Author Topic: MCT Speculation and Discussion Thread 4  (Read 610849 times)

Offline GregA

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Re: MCT Speculation and Discussion Thread 4
« Reply #1060 on: 11/03/2015 12:34 AM »
But why would the passenger version of MCT use the super heavy legs of the full payload version? F9R's legs are removable and added at the launch site, so it would make perfect sense to be able to use less massive legs if you aren't landing a huge payload and that was a big mass driver.

I haven't noticed this distinction before. Dramatically different masses for passenger vs cargo MCT - is that what you meant?

Earlier I'd raised this in a different way, saying that a cargo MCT would contain denser materials than a passenger MCT,  and thus to have the same mass in the same volume would have "free space" that would be better shared with a small passenger group who would greatly appreciate free space. Some people responded that the cargo MCT would just be packed with extra space.

But you seem to be saying they'd actually just have a heavier version. More fuel would be part of that too of course. A higher mass for cargo MCT would impact its velocity (is it a slow trajectory?) and EDL issues/differences as well as launch differences.

Actually... perhaps you're saying the opposite. You'd make a regular cargo MCT, but there'd be a low-load light version too for passengers? I would think you wouldn't travel light in a system capable of carrying more.

I'd like to read the arguments for this... where/when was it?

Offline oiorionsbelt

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Re: MCT Speculation and Discussion Thread 4
« Reply #1061 on: 11/03/2015 12:38 AM »

and you land on a prepared surface.
Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #1062 on: 11/03/2015 01:16 AM »
But why would the passenger version of MCT use the super heavy legs of the full payload version? F9R's legs are removable and added at the launch site, so it would make perfect sense to be able to use less massive legs if you aren't landing a huge payload and that was a big mass driver.

I haven't noticed this distinction before. Dramatically different masses for passenger vs cargo MCT - is that what you meant?

Earlier I'd raised this in a different way, saying that a cargo MCT would contain denser materials than a passenger MCT,  and thus to have the same mass in the same volume would have "free space" that would be better shared with a small passenger group who would greatly appreciate free space. Some people responded that the cargo MCT would just be packed with extra space.

But you seem to be saying they'd actually just have a heavier version. More fuel would be part of that too of course. A higher mass for cargo MCT would impact its velocity (is it a slow trajectory?) and EDL issues/differences as well as launch differences.

Actually... perhaps you're saying the opposite. You'd make a regular cargo MCT, but there'd be a low-load light version too for passengers? I would think you wouldn't travel light in a system capable of carrying more.

I'd like to read the arguments for this... where/when was it?

It would be the same base MCT, but since a load of passengers wouldn't have as much mass as a load of cargo, they could reduce some vehicle mass by using lighter landing legs. Maybe change out some other components too. Then you get better performance from the engines with an overall lighter vehicle.

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1063 on: 11/03/2015 02:03 AM »
...The size of boulders that would pose a problem for an MCT landing would be faaar to large to handle with a small rover. And boulders are only part of the issue, dust blasting may be more of a problem, and that certainly cannot be addressed by a small rover....

Boulders big enough to cause problems for MCT landing are easily spotted using MRO. You can just avoid them in the planning stages.


1) MRO can resolve objects of "about a meter across"...
MRO has a resolution of ~30cm per pixel. Multiple exposures of the same site from different angles and ESPECIALLY with the Sun at high angles (thus casting long shadows) can identify hazards.

Perhaps, but that kind of coverage does not exist.
What do you mean? You can, right now, go and request new images be taken of a certain area. There's plenty of "coverage" to take multiple images of the same small area at multiple times of day.

No, it does not exist *now*. If you watched the streams, you might have missed the talk about additional coverage needed to get better coverage during various times of the day, including the benefit of satellites in near-polar orbits for this very purpose. Better coverage of the ground is needed for final analysis of landing sites... What we have now is not sufficient as you could not tell the difference between a flat ground and an even boulder field.

You may not recall this, but the MER scientists were *quite* surprised that the landscape was as flat and featureless as it turned out to be. (See image below) They expected lots of smaller boulders, but found practically nothing. And vice versa at the other rover site.  It was a real surprise, despite an in-depth analysis of the landing sites.

