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I'm guessing the first cargo lander is going to have some specially adapted legs to ensure stability. I presume, further that Space X will choose to land in an area they reconnoitre first. Once a cargo lander is on the ground, it should be possible to then get a robot rover on the surface which can clear a landing area for the first human lander, ensuring there are no stones or boulders. Let's remember we now have technology on modern cars that can park vehicles to a centimetre. I really don't think it will be that difficult to build a mini rover-dozer that can clear boulders and rocks from a a defined area at a suitable distance from the cargo landers.
On the question of propulsive landing.
Thus far landings of large stages have taken place on prepared pads or barges. The latter seems to work even when the vehicle is burning! Only small devices have been landed on rough ground and regolith, eg Lunar lander module and small rovers, The last one used a skycrane device. Large rockets will have very large exhaust volumes at high velocity which will blow rocks, sand and dust a long way. The first arrival will be fine with that, it will just clear an area around its landing spot. Subsequent landings cannot be made close to the first, we cant have flying rocks or sand blasting damaging the reuseable earlier vehicles. So there must be a radius where this problem is minimised which will depend on gravity and atmospheric conditions. The problem recurs when the reusable craft departs, using a lot more thrust than when it landed, exposing remaining craft and structures to damage from this cause. Unlike earth launches there won't be a prepared pad with hold downs and exhaust ducts. It will just be lightup and go!
Having established a safeish radius, refuelling will need hoses/pipework long enough to reach from the fuel production point to the receiving vehicle and transfer pumps to refuel in a reasonable time to avoid excessive boil off. The only alternative that I can see would be for each early lander to contain its own fuel and LOX plant independently.
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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This is another reason for an explorative precursor landing with a somewhat smaller vessel--bringing a Bobcat-style excavator with a front loader. To prepare a landing site for the BFR.
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The lunar lander was not good for significantly-rough ground. Avoiding boulders as big as the spacecraft's legs was why Armstrong nearly ran out of propellant during the Apollo 11 landing. There was something like 20-30 seconds of burn left when he shut the engine down.
If you noticed, the span between the legs was just about equal to the vehicle height with the lunar lander, and with the two Vikings at Mars, and the old lunar Surveyor landers, and a whole lot more. That proportion enables landings on not-flat, not-level ground without tipping over. Taller proportions require very flat, very level ground, period. It's just physics of cg vs footprint.
I think Oldfart1939 is quite right when he says ITS needs a pre-prepared field to land on. That means a small one-way vehicle with some sort of dozer aboard. That thing can be a robot. This BFR/ITS thing is still far enough off that they haven't really thought that part of the problem through yet. Or they wouldn't be proposing to land at height/leg span > 4 on unprepared ground.
GW
Last edited by GW Johnson (2017-11-02 10:58:49)
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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This brings me to the conclusion that we need a pioneering mission to survey, select and characterise the landing site and to prepare for the arrival of a BFR with heavy equipment. Until the site is visited we cannot know what is really there.
Even if a robot drilling rover has been deployed there will be gaps between holes. And drilling rigs on earth are prone to breakdowns. How much more prone will be a small, rocket transportable machine? You have to have people on site!
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Some points here:
1.Compared with 20 years ago, we now have far more data on ground conditions on Mars than we ever had before.
2. All we are really talking about is getting the cargo landers safely on the ground. If the worst came to the worst and one or both of the cargo landers failed in their landings then that would be a major setback but it would not be a tragedy involving loss of life. All it would mean is a two year delay in the mission. No more significant than one of the many rockets that have blown up in development stage.
3. Once the cargo landers are down, well there you have your precursor mission running at the same time as the cargo landing, as you can then have a robot rover-dozer go clear a space and test the ground for a landing by the human-passenger BFRS.
4. We have the option of landing in an area that has already been surveyed e.g. close to the Viking Lander sites.
This brings me to the conclusion that we need a pioneering mission to survey, select and characterise the landing site and to prepare for the arrival of a BFR with heavy equipment. Until the site is visited we cannot know what is really there.
Even if a robot drilling rover has been deployed there will be gaps between holes. And drilling rigs on earth are prone to breakdowns. How much more prone will be a small, rocket transportable machine? You have to have people on site!
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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I have a suspicion that the final BFR landers will be quite different from those illustrated in the released videos; not so much the overall appearance but in the mechanical configurations. We shouldn't expect to see something too tall and "tippy." Most likely with a much broader "footprint."
