to work with this package, and I appreciate it. There is a separate forum where people who run FluxBB congregate and pass tips back and forth. I have only learned a tiny fraction of what is knowable, because my PRIMARY interest is trying to follow the discussions here, and to participate if that seems to make sense in a particular situation.

I have found that it is possible to design search strings that allow a particular post to be brought back as a unique result. I use this capability every day, in order to find a particular post which is the foundation for the calendar updates.

Every now and then, I'll toss a search request into FluxBB, and it comes back with history from this forum that I often find intriguing, so that I lose track of what I was originally trying to do. Almost everything void contributed falls into that category, for example, but there are many other authors who have a permanent place in the historical record who I find timely or interesting or enlightening or some combination.

All that said, I'll try to think of text strings that would help to find specific posts.

Hope this helps!

(th)

louis · 2019-10-08 12:46:46

In one job I had I used to receive about 100,000 journals - a few tons - for local distribution by 40 staff and manhandled a few hundred kgs of the stuff myself, so less of the ad hominems, I do know a little about unloading and logistics!

You know how long it takes to get in and out of a spacesuit I presume? What kind of toll would it take on pioneers having to don and remove spacesuits sol after sol after sol at the start of a two year mission? How long could they operate an EVA, given that gravity makes life support equipment heavier? How long would it take to shower off Mars dust each time? And dry off/decontaminate space suits?

Well even on your description you have 100 large packaged items that will require unpacking - on the Mars surface...unless you're planning on having 100 habs for your KP units. So how long will that surface unpacking take?

Remember, the PV system can be unloaded robotically.

I hope we do get MCPs but my approach to Mars Mission One is that any EVAs would be purely for the cameras, for the PR. Generally humans stay inside habs or inside pressurised rovers.

You seem to be changing tune and now arguing that even if you arrive in the middle of a dust storm, solar will be able to power your Starship from solar panels, even though you can't operate your nuclear reactors. At least on my approach, you have meth-ox on board to power electricity generators should the dust storm create a problem in your PV supply.

I think you misread what I wrote - I was suggesting the Starships will eventually have solar "wings" and that the Mk 1 and 2 don't because they don't need them.

I never bet against Musk. The six landing legs don't look convincing, but we'll see.

GW Johnson wrote:

Louis:
It is pathetically obvious that you have never gotten your hands dirty loading and unloading anything bigger than a sack of groceries.
I can (and I do) move a ton of steel across the floor of my shop all by myself, with a simple pallet jack. I could do that wearing an idiotically clumsy spacesuit. These things are designed to make such activities easy.
The stuff you ship to Mars must be banded to pallets (which in turn must be secured to the cargo deck during the flight). When you lower your crane elevator the first time, the biggest items on it are a roll of landing mat material, a big roll of sheet metal, and a pallet jack. You roll out the landing mat onto the dirt as a firm substrate, unroll the sheet metal on top of it to provide a smooth surface. Then you can use the pallet jack to unload onto the surface from the crane elevator platform.
Your next trip down is the small electric bulldozer/front end loader vehicle, which you drive onto the crane platform, and you drive it off when at the surface. It will pick up and move the cargo packages offloaded onto the landing mat structure.
The rest of it is just packages that are palleted. Use a pallet jack in the cargo bay to roll the pallets onto the crane platform. Then lower it. And use the other pallet jack to roll that pallet onto the landing mat, where the bulldozer/loader picks it up and moves it where it is needed. You can accumulate palletized packages on the landing mat surface while the bulldozer/loader is taking them to wherever they go, one by one.
You need one guy to operate the crane, who can double (or not) as the pallet jack operator in the cargo bay. You need another guy operating the pallet jack on the landing mat. You need a driver for the dozer/loader vehicle.
That's how a crew of 3 to 4 can unload 100 tons from a "Starship" in a day, while wearing those idiotic balloon spacesuits. I find it rather pathetic that I have to explain this operation to you. If you but look around, you can watch people doing this sort of thing at construction sites and freight ports every single day!
Why aren't you worried about getting better, more supple spacesuits, instead of worrying about this unload operation? That's my question to you.
And no I DID NOT say to fire up Kilopower units to supply electricity aboard "Starship" during the transits. I said they will have to do what was pictured in the 2017 and 2018 presentations: big solar panel wings. There is no place in the "Starship" concept to safely operate a nuclear reactor, even a compact one like Kilopower. I've said that multiple times on these forums. You simply refuse to read and understand it.
The fact that the so-called "Starship Mark 1" vehicles do not have this feature is meaningless. Flight test experimental vehicles rarely resemble the finished products they finally generate. I've written this before on these forums. You simply refuse to read and understand that, too.
And by the way, the six-leg landing leg design, of diameter only 9-ish meters, may double landing pad-foot area, but it makes the topple-over risk far worse. The footprint is just way too narrow for the cg height. That Mark 1 experimental flight test vehicle will NEVER successfully land off of a thick, steel-reinforced concrete pad! It looks NOTHING like whatever might successfully make a rough field landing on Mars or the moon!
GW