Quote
But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.
I am in favor of actual abort capability, instead of super heavy landing gear. Why do I need to "admit" something that has no basis in actual SpaceX communications, just some people's opinion on the internet? My mental model of MCT most resembles DC-Y and DC-I, which used fairly stubby landing gear and had real abort capability. (And yes, you'd need Soyuz-style cushioning thrusters on Mars in addition to a parachute. But the actual delta-v for that cushioning thrust is VERY low.)

DC-X (and the planned derivatives) had smaller tiny gears because they were designed to land on... drum roll... hard concrete pads. (and they could not land outside a prepared pad)

This is not what early MCT's will encounter. Even if your fantasy of tiny ground-clearing robots delivered by Red Dragon's materialize, the ground is not hard as diamond. It won't be even close to a prepared landing pad on Earth. And you continue to dodge the fact that the MCT legs also need to support a full propellant load, this will require something significantly more than an "elephant on stilts" approach.

My view of the legs is that they need to be fairly short (to conserve mass) - but have very wide feet, to distribute the load.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1064 on: 11/03/2015 02:32 AM »
....
You may not recall this, but the MER scientists were *quite* surprised that the landscape was as flat and featureless as it turned out to be.
So what? MER was before MRO....

Quote
But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.
I am in favor of actual abort capability, instead of super heavy landing gear. Why do I need to "admit" something that has no basis in actual SpaceX communications, just some people's opinion on the internet? My mental model of MCT most resembles DC-Y and DC-I, which used fairly stubby landing gear and had real abort capability. (And yes, you'd need Soyuz-style cushioning thrusters on Mars in addition to a parachute. But the actual delta-v for that cushioning thrust is VERY low.)

DC-X (and the planned derivatives) had smaller tiny gears because they were designed to land on... drum roll... hard concrete pads. (and they could not land outside a prepared pad)

Quote
This is not what early MCT's will encounter. Even if your fantasy
Give me a break.
Quote
of tiny ground-clearing robots delivered by Red Dragon's materialize,
If you think a 10-ton bulldozer is "tiny"
Quote
the ground is not hard as diamond. It won't be even close to a prepared landing pad on Earth.
Yes, there will continue to be improvements, not just clearing land.
Quote
And you continue to dodge the fact that the MCT legs also need to support a full propellant load, this will require something significantly more than an "elephant on stilts" approach.
Enough with the dramatic rhetoric! I've not "dodged" anything. I haven't seen it come up to need to address it, though I have thought of it. Jeez. No, you wouldn't launch an MCT on its legs. You also need supports placed under the MCT before you fuel it up for launch, just like with DC-X (DC-X's legs could support a full load, but they were practicing for DC-Y/I).



It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).


Musk talks about 80,000 people per year going to Mars (look it up), which would mean hundreds if not thousands of MCTs going every synod. It's ridiculous to think that the first couple MCTs are going to dominate the design of the vehicle and its operational dry mass. If the first few MCTs need beefier/taller legs because they won't be landing on a fully prepared surface, then so be it. But such early mods won't be driving up the dry mass for the vehicles making Earth-to-Mars transits with 50-100 passengers in ~100 days.
« Last Edit: 11/03/2015 02:42 AM by Robotbeat »
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1065 on: 11/03/2015 02:40 AM »

and you land on a prepared surface.
Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?
There are several methods. As I posted earlier, some of the guys from "Swampworks" are establishing a technique that could be built with a rover. I guarantee that SpaceX is working the problem and has their own ideas of how to solve the problem.
« Last Edit: 11/03/2015 02:47 AM by Robotbeat »
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Offline stoker5432

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Re: MCT Speculation and Discussion Thread 4
« Reply #1066 on: 11/03/2015 02:45 AM »

and you land on a prepared surface.
Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?

NASA has some ideas so maybe SpaceX will get some advice from from them.

http://www.nasa.gov/content/landing-pads-being-designed-for-extraterrestrial-missions

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1067 on: 11/03/2015 02:52 AM »

and you land on a prepared surface.
Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?

NASA has some ideas so maybe SpaceX will get some advice from from them.

http://www.nasa.gov/content/landing-pads-being-designed-for-extraterrestrial-missions

Exactly, Dr. Phil is the guy I was thinking of.

Here's some quotes from the article:

"Robotic landers would go to a location on Mars and excavate a site, clearing rocks, leveling and grading an area and then stabilizing the regolith to withstand impact forces of the rocket plume," Mueller said. "Another option is to excavate down to bedrock to give a firm foundation. Fabric or other geo-textile material could also be used to stabilize the soil and ensure there is a good landing site."