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If space x can go from Falcon 1 to Flacon 9 in x years and then we are looking a falcon heavy in just a little while what do we think it will take to get to BFR, Tankers, ITS, mars colony transport....
What do you think will be that time frame.......Not holding breath that I will be alive then....
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Space X went from virtually "nothing" to Falcon 9 in ten years. I have read Musk couldn't get established rocket engineers to come to work for his new outfit so he ended up doing a lot of the design work himself!
But now they are well established...they know how to (a) build rocket engines (b) pack cargo into holds (c) dock vehicles in orbit (d) make rocket casings (e) build huge propellant tanks (f) do all the coms and (g) achieve propulsive landing.
On that basis, I don't think it is much of a leap to the BFR. Musk self-declared target is having a Mars BFR cargo lander operational in 2022.
Which probably means he needs an operational prototype by 2020 I would have thought. He says they will start building next year, so it does sound doable to me.
If space x can go from Falcon 1 to Flacon 9 in x years and then we are looking a falcon heavy in just a little while what do we think it will take to get to BFR, Tankers, ITS, mars colony transport....
What do you think will be that time frame.......Not holding breath that I will be alive then....
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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As I have said before, Musk usually does what he says. But it takes about a factor 1.5-to-2 longer elapsed schedule time to get the job done. Falcon-Heavy was supposed to have flown in 2013, but turned out to be much harder than anticipated. Staging supersonic in the thin air without collisions turned out to be harder than they thought, which is why Falcon-1 took longer than thought, and a couple of them failed.
That's the history. I think Musk/Spacex will fly something that looks a lot like the BFR/ITS-tanker thing, but I really doubt he can do it by 2022. If he does it by 2030, he will still beat the pants off NASA to Mars with men. (That emperor hasn't had any clothes since it built ISS.)
Give them time to think the unprepared landing field problem through. They haven't done that yet, or they would NOT be proposing a height/leg span ratio exceeding 4 for ITS. The vehicle shape will have to change (although it can't change a lot and still enter as they have depicted), and the legs must be much larger (and heavier) than they are now.
But, the REAL problem that will slow them down is the thing that they (and nobody else) have ever done before: the nose-for-tail flip, down in the dense air at transonic speeds. That is required to enter nose-first at high AOA, and still go tail first for the final retropropulsive landing. That must be made to work, or the whole concept will prove infeasible. THAT is the unreality in the development schedule calling for a flying article around 2022, with a first trip to Mars by about 2024.
GW
Last edited by GW Johnson (2017-11-05 10:40:58)
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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Do you think a small parachute could be used from the nose end just to reverse the vehicle, GW?
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My hunch, and it is just a hunch, is that a lot more detailed work than people think has already been done on the Mars plan. I really can't accept that they have not yet thought through the problem of landing on a boulder-strewn plain.
What in a nutshell is the problem with the nose/tail flip? Is it the speed - so that any slight deviation leads to gross error? Or is it the amount of force which could damage something? Or something else?
As I have said before, Musk usually does what he says. But it takes about a factor 1.5-to-2 longer elapsed schedule time to get the job done. Falcon-Heavy was supposed to have flown in 2013, but turned out to be much harder than anticipated. Staging supersonic in the thin air without collisions turned out to be harder than they thought, which is why Falcon-1 took longer than thought, and a couple of them failed.
That's the history. I think Musk/Spacex will fly something that looks a lot like the BFR/ITS-tanker thing, but I really doubt he can do it by 2022. If he does it by 2030, he will still beat the pants off NASA to Mars with men. (That emperor hasn't had any clothes since it built ISS.)
Give them time to think the unprepared landing field problem through. They haven't done that yet, or they would NOT be proposing a height/leg span ratio exceeding 4 for ITS. The vehicle shape will have to change (although it can't change a lot and still enter as they have depicted), and the legs must be much larger (and heavier) than they are now.
But, the REAL problem that will slow them down is the thing that they (and nobody else) have ever done before: the nose-for-tail flip, down in the dense air at transonic speeds. That is required to enter nose-first at high AOA, and still go tail first for the final retropropulsive landing. That must be made to work, or the whole concept will prove infeasible. THAT is the unreality in the development schedule calling for a flying article around 2022, with a first trip to Mars by about 2024.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Maybe you just get the vehicle to climb after it drops to mach 2 or 3. It does have rudimentary winglets, perhaps this is why they are there. Then you can slow against gravity and change the attitude to vertical with engine downwards.
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From what I read, the rudimentary winglets are for control of center of gravity versus center of pressure, as possible payload loadings vary. They had to have them for static stability across a range of Mach numbers from subsonic to hypersonic. The center of pressure typically moves around drastically across Mach 1.