Last edited by louis (2019-10-08 15:21:47)

Oldfart1939 · 2019-10-08 15:01:39

Louis-
In response to your comment about betting heavily on Elon Musk; I'll bet on the Laws of Physics, regarding the stability or lack thereof, in Starship with 6 small landing legs. You really need to go take a University course called "Statics," which is the analytical mechanics of bodies at rest. In that course, you will learn of something called a Center of mass of---say the Starship. The area within the feet of the starship represents the region above which the center of mass must remain for stability. What we will have if the design moves forward is a hexagon with points of the hexagon each 15 meters from either adjacent point. If the starship happens to land on a slope, or several of the points of landing contact are soft the, ship tilts to the side, then that center of mass better be directly above the hexagon and remaining within it's bounds. If not--simple laws of physics, which cannot be repealed by Elon, dictate that it topples over into a heap of once beautiful S.S. 301.

GW Johnson · 2019-10-08 15:43:28

Louis:

What I described was a way for 3 or 4 spacesuited crew to unload 100 tons in only 1 day, given the right tools. Or two. Make it easy on them.

The tools we would send with the ship to Mars would be high-tech lightweight versions (and therefore quite expensive, too) of the sorts of steel handing tools we have used for over a century here on Earth. Unloading is JUST NOT A BUGABOO, whether anybody at Spacex has yet addressed this or not!

And I think not yet, as they have their hands full making Starship work at all, simultaneous with recovering from that screw-up with Crew Dragon.

I don't know what effort and time the surface unpacking might take. NO ONE DOES, because planning details like that are premature, until the ship is proven in flight tests. Actually it doesn't matter much; the crew is living in the Starship until they get habs set up and operational. If that takes months, so what? As long as they have power for life support in that Starship they are living in!

What I said about power for Starship life support related to the transit between planets. That takes solar panel wings, something Musk had in his 2017 and 2018 concept presentations. Out in space away from any planet's shadow, all you have to do is orient these solar panels sunward to get "24/7 electricity no matter what".

There has to be a way to deploy these things to one extent or another on the ground, with batteries to get you through the night, built into any such vehicle that goes to Mars. You could unroll a set of thin film PV's, relying on the ship's batteries while you do this, knowing the desert grit will destroy them "soon", but what if there's a dust storm that makes them too ineffective right off the bat?

Instead, you set up as fast as you can one kilopower unit and hook it to the ship. Then use your bulldozer to bulldoze a berm around it and turn it on. That works, dust storm or no. And it relieves the ship's batteries soonest, and with better reliability. This shouldn't take but a day or two. Which sizes the ship's batteries.

Bear in mind that my spacesuited crew are mostly just pushing buttons or driving a dozer to unload. The hardest work they do is manually pushing that pallet jack around to load and unload the elevator platform. Unpacking is very likely the more demanding work for somebody in a spacesuit.

And, no, I do NOT believe a working PV system can be unloaded and set up robotically! If it can, then so can a Kilopower unit! But I do believe both will require humans there connecting cables and using meters to test connections before throwing the "on" switch. That can be done in a spacesuit; and a supple one means you can do more in day before getting too tired.

People will have real work to do outside in a suit. That is inevitable. You are too trusting of robotics and remote manipulators. In my experience, that is just more failure modes. Such are useful, but you must plan on their failure, with manual recovery.

GW

Last edited by GW Johnson (2019-10-08 15:45:41)

louis · 2019-10-08 15:44:58

Well I would respond:

1. It won't be landing on anything more than a 5% slope. That will be guaranteed by satellite surveys.

2. It looks like the legs might be hydraulic, so it's possible Space X have a gyro technology that can offer virtually instant stabilisation.