...
""We've tested several types of materials and it seems that basalt regolith mixed with polymer binders hold up well," Metzger said."
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Offline Pipcard

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Re: MCT Speculation and Discussion Thread 4
« Reply #1068 on: 11/03/2015 02:58 AM »
It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).
I think it's about having "conservative" mass estimates instead of "optimistic" ones, a.k.a. the reason why Mars Direct in its original form was criticized, then revised (e.g. Semi-Direct, DRM 3.0).
« Last Edit: 11/03/2015 03:12 AM by Pipcard »

Offline oiorionsbelt

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Re: MCT Speculation and Discussion Thread 4
« Reply #1069 on: 11/03/2015 02:59 AM »

and you land on a prepared surface.
Robotbeat, you have mentioned this 'prepared surface' in several posts. How this would be accomplished?

NASA has some ideas so maybe SpaceX will get some advice from from them.

http://www.nasa.gov/content/landing-pads-being-designed-for-extraterrestrial-missions


Quote
"Of all the substances we studied, ablative materials seem to work best," Metzger said.

:) PICAX landing sites

Edit: or is it PIRAX Polymer Impregnated Regolith Ablator.
« Last Edit: 11/03/2015 03:03 AM by oiorionsbelt »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1070 on: 11/03/2015 03:03 AM »
It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).
I think it's about having "conservative" mass estimates instead of "optimistic" ones, a.k.a. why Mars Direct was revised (e.g. Semi-Direct, DRM 3.0).
The problem with that is "conservative" mass fractions are NOT realistic!

If NASA, for instance, were doing initial design of F9 v1.1 full thrust and Falcon Heavy, no way would they contemplate giving the boosters mass fractions of 25 and 30, respectively.

Heck, when NASA was trading ULA's Centaur for ESAS, they literally sandbagged the dry mass of Centaur 50% more than it actually weighs today! How they managed to do that, I do not know.

But "conservative" mass fractions are the least likely for a company that both prides itself in world-class mass-fractions and that can't afford building something much larger and with extra stages than necessary. SpaceX doesn't need to spread work amongst several Centers, there's no reason they'll want to expand the number of stages beyond what they think they can achieve. "Conservative" is not realistic when you don't have infinite money.
« Last Edit: 11/03/2015 03:04 AM by Robotbeat »
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1071 on: 11/03/2015 04:24 AM »
It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).
I think it's about having "conservative" mass estimates instead of "optimistic" ones, a.k.a. why Mars Direct was revised (e.g. Semi-Direct, DRM 3.0).
The problem with that is "conservative" mass fractions are NOT realistic!

If NASA, for instance, were doing initial design of F9 v1.1 full thrust and Falcon Heavy, no way would they contemplate giving the boosters mass fractions of 25 and 30, respectively.

Heck, when NASA was trading ULA's Centaur for ESAS, they literally sandbagged the dry mass of Centaur 50% more than it actually weighs today! How they managed to do that, I do not know.

But "conservative" mass fractions are the least likely for a company that both prides itself in world-class mass-fractions and that can't afford building something much larger and with extra stages than necessary. SpaceX doesn't need to spread work amongst several Centers, there's no reason they'll want to expand the number of stages beyond what they think they can achieve. "Conservative" is not realistic when you don't have infinite money.

You've brought up this distinction before and I think it's a semantic waste of time.

Nobody's dropping the performance numbers they hope to achieve for the sake of "being conservative".  They're trying to build in a performance margin between "What we would need for this mission to go forward at all" and "What we're hoping to achieve".  They're trying to *overbuild*, because they know that any hundred out of ten thousand things could turn out to just not be achievable - this possibility is the price you pay for being on the forefront of engineering, for trying new things that have not been done before.

There are about four or five orders of magnitude of price reduction that SpaceX is hoping to achieve - right now we could expect to field around 5 billion dollars (if not 50 billion dollars) a ticket for tens to hundreds of tickets to Mars aboard an Apollo-like effort, while SpaceX wants to eventually achieve 500,000 dollars.

Let me say that again.

Four.  Or.  Five. Orders.  Of.  Magnitude.