The problem tipping nose for tail is that as you turn, you go broadside to the air. This vehicle is designed like other rocket stages (meaning as fragile as can be tolerated) to reduce inert weight fraction. They are a lot stronger to air loads end-on than they are broadside.
No other stage hitting the atmosphere tumbling (meaning it goes broadside) has ever survived entry intact. They usually break up lower down nearer Mach 1, where the wind pressures get very high (the so-called "max-q").
So, the ITS runs the same risk of being crushed by the wind pressure difference between windward side and leeward side, while broadside. Now, Elderflower has identified one thing that might work here on Earth: adjusting the trajectory into a vertical climb just as it decelerates to zero speed, leaving the craft ready to descend vertically tail first in retropropulsion. That exposes the craft to severe lateral airloads at high angle of attack, but not the highest, because it never goes fully broadside to the air flow.
I rather doubt that would work in the much thinner (0.6% at the surface, similar scale height) atmosphere of Mars, because I doubt enough lift can be had to turn the descending momentum vector significantly. But, max-q airloads are also lower, so what might survive a climbing pitch-up at Earth, might survive the dead-broadside turn at Mars.
Strength against airloads costs weight. That is inherent. It's worse at large sizes due to square-cube scaling effects, too. We'll see what their aero-structures guys can come up with.
As I said elsewhere, this is a big deal that no one has ever done before. The arrogance of ignorance has shot them in the foot before. Supersonic staging and some other troubles.
The unprepared landing field problem is something JPL has faced since Surveyor, and the now-dead-or-retired designers of the lunar module faced, but Spacex has never, ever faced. The pitching and rolling motions of a barge at sea complicated their stage landings, yes. But they have NOT yet faced uneven ground or obstructions (boulders and worse) getting in the way.
Again, the arrogance of ignorance leaves one vulnerable. Even the very best can suffer from it. And often do. DeHavilland certainly did with the then-unknown phenomenon of metal fatigue in their Comet jet airliner of the early 1950's.
GW
Last edited by GW Johnson (2017-11-06 12:17:56)
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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The mars landers change there angle of attack by ejecting a center of balance mass...
http://www.nar-associates.com/technical … screen.pdf
Aerodynamics for the Mars Phoenix Entry Capsule
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Thanks for that explanation of the problem. I am sure Space X must be aware of these issues and have at least some conceptual ideas on how to tackle them. Of course, proof of the pudding is in the eating, so we shall see what they come up with. I guess they have some mass available in the cargo load at 150 tonnes which could be converted into structure?
From what I read, the rudimentary winglets are for control of center of gravity versus center of pressure, as possible payload loadings vary. They had to have them for static stability across a range of Mach numbers from subsonic to hypersonic. The center of pressure typically moves around drastically across Mach 1.
The problem tipping nose for tail is that as you turn, you go broadside to the air. This vehicle is designed like other rocket stages (meaning as fragile as can be tolerated) to reduce inert weight fraction. They are a lot stronger to air loads end-on than they are broadside.
No other stage hitting the atmosphere tumbling (meaning it goes broadside) has ever survived entry intact. They usually break up lower down nearer Mach 1, where the wind pressures get very high (the so-called "max-q").
So, the ITS runs the same risk of being crushed by the wind pressure difference between windward side and leeward side, while broadside. Now, Elderflower has identified one thing that might work here on Earth: adjusting the trajectory into a vertical climb just as it decelerates to zero speed, leaving the craft ready to descend vertically tail first in retropropulsion. That exposes the craft to severe lateral airloads at high angle of attack, but not the highest, because it never goes fully broadside to the air flow.
I rather doubt that would work in the much thinner (0.6% at the surface, similar scale height) atmosphere of Mars, because I doubt enough lift can be had to turn the descending momentum vector significantly. But, max-q airloads are also lower, so what might survive a climbing pitch-up at Earth, might survive the dead-broadside turn at Mars.
Strength against airloads costs weight. That is inherent. It's worse at large sizes due to square-cube scaling effects, too. We'll see what their aero-structures guys can come up with.
As I said elsewhere, this is a big deal that no one has ever done before. The arrogance of ignorance has shot them in the foot before. Supersonic staging and some other troubles.
The unprepared landing field problem is something JPL has faced since Surveyor, and the now-dead-or-retired designers of the lunar module faced, but Spacex has never, ever faced. The pitching and rolling motions of a barge at sea complicated their stage landings, yes. But they have NOT yet faced uneven ground or obstructions (boulders and worse) getting in the way.