3. I don't think any of the ground will be soft. Again I think this will be established by rigorous statellite surveys.

4. Unlike on Earth, in terms of weather, the worst Mars has to offer is 16 MPH equivalent force winds.

5. A lot may depend on the pad design. I can think of ways you might add stability through pad design.

Oldfart1939 wrote:

Louis-
In response to your comment about betting heavily on Elon Musk; I'll bet on the Laws of Physics, regarding the stability or lack thereof, in Starship with 6 small landing legs. You really need to go take a University course called "Statics," which is the analytical mechanics of bodies at rest. In that course, you will learn of something called a Center of mass of---say the Starship. The area within the feet of the starship represents the region above which the center of mass must remain for stability. What we will have if the design moves forward is a hexagon with points of the hexagon each 15 meters from either adjacent point. If the starship happens to land on a slope, or several of the points of landing contact are soft the, ship tilts to the side, then that center of mass better be directly above the hexagon and remaining within it's bounds. If not--simple laws of physics, which cannot be repealed by Elon, dictate that it topples over into a heap of once beautiful S.S. 301.

GW Johnson · 2019-10-08 15:49:49

Louis:

Your point number 3 is wrong. Satellite remote sensing CANNOT establish surface bearing strength. That satellite result is inference only, and inference is not reliable enough to risk lives upon. Throughout the space program, ground truth has always differed from remote sensing results, true to this very day.

And Oldfart1939 is correct about the static stability problem. The ship that lands on Mars (or the moon) will NOT look like that 6-legged prototype. What they have for a prototype will have to be blown up in an abort, since it cannot land on a wet sand beach.

GW

Last edited by GW Johnson (2019-10-08 15:53:40)

GW Johnson · 2019-10-08 16:02:21

A Note on Solar vs Dust Storms on Mars

NASA published on its website for public consumption an informational report on the Opportunity and Spirit rovers late in July 2007, when both were still running (although Spirit was no longer able to move). This public information document gave some actual numbers in the text, and included some comparative photos taken as the dust storm darkened the sky.

You can still find it on the internet at https://www.nasa.gov/mission_pages/mer/ … 70720.html. It was after this publication date of 7-20-07, that the Opportunity rover died in the cold, because its batteries could not be charged enough to keep it from freezing. Which means that the sunlight was even dimmer than is shown in this public release document.

Data from the article: normal 700 Wh/day, dropped to 128 Wh/day (18%). The engineers calculated they needed 130 Wh/day with everything shut down, just running the heaters. This data in this document was taken before the batteries drained and the rover died. Quoting NASA: “99% reduction of direct beam radiation, leaving only diffuse radiation”. Diffuse is usually at most 10% of total.

It looks like recoverable power levels went down to something in the 5-10% range (all diffuse) for too long, which is what drained the batteries and “killed” the rover in the cold. Looking at the published photos in the NASA document, that one on the right is just plain dark. And that’s what it looked like before it got so dark as to drain the batteries and “kill” the rover in the cold. No one can argue with that data, regardless of its academic pedigree, or lack thereof in this case.

So, regardless of it being a scholarly paper, that “35% of normal sunlight” figure in the paper Louis cited is just utter nonsense. What that tells you is something you really ought to know already: academic credentials actually mean little as far as veracity is concerned; I know, I’ve been one, among many other things. Insolation (and solar panel recovery of it) actually got down to a lot lower than that 128 Wh/day figure quoted by NASA, and stayed there for a long time, or else Opportunity would not have died in the cold and the dark!

And the Mariner 9 event in 1969 was worse still: no one since has reported the volcanoes on Tharsis Mons to be obscured in the dust layer. But in 1969, Mariner 9 observed that they were completely obscured, for some 6 months. The Martian atmosphere didn’t fully clear for 9 months. THAT IS the Mariner 9 data. And no one really knows when it actually started: Mariner 9 found Mars that way when it arrived! THAT is the worst case known! Bar none! So far.

So, for design purposes, you plan on zero solar available for up to (at least) 9 months. Or you kill your crew on Mars if there is no other source of power besides solar. Take your pick.

GW

louis · 2019-10-08 16:05:45

Unloading is indeed a bugaboo, as is deployment when it comes to nuclear power reactors. No use trying to blithely say things like "hook up a KP unit to the Starship". It's going to be radioactive. How far away does it have to be? I think you are totally underestimating the strain of working in space suits in an unfamiliar landscape on repetitive tasks (not on interesting science experiments or mini-expeditions as happened under the Apollo programme).

You can't compare a KP unit with a reel of flexible PV in terms of unloading. You could roll off a PV reel on a chute on to an inflatable bed and it could roll on to the surface with minimal packaging. Robot rovers could collect them whereever they land and then roll them out like the Rapid Roll PV. No way could you treay KP units like that.