Putting a 20% margin in (or a 50% margin) as allowance for unexpected roadblocks is not going to break the bank.  Assuming that every performance estimate you have will ultimately take the upper 95% error bound is dumb.  They're not going to get there on audacity alone.

People use "conservative" because they want to demonstrate that yes, even with what we know that we don't understand, we're pretty sure this can be done.  It shouldn't have to have [TRIGGER WARNING] attached.
« Last Edit: 11/03/2015 04:29 AM by Burninate »

Online Lars-J

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Re: MCT Speculation and Discussion Thread 4
« Reply #1072 on: 11/03/2015 04:26 AM »

Fix your quoting!  I'can't even reply to the rest with that quoting mess, but these nuggets...:

Quote
This is not what early MCT's will encounter. Even if your fantasy
Give me a break.
Quote
of tiny ground-clearing robots delivered by Red Dragon's materialize,
If you think a 10-ton bulldozer is "tiny"

10 tons? Since when can Red Dragon deliver a 10 ton payload to Mars?!? I'm not the one who raised the "Red Dragon" approach - you did. The Red Dragon proposal can only put 7.5t on the surface of Mars, and 6.5t would be the Dragon dry mass. That's one ton remaining.

It's as if you and Impaler want MCT to be a non-viable, heavy monstrosity that barely can push its own weight around. It's almost as if you're not trying to imagine how to reduce dry mass in meaningful ways that just about any aerospace undergrad would think of (let alone a team of seasoned professionals).

You couldn't be more wrong. (Heck, Impaler and I hardly agree on anything) I'm just trying to imagine a *VIABLE* vehicle with sufficient margins to be a reusable tool for opening up Mars and the solar system. Something closer to a utility truck rather than an extreme sports car with razor thin margins.
« Last Edit: 11/03/2015 04:49 AM by Lars-J »

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #1073 on: 11/03/2015 04:30 AM »
How many times we we have to explain that conservative means 'what were sure we can do at reasonable time and money' not 'an incorrect estimate of the best we could ever do', being conservative dose not cut off the future potential to exceed original goals either in the detailed design process or in subsequent upgrading campaigns.  Maybe your absurd dry mass fractions will be achieved some day, but no sane person would bet the company on hitting those targets on the initial design.

Your claim that single digit mass fractions and 8 km/s DeltaV were achievable in the MCT is absurd (It seems you decided not to back off to 6 km/s and 20% so be it)  because you ignore all the parasitic mass involved in keeping a vehicle alive during interplanetary transit, EDL, rough surface landing and launch, cargo-hold and unloading access.  If the MCT were just tanks and engines you would be right, but it's not.

In my own estimations if you removed all dry mass other then tanks and engines the dry mass fraction when fully fueled would be just 6%, right on par with SpaceX's established performance, but the total vehicle dry mass is 20% of the GTOW once you account for real world needs (and in all likelihood I am underestimating the dry mass fraction).  You have been repeatedly accusing me of sand-bagging SpaceX, but your wrong, I started with the same basic rocketry you did and accounted for real needs and the parasitic mass resulting from it.

« Last Edit: 11/03/2015 04:34 AM by Impaler »

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #1074 on: 11/03/2015 04:33 AM »
To put it into another frame:

If you have estimates of achievable performance, and you hang error bounds on those estimates, and you decide that you're going to rely on the 95% high error bound being achieved, or the mission fails;  And you do this several times on several (n) different variables.  And the mission doesn't fail?

It's almost certain (1-0.05^n probability)  that the people who wrote the estimates were lying to you about what they thought was achievable.  Because you're a bad manager who won't accept reality.

Like James Tiberius Kirk and his "Two hours to fix the engines?  You have 30 seconds or we all die!"


« Last Edit: 11/03/2015 04:47 AM by Burninate »

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1075 on: 11/03/2015 12:10 PM »
Lars-J:
I said: "If you think a 10-ton bulldozer is "tiny""

A typical 10-ton bulldozer on Earth is built using steel. Aerospace grade materials have 20 times the strength-to-weight ratio, so fitting something the size of a 10 ton bulldozer into the ~2 ton payload capability of Red Dragon is certainly feasible, especially with careful, topologically-optimized design. To gain the traction you'd need in the lightweighted bulldozer, use ballast boxes like is commonly used with Bobcats.