Again, the arrogance of ignorance leaves one vulnerable. Even the very best can suffer from it. And often do. DeHavilland certainly did with the then-unknown phenomenon of metal fatigue in their Comet jet airliner of the early 1950's.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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Increased structure mass comes at the expense of payload or fuel. If you carry less fuel you can take a longer time to get there, but that is unlikely to be acceptable for a manned mission. Cargo missions already take a long time so you may have to reduce payload.
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Another possibility that occurred to me is that the turn is made outside the atmosphere and entry is made with engines running so that the plasma is kept clear of the vehicle by the engine exhaust and the main shock is in front of the bubble. I dunno. My aerodynamics and thermodynamics are not up to working that one out!
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If you scroll down a little there is a video called "Mars Entry" here.
It clearly states the vehicle has an ablative heat shield designed initially for the dragon craft.
So, at least we know they've been working on this.
It appears to do the flip when speed drops below 3000 metres per second.
Edited to add:
And here's some background on the ablative heat shield.
http://spacenews.com/spacexs-high-veloc … hield-guy/
Last edited by louis (2017-11-07 09:38:30)
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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SpaceX must be getting loads of data regarding hypersonic retropropulsion based on the reentries of the Falco 9 first stages. OK, it may only be supersonic data, but they probably have more raw data collected than any other operator.
Another case of "NASA strikes again." causing SpaceX to abandon the retropropulsive Crew Dragon landings was really a severe blow to the plans Elon had for the company.
Last edited by Oldfart1939 (2017-11-07 13:59:25)
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Yes not invented here mentality for Nasa... as they wanted to safe guard the lander sample return unit... rather than see it smack into the ground....
Well then do not try the retropropulsion on the first try with anything that you can not afford to lose...
In fact Musk should send his own just to put egg on Nasa's face.
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In fact Musk should send his own just to put egg on Nasa's face.
I wholeheartedly agree! NASA has become far too risk adverse. It's actually stultifying progress.
Last edited by Oldfart1939 (2017-11-07 20:36:11)
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What saw from the video (for which Spacenut provided the link) was a down-lifting direct hypersonic reentry from a speed exceeding escape, rolling from nose down to nose up for up-lift below 3 km/s. This actually climbed from near 5 km to near 10 km altitude while slowing to ~Mach 2.5 (about 0.6 to 0.7 km/s). Then and only then was the pitch-up to tail first (without any engines burning), but they were below 5 km before actually starting the landing burn.
For comparison, 3000 m/s = 3 km/s is only a little below low Mars orbit velocity, which is just about 3.5 km/s. Sound speed is in the vicinity of 220-240 m/s (varies as the square root of the local "air" temperature.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Sorry had missed Spacenut's earlier link then.
But my point really was that these Space X guys must think this is possible and I have a lot of faith in their technical ability and speed of development. As far as I can make out, a lot hangs on the ablative heat shield.
What saw from the video (for which Spacenut provided the link) was a down-lifting direct hypersonic reentry from a speed exceeding escape, rolling from nose down to nose up for up-lift below 3 km/s. This actually climbed from near 5 km to near 10 km altitude while slowing to ~Mach 2.5 (about 0.6 to 0.7 km/s). Then and only then was the pitch-up to tail first (without any engines burning), but they were below 5 km before actually starting the landing burn.
For comparison, 3000 m/s = 3 km/s is only a little below low Mars orbit velocity, which is just about 3.5 km/s. Sound speed is in the vicinity of 220-240 m/s (varies as the square root of the local "air" temperature.
GW
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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The pica X shield is just about scale of size, panel counts and the means to adhere it to the shell of the section that it is applied to.
As far as composite this and that to save mass just stick with what works and make small changes as you go with these mass saving materials.
While I like some of the concepts of refueling the second stage in orbit to act as the EDS its a proble for the other end since to reuse it again once it leaves mars surface another tanker of fuel must be already in mars orbit with the consumables for the return trip to make it back home as you would not want to land the craft on mars surface just to try and haul it back up. Leave the return trip mass in orbit and substitue that mass with what you want on the surface instead that will not be coming back up.
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Just a quick note on Space X...Forbes state that the company is now valued at more than $20 billion. That is a big operation, albeit valuations in the world of rocketry are perhaps less meaningful than in some other fields.
Last edited by louis (2017-11-09 04:57:57)
Let's Go to Mars...Google on: Fast Track to Mars blogspot.com
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