GW Johnson wrote:

Louis:
What I described was a way for 3 or 4 spacesuited crew to unload 100 tons in only 1 day, given the right tools. Or two. Make it easy on them.
The tools we would send with the ship to Mars would be high-tech lightweight versions (and therefore quite expensive, too) of the sorts of steel handing tools we have used for over a century here on Earth. Unloading is JUST NOT A BUGABOO, whether anybody at Spacex has yet addressed this or not!
And I think not yet, as they have their hands full making Starship work at all, simultaneous with recovering from that screw-up with Crew Dragon.
I don't know what effort and time the surface unpacking might take. NO ONE DOES, because planning details like that are premature, until the ship is proven in flight tests. Actually it doesn't matter much; the crew is living in the Starship until they get habs set up and operational. If that takes months, so what? As long as they have power for life support in that Starship they are living in!
What I said about power for Starship life support related to the transit between planets. That takes solar panel wings, something Musk had in his 2017 and 2018 concept presentations. Out in space away from any planet's shadow, all you have to do is orient these solar panels sunward to get "24/7 electricity no matter what".
There has to be a way to deploy these things to one extent or another on the ground, with batteries to get you through the night, built into any such vehicle that goes to Mars. You could unroll a set of thin film PV's, relying on the ship's batteries while you do this, knowing the desert grit will destroy them "soon", but what if there's a dust storm that makes them too ineffective right off the bat?
Instead, you set up as fast as you can one kilopower unit and hook it to the ship. Then use your bulldozer to bulldoze a berm around it and turn it on. That works, dust storm or no. And it relieves the ship's batteries soonest, and with better reliability. This shouldn't take but a day or two. Which sizes the ship's batteries.
Bear in mind that my spacesuited crew are mostly just pushing buttons or driving a dozer to unload. The hardest work they do is manually pushing that pallet jack around to load and unload the elevator platform. Unpacking is very likely the more demanding work for somebody in a spacesuit.
And, no, I do NOT believe a working PV system can be unloaded and set up robotically! If it can, then so can a Kilopower unit! But I do believe both will require humans there connecting cables and using meters to test connections before throwing the "on" switch. That can be done in a spacesuit; and a supple one means you can do more in day before getting too tired.
People will have real work to do outside in a suit. That is inevitable. You are too trusting of robotics and remote manipulators. In my experience, that is just more failure modes. Such are useful, but you must plan on their failure, with manual recovery.
GW

louis · 2019-10-08 17:07:23

Well everything's inference until you get there!

https://en.wikipedia.org/wiki/SHARAD

I think NASA and ESA are capable, with the ground penetrating satellite radar and other measurements, to establish if for instance you are landing on a mass of volcanic rock, or sand or gravel or shale.

Gathering all the evidence together, the nature of the landing site can be established with extremely high accuracy. But more than that before any humans land, you will have cargo ships at the location which will be able to carry out further remote surveying to give you something v. close to 100% accuracy.

GW Johnson wrote:

Louis:
Your point number 3 is wrong. Satellite remote sensing CANNOT establish surface bearing strength. That satellite result is inference only, and inference is not reliable enough to risk lives upon. Throughout the space program, ground truth has always differed from remote sensing results, true to this very day.
And Oldfart1939 is correct about the static stability problem. The ship that lands on Mars (or the moon) will NOT look like that 6-legged prototype. What they have for a prototype will have to be blown up in an abort, since it cannot land on a wet sand beach.
GW

SpaceNut · 2019-10-08 17:36:16

If the rover had been able to tilt its panels even 5 degrees toward the sun from flush or level then it could have gotten the heat it needed to make it through the winter but it had already been dragging its wheel and such other techiques to move as it was.

What is needed is the means to move the unload cargo off from the crane hook and take it from that location to where it needs to be without any manpower in a suit required. A sort of a robotic controlled crawler much smaller that what we used for the apollo program, shuttle and now for the sls.

https://en.wikipedia.org/wiki/Crawler-transporter

Mounted on it is the crane to set its payload on the ground so that it can go back and get another load.

GW Johnson · 2019-10-08 18:20:39

Louis:

Your denial of facts is not acceptable. I like the questions you raise, but I hate your denial of facts that you want to disagree with.

A KP unit exposes you to LESS than a chest X-ray if you get close (within a few feet) before it is turned on. THAT'S A FACT! And that exposure is range-dependent, at the usual 1/r-squared law! THAT'S ALSO A FACT! There simply is no credible radiation hazard until you turn the damned thing on!