So you can build something the size of a 10-ton bulldozer using just 2 tons of payload mass (and yes, taking the rough physical dimensions of Red Dragon into account).

I am just repeating what I said earlier, but you must have missed it.
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Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #1076 on: 11/03/2015 12:12 PM »
A heavy-duty "utility truck" would require multiple stages, thus losing all of its utility.

My opinion is that MCT may be capable of 6-8km/s. 8km/s is certainly within the range of possibility. DC-I was more ambitious than even that. But 8km/s is the upper range.
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Offline spacenut

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Re: MCT Speculation and Discussion Thread 4
« Reply #1077 on: 11/03/2015 02:30 PM »
Construction equipment taken to Mars will probably be made from aircraft aluminum or titanium to reduce weight.  If extra weight is needed, Martian regolith, or even a lox tank for extended durations on Mars can be added to it.  Also, Martian gravity being only 38% of earths will make lighter equipment capable of doing the same proportion of work as a steel earth moving piece of equipment.  Equipment will also probably be using something like a Tesla battery package with a solar charging station. 

Also landing on a fairly level plain with nearby mountains or craters for water access shouldn't be too hard to find. 

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #1078 on: 11/03/2015 03:09 PM »
Construction equipment taken to Mars will probably be made from aircraft aluminum or titanium to reduce weight. 

I am not sure they will go that way. Such equipment will be extremely expensive to develop and build. Also regolith is not that dense. It will not be easy to add enough mass. Maybe they just send more standard equipment, adjusted to Mars conditions with electric drive. It does not need to be the 20 ton caterpillar early on. They can still go big when they are able to make at least the biggest heaviest components locally. But even if they go that heavy it would only be 20% of a full MCT cargo load. Maybe the beams of large cranes are worth making them of aluminium.

Offline meekGee

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Re: MCT Speculation and Discussion Thread 4
« Reply #1079 on: 11/03/2015 03:10 PM »
Where is the 12 flight requirement coming from?

And suppose there is some requirement.  You're implying customer flights, and revenue-generating ones at that, are cheaper than SpaceX flight?   

For a reusable rocket, test flights are the cheapest option.   

Quote
"A rocket is typically tested only once with a dummy payload with subsequent launches being with paid customers who have satellites."

But the whole point was that this is a reusable rocket.  Your "typically" applies to expendable rockets that operate in a field where there are worthy second-rate payloads that can both generate revenue and be of lesser value if lost.

None of that applies here.

If you're building a reusable rocket, then you can test it like you test any aircraft.  Send it up (you can test the first stage independently of the rest of the rocket), certify it, then start using it.

You can't use "typically" across a paradigm shift.


But we know that SpaceX, even in the fast build-up scenario, needs to send multiple unmanned payloads to Mars first.

BFR is an Earth-to-orbit vehicle, and so you're not risking a two-year delay when you first launch it.  Yes, the first launch may not carry an MCT, but by the same token, it won't carry any other one-of-a-kind payload.  Being reusable, it's a complete non-brainer to fly a dummy payload.  If it works, you you relaunch.  It it doesn't, good thing you didn't.

Actually, once you're shifted to a reusable rocket, test flights are cheap enough that you could do multiple dummy payloads before you put an MCT on top - and it's not like you wasted multiple rockets doing it.

So I think the "build up flight history" argument doesn't apply.

It will be cheaper per kg to LEO, but it is by no means going to be cheap enough to build an adequate flight history (~12 launches) with nothing but dummy payloads all at SpaceX's own expense.  A rocket is typically tested only once with a dummy payload with subsequent launches being with paid customers who have satellites.  I expect prices of ~200 million per launch even with recovery (about $1000 per kg, Musk's optimistic goal) so it would cost Billions to do these launches without customers.

I expect that 1st stage recovered will be attempted on every single BFR flight with very likely full success from the start.  If first stage recovery works on the dummy flight and is declared a solved problem then the price point will likely be set such that SpaceX is fully covering the cost of the 2nd stage in case it is lost as I expect 2nd stage recover to require a long campaign of attempts with lots of failures and redesigns as we have seen with F9, the customer will not care any more about the success or failure of these recovery attempts any more then they care about 1st stage recovery attempts now. 

By the time you have your 12 flight history your close to getting the 2nd stage to recover reliably and can drop the price to perhaps 100 million and try to get more volume.
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