I got 10-ish times that exposure in the early 1950's watching early model TV's with unshielded Klystron tubes, simply because I'm an old man. Nobody knew any better back then. My father-in-law got 10-100 times my exposure (100-1000 times what early TV caused) doing 1940's-1950's radio and radar work in the Navy. Neither of us ever came down with cancer from it, much less any other sign of radiation poisoning at all! Your fears of properly-engineered nuclear power sources are just groundless.

Landing pad size: "Well everything's inference until you get there!" -- which is PRECISELY my point! Weight divided by pad area may not exceed the min credible value for surface bearing strength (a thing measured in pressure units). That is something FUNDAMENTALLY not knowable until AFTER you get there and do the tests. Yourself. In person. For what Mars generally seems to be (fine loose, dry sand, with or without rocks), that number is 0.1 to 0.2 MPa. DEAL WITH IT!

If that applied bearing pressure (weight/area) exceeds the safely allowable bearing pressure (0.1 to at most 0.2 MPa), your landing legs sink into the surface like tent stakes instead of snowshoes. Unevenly! Inherently unevenly! And the ship then topples over and explodes. PERIOD!!! Spacex has yet to deal with that issue.

As for radioactivity in general, which you so vociferously fear, the natural background world-wide is 0.3 REM per year. (100 REM = 1 Sievert, FYI.) It varies from that, to up to 10 times larger (!!!!), depending upon where you are. Higher altitudes and metal mines increase the exposure. And about a third of that natural background actually comes from coal power plants, because there are radioactive materials that are inherently part of natural coal! No way around that fact! And FACT it is!!! Hasn't hurt you yet, has it?

GW

Last edited by GW Johnson (2019-10-08 18:22:48)

SpaceNut · 2019-10-08 18:31:21

Here is the Landing legs for the BFR topic

Still searching for active numbers for the reactor units...

Oldfart1939 · 2019-10-08 18:34:07

Here is a NASA paper on Kilopower nuclear reactors:

https://ntrs.nasa.gov/archive/nasa/casi … 011723.pdf

SpaceNut · 2019-10-08 18:47:38

Here is the one line in the document

conical LiH/W shadow shield provides electronics-rated radiation protection at the
science payload, assumed to be 10 m from the core.

https://www.theguardian.com/news/databl … vels-guide

Edit 2nd time to add content
https://permalink.lanl.gov/object/tr?wh … R-15-25540
Los Alamos KiloPower Project - KRUSTY Experiment Nuclear Design

louis · 2019-10-09 02:06:58

I don't think I said there was health threat until it's turned on. That was my understanding - now from what you say I am not so sure... even a small dose over six months could amount to a large exposure, so again this might affect where you store them on a Starship.

I don't particularly have fears about the KP units. I have fears about the management systems required to deploy them affecting mission efficiency. No one has yet come up with a credible plan for deployment.

re the landing area (a) as already stated, when humans travel there you will know about ground conditions from the cargo ships with a high degree of uncertainty (b) I think when you get a mass of volcanic rock hundreds of metres thick and penetrating radar shows to be solid, I think the chance of a collapse due to a Starship landing or being in position is effectively zero. I've never heard of for instance a granite mountain suddenly collapsing. Sink holes appear where you have loose regolith, certainly. Limestone can be dissolved by water. But volcanic rock? I'm not saying it's absolutely impossible but it must be so rare...and where it does happen there would probably be warning signs that scientists could observe.

GW Johnson wrote:

Louis:
Your denial of facts is not acceptable. I like the questions you raise, but I hate your denial of facts that you want to disagree with.
A KP unit exposes you to LESS than a chest X-ray if you get close (within a few feet) before it is turned on. THAT'S A FACT! And that exposure is range-dependent, at the usual 1/r-squared law! THAT'S ALSO A FACT! There simply is no credible radiation hazard until you turn the damned thing on!
I got 10-ish times that exposure in the early 1950's watching early model TV's with unshielded Klystron tubes, simply because I'm an old man. Nobody knew any better back then. My father-in-law got 10-100 times my exposure (100-1000 times what early TV caused) doing 1940's-1950's radio and radar work in the Navy. Neither of us ever came down with cancer from it, much less any other sign of radiation poisoning at all! Your fears of properly-engineered nuclear power sources are just groundless.
Landing pad size: "Well everything's inference until you get there!" -- which is PRECISELY my point! Weight divided by pad area may not exceed the min credible value for surface bearing strength (a thing measured in pressure units). That is something FUNDAMENTALLY not knowable until AFTER you get there and do the tests. Yourself. In person. For what Mars generally seems to be (fine loose, dry sand, with or without rocks), that number is 0.1 to 0.2 MPa. DEAL WITH IT!
If that applied bearing pressure (weight/area) exceeds the safely allowable bearing pressure (0.1 to at most 0.2 MPa), your landing legs sink into the surface like tent stakes instead of snowshoes. Unevenly! Inherently unevenly! And the ship then topples over and explodes. PERIOD!!! Spacex has yet to deal with that issue.
As for radioactivity in general, which you so vociferously fear, the natural background world-wide is 0.3 REM per year. (100 REM = 1 Sievert, FYI.) It varies from that, to up to 10 times larger (!!!!), depending upon where you are. Higher altitudes and metal mines increase the exposure. And about a third of that natural background actually comes from coal power plants, because there are radioactive materials that are inherently part of natural coal! No way around that fact! And FACT it is!!! Hasn't hurt you yet, has it?
GW

louis · 2019-10-09 02:08:07

Presumably that's electronics and not human rated protection.

SpaceNut wrote:

Here is the one line in the document
conical LiH/W shadow shield provides electronics-rated radiation protection at the
science payload, assumed to be 10 m from the core.
https://www.theguardian.com/news/databl … vels-guide
Edit 2nd time to add content
https://permalink.lanl.gov/object/tr?wh … R-15-25540
Los Alamos KiloPower Project - KRUSTY Experiment Nuclear Design

louis · 2019-10-09 02:16:29

As always with Kilopower, things are not at all clear. That paper says the overall system will give 2 W per kg. So for a 10 Kw system that would be 5 tons not 1.5 tons!

The paper is 5 years old so not that useful probably. More recent stuff I've read suggests 1.5 tons is nearer the mark.

Oldfart1939 wrote:

Here is a NASA paper on Kilopower nuclear reactors:
https://ntrs.nasa.gov/archive/nasa/casi … 011723.pdf

louis · 2019-10-09 02:43:56

One other point about the landing stability of the Starship - it was redesigned, was it not, to provide for some cargo at the base. Presumably this would act like ballast. At least theoretically (assuming you haven't landed on a potential sinkhole) on a sub 5% slope and with sufficient ground ballast the thing could not fall over could it? So it then becomes a question of how much ground ballast do you need and to what height to ensure that theoretical stabilty? I'm guessing if the whole thing weighs 250 tons on landing but you've got 75 tons of "ballast" cargo, nearly a third of the mass, at the base of the rocket maybe that would be enough...

elderflower · 2019-10-09 07:04:33

There could be widespread duricrust with sand and dust on top and god knows what underneath. The only way of finding out is to drill a lot of holes through the crust in the area where you intend to land, and examine the cores. That way you can determine a design figure for the bearing strength of the soil.

louis · 2019-10-09 07:22:36

Wikipedia tells me duricrust is formed over sedimentary bodies, not volcanic rock.

https://en.wikipedia.org/wiki/Duricrust

We're talking I think about plaforms of volcanic rock that are fully exposed. These will have been surveyed by ground penetrating radar to ensure there are no anomalies below the surface. They will be sand free and photography will establish the size of any rocks and stones down to 6 cms.

elderflower wrote:

There could be widespread duricrust with sand and dust on top and god knows what underneath. The only way of finding out is to drill a lot of holes through the crust in the area where you intend to land, and examine the cores. That way you can determine a design figure for the bearing strength of the soil.

elderflower · 2019-10-09 07:27:58

In order to land Musk's ship we need a lot of atmospheric travel in a relatively high density to bleed off energy. That means the landing zone will be at low level and therefore quite likely to be sedimentary.

louis · 2019-10-09 07:58:15

Well if so, I would agree that would be more problematic.

elderflower wrote:

In order to land Musk's ship we need a lot of atmospheric travel in a relatively high density to bleed off energy. That means the landing zone will be at low level and therefore quite likely to be sedimentary.

GW Johnson · 2019-10-09 12:49:43

The ballast idea in post 93 is at odds with flight vehicle weight and balance realities. You just can't load a bunch of cargo into the extreme tail, only a little bit. Which means the prototypes so far are demonstrators for hard-pad landings only, not any sort of rough field stuff! They really need the rough field capability right here on Earth, for emergency abort landings far from base. It is something Spacex must face up to, but hasn't yet.

During the Mars entry sequence, starting at about 140 km altitude, 7.5 km/s speed, and a flight path angle between about 1 and 2 degrees below local horizontal, the vehicle must be inherently stable enough to hold an angle of attack (off the relative wind vector) of something like 40 degrees. Not much less and not much more, or those windows are toast in a single handful of seconds. Center of gravity just a small margin ahead of center of pressure is utterly crucial for this angle of attack controllability. That is exactly what the combination of fins and canards (and attitude control thrusters) is all about.

90+% of Mars is NOT rock outcrops, it is sand-plus-rocks desert resembling west Texas, New Mexico, Arizona, Chihuahua, and Sonora; or to a lesser extent sand-only, to include giant shifting dunes, that resembles the Arabian peninsula or the Sahara. Why on God's green Earth would you want to restrict your feasible landing zones to a tiny minority of Mars, especially when it is so easy to miss your landing target, if anything whatsoever goes amiss during entry, major or minor? That is stupid indeed!

Scott Carpenter had that problem on the second Mercury capsule orbital flight in 1962, and missed his target by over 400 nautical miles (about 800 km). The initial shuttle landings were at Edwards in California for exactly that risk, and one missed and landed at White Sands in New Mexico instead. There is a demonstrated history of such landing errors, even if it is not frequent.

That risk says you had better be able to land on soft sand at 0.1 to 0.2 MPa bearing strength, and if you ever intend to leave that site, you had better plan on supporting the fully-fueled launch weight, on that same material. If you thought load/unload was hard for a crew, try raising the leg of a tail-sitter rocket to put something hard under its landing pad. And don't try to tell me some robot or some hydraulic system is going to do it for the crew. That's BS and everyone instinctively knows it! Such things assist, but the crew must go and do.

It doesn't matter whether there is duricrust or some analog of it on Mars, and there is, we've already seen it with the various rovers. Not every stretch of Martian desert has it, and we've already seen that, too. Duricrust or something like it is what gets you to the 0.2 MPa bearing strength, instead of just 0.1 MPa for fine, loose, dry sand.

Those are Earthly values for Earthly desert materials, but they are the best we have at this time, and we have them at far better confidence than anything inferred from orbital remote sensing. While the correspondence between ground truth and remote sensing is far better today than it was 50 years ago, there is still a significant disparity, even today, enough for somebody to get into real trouble, or even die. And that's a fact, Jack!

The entry sequence is flown at AOA ~40 deg, lift vector rolled downward, until speed drops below orbital velocity 3.5 km/s or thereabouts. That downward lift vector is what keeps the ship from bouncing off the atmosphere into space at above Mars escape speed (5 km/s). Once decelerated to about 3.5 km/s, the lift vector at AOA ~40 deg is rolled upward, to keep the trajectory from prematurely steepening downward as momentum is lost. If AOA gets too low, or too high, the windows are toast in a single handful of seconds, followed by vehicle breakup. Control of AOA is crucial to survival.

This continues to the end of hypersonics at about local Mach 3 (about 1 km/s velocity), which for effective ballistic coefficients that large, will be at altitude near 5 km, mere seconds from impact if nothing is done. The trajectory has steepened downward to something near 35 degrees below horizontal, maybe a bit more. That low altitude is what I got independently, and it is exactly what Spacex shows in its Mars landing simulations.

At this point heating is no longer a risk, and AOA is actually increased to beyond 40 deg on Mars, to generate more body lift, which bends the trajectory upward (see next paragraph), leading to velocity bleed-off as the altitude climbs nearer 10 km. At peak altitude and essentially sonic velocity (about 0.3 km/s), the ship is rotated tail first and the engine (or engines) lit for touchdown. It rapidly steepens to vertical just as speed reduces to zero at touchdown.

I think Spacex is overoptimistic about bending the trajectory upward with only body lift in that thin near-vacuum of a Martian atmosphere. I think they will have to light the landing engine (or engines) sooner, in order to add a thrust component to that lift. If so, it means the landing fuel allowance is substantially larger, by somewhere around my guesstimated factor 1.5 "cheat factor" that I use for estimating things. We'll see.

I think Spacex is currently planning on one engine for landing thrust. Engine-out reliability suggests they do it with two, throttled-down, so that if one quits, the other can be throttled up faster than another engine could be ignited. But then I was trained to worry about stuff like that.

The Earth entry sequence from orbit is quite similar: 140 km, 8.0 km/s, 1-2 deg below horizontal at entry interface. It all looks the same to end-of-hypersonics at Mach 3 (about 1 km/s), just at 40-45 km altitude instead of 5 km. From there, they steepen downward into increasingly-dense air as they increase AOA to 90 deg. That puts them into a supersonic vertical belly-flop, which decelerates to subsonic somewhere around 6-8 km. Nearing the surface, they are well-subsonic at about 0.3 Mach (100+ m/s) vertically downward, and they rotate the ship to tail-first, and fire up about 3 engines to touch down. That's what the Spacex Earth entry simulation shows.

Coming back to Earth from Mars, it's essentially the same as the Earth orbit entry sequence, except the entry interface velocity is much higher: at least 12 km/s (Earth escape is 11 km/s), and potentially as high as about 17 km/s. The sequence is longer, and at a very slightly shallower average angle below local horizontal. It ends at the same end-of-hypersonics conditions: Mach 3-ish about 40-45 km up.

I think the Mars entry sequence would likely work anywhere on Mars, except not to land atop some mountain. Anywhere the surface pressure was near or higher than about 6 mbar. But I also think they will be firing up the landing engine(s) sooner than they currently plan, to get the trajectory bent up steep enough to manage their energy the way they want. I already covered that.

The real question is this: why restrict your landing site to a tiny minority of Mars and run the risk of a fatal crash if you miss it, instead of just being able to handle landing on sand?

You can get the extra landing pad area by fold-out panels if your landing legs are fins. They hinge at the trailing edge, and fold out flat to the surface. Something like 40-45 sq.m total pad area is adequate for 0.1 MPa sand, at the masses I've seen quoted so far for Starship.

That’s two panels roughly 2.6 x 2.6 m folded out from each side of the fin, for 3 such fins as landing legs. It’s just hydraulics, rather similar to landing gear bay doors. I’d rather do it with 4 fins flown X-configuration during entry, so as to get the square instead of the triangle for my pad footprint. And I prefer the fins to get the wider stance relative to cg height, a very serious influence on stability.

The “4-poster” is just substantially more stable on sloped or undulating surface contours. We’ve already seen it with 3 wheel mini cars that turn over too easily, just like a child’s tricycle. Increasing that stability relaxes your constraints on feasible landing sites some, and opens up more sites you could safely visit. But adding soft sand landing and takeoff capability with the fin-mounted folding panels opens up the vast bulk of Mars!

By the time Spacex actually faces this issue, and they will have to, for routine Earth orbital flights because of the off-site abort landing risk, that’s very likely what the Starship design will have come to resemble.

GW

Last edited by GW Johnson (2019-10-09 13:00:10)

elderflower · 2019-10-10 06:17:32

If you use fold out panels at the fins, particularly the underside, there has to be a seam exposed to the hypersonic flow. This might be attacked by the hot plasma resulting in failure. If you use legs the seams are tucked in close to the body where they are exposed to the boundary layer, and therefore a bit less likely to lead to disassembly in re-entry.

GW Johnson · 2019-10-10 08:04:45

Elderflower:

Why would the panel "seam" be a problem? There were landing gear bay doors on the windward side of the space shuttle heat shield. There was also a hatch in the heatshield of the Gemini-B capsule (one did fly for the MOL program in 1969).

There is a boundary layer on a fin, too, but that is not how you protect a door "seam", which is really a gap between the panel and the adjacent skin. You design-in a way for that door or panel to seat upon a gas seal, that stops the infiltration of hot gas through the gap. With throughflow stopped, the static gas column in that gap is a better insulator than any solid structure we could build.

This technique works fine on gaps in windward-facing heat shields, where conditions are far more extreme that lateral facing skins on the side of a body or a fin.

There is no technical obstacle to using fold-out panels at the trailing edges of fins to be enormous landing pads. That is how you achieve the low bearing pressures necessary for rough-field landing capability on soft sand, appropriate to the moon or Mars.

I posted an article on exactly how to do this, right down to sizing the pads, on "exrocketman" several months ago. That article is dated 4 February 2029, and titled "Designing Rough-Field Capability Into the Spacex Starship". That site is http://exrocketman.blogspot.com, and there is a fast navigation tool on the left of the page. Click on the year, then on the month, then on the title.

During the entry sequence, when angle of attack is near 40 degrees to the relative wind axis, the panel gaps on the windward sides of the fins are somewhat exposed to slipstream blast pressures. That is why the seating of the closed panel upon a gas seal inside, is so important.

On the leeward side, this is less important, but I still recommend sealing, because you never know what transient orientation your vehicle might get to, during recovery from a flight upset.

GW

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