Author Topic: How BFR Earth-to-Earth Might Actually Get Started  (Read 110641 times)

Offline cppetrie

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #40 on: 04/02/2018 06:11 pm »
BFS will hold far more than 100 passengers. Also, remember that these flights are only 30-60 minutes in length. Individual passengers don’t require near as much niceties and space for very short haul flights. Think business class space on a regional jet.

Offline speedevil

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #41 on: 04/02/2018 06:19 pm »
I think that the best argument for a BFS based SSTO with a small payload is for SpaceX to gather a lot of experience with it relatively quickly by using it to launch their Starlink constellation. It could potentially allow for a lot of launches in a relatively short time frame.

Starlink polar orbit requires considerable more delta-v than just "barely reaching some orbit" which BFS might be able to do.

No way BFS will reach starlink polar orbit without the booster, so could we stop polluting all threads with this BFS-SSTO nonsense, please.

It requires of the order of 200 tons of fuel (burn to GTO, plane change, aerobrake, circularise) to get a nominal payload in polar orbit.

This is systematically very very inefficient, as with optimistic assumptions, this is 20 launches or so. (you'd want to do more than a nominal payload).

The only scenario I can see this happening is if this is part of an effort to qualify BFS for passenger service, for which a few thousand flights might be a very nice side-benefit.

Offline Alkan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #42 on: 04/02/2018 06:41 pm »
What’s the business case for P2P on earth? If the per flight cost is $5M, which is fantastically low for space launch purposes and quite low enough for space tourism, it still doesn’t seem to fit P2P.

It doesn't.
The basis of P2P on earth is launch costs well under $1M, and lots of people fly.
'Cheaper than economy air fare' - for around a thousand people, flying the airframe a dozen times a day, can really add up to a profitable vehicle.

May it start out rather more expensive than this - likely.

What this actually says to me is that BFR is probably oversized for an initial P2P suborbital passenger service. What you need is a fully reusable vehicle that can carry maybe 50-100 people at super premium fares. It will be cheaper to build, cheaper to operate, require cheaper infrastructure, and generate the same revenues as 1000 people paying an economy fare.

No, building ANOTHER COMPLETELY DIFFERENT VEHICLE is not cheaper. It is much more expensive.

And for super premium fares, the people also want to have super premium conditions. Lots of space/person.

It seems as though the reusable payload actually shrinks pretty rapidly to the point where it makes little sense to make much smaller of a vehicle. You shouldn't in principle be able to figure this out based on the rocket equation alone since you dump a bunch of reentry velocity using a heat shield.

So, it might be something like your payload goes waaaaay down if you don't use a huge spaceship. You know, since your new delta-v has a big spaceship instead of just the payload inside of it. Your relative mass of the spaceship to your fuel goes down as the size increases since the dry mass would roughly increase quadratically and your fuel mass would increase cubically (surface area containing the fuel, which has a volume component). I.e. you might only be able to launch a few tons into orbit on a falcon-9 sized BFR. Or, it might not even reach orbit unloaded.

But going up in size allows you to get around this problem. So, very, very large rockets could be a pretty useful thing in the future. I don't know how badly the size effects things. I think BFR will justify things like developing significantly more efficient tools to make absolutely massive carbon fiber fuel tanks.

Offline Arch Admiral

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #43 on: 04/02/2018 07:29 pm »
"since you dump a bunch of reentry velocity using a heat shield."

How is this possible? The entry angle from a minimum-energy ICBM trajectory is around 20 degrees. Mercury and Gemini entered at about 8 degrees at Mach 25 and subjected the astronauts to about 8g. At Mach 22 and 20 degrees the passengers will be smashed against the nose. If you use aerodynamic lift to shallow out the trajectory, the passengers will be smashed again the floor. Has Elon explained how he plans to deal with reentry?






Offline speedevil

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #44 on: 04/02/2018 07:41 pm »
"since you dump a bunch of reentry velocity using a heat shield."

How is this possible? The entry angle from a minimum-energy ICBM trajectory is around 20 degrees.
It's not minimum energy, but basically fully orbital, meaning lifting reentry works.

Offline Alkan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #45 on: 04/02/2018 07:59 pm »
"since you dump a bunch of reentry velocity using a heat shield."

How is this possible? The entry angle from a minimum-energy ICBM trajectory is around 20 degrees. Mercury and Gemini entered at about 8 degrees at Mach 25 and subjected the astronauts to about 8g. At Mach 22 and 20 degrees the passengers will be smashed against the nose. If you use aerodynamic lift to shallow out the trajectory, the passengers will be smashed again the floor. Has Elon explained how he plans to deal with reentry?

Retro propulsion before you start reentry.

Offline speedevil

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #46 on: 04/02/2018 08:08 pm »
Retro propulsion before you start reentry.

This is not required if you are doing near-orbital entries.
The BFS gently settles into the atmosphere, its effective weight counteracted by lift until it's at around half or so of orbital speed.
After this, it gradually sinks, all at fairly low G.

Offline guckyfan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #47 on: 04/02/2018 09:22 pm »
BFS will fly orbital, not a ballistic trajectory like ICBM. It will be able to brake quite gentle.

Offline Alkan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #48 on: 04/02/2018 09:41 pm »
Retro propulsion before you start reentry.

This is not required if you are doing near-orbital entries.
The BFS gently settles into the atmosphere, its effective weight counteracted by lift until it's at around half or so of orbital speed.
After this, it gradually sinks, all at fairly low G.

Well, we don't know the reentry forces on the BFS.

The Space Shuttle's reentry g-force is list as 3gs in this wikipedia page:

https://en.wikipedia.org/wiki/G-force

But it's not backed up by a citation and I'm having a hard time finding good information. A lot of people make layman's cases for the acceleration being low, but I can't find a solid source. I'd imagine the wing helps keep reentry smooth since it can take a longer trajectory. But I don't work with hypersonic aerodynamics (of which trajectories only solvable numerically), so I can't say for sure.

Offline Ludus

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #49 on: 04/04/2018 06:15 am »
BFS will hold far more than 100 passengers. Also, remember that these flights are only 30-60 minutes in length. Individual passengers don’t require near as much niceties and space for very short haul flights. Think business class space on a regional jet.

Even 100 passengers is pretty high for premium air travel. Boom is making a Supersonic airliner that holds about 50 passengers based on the idea that’s about the business class demand for long distance routes. The Boom SST is supposed to cost about $200M, in the ballpark of other airliners or a BFS. The assumption is that Concorde failed to pay it’s way because it carried too many passengers.

I get that there may be a theoretical break even with large numbers of passengers at relatively low prices. Say 500 at $5000 business class each gets to $2.5M, about halfway to $5M. It’s just not clear that there is that much demand for those tickets. How much more other inconvenience will a passenger tolerate for the convenience of a much shorter travel time? Different departure and arrival cities? Different travel day? In a sense it’s a bet on hub and spoke for long distance which didn’t work out well for the A380.

In questioning the business case, I’m asking how do you start up such a service with plausible pricing and demand if the numbers don’t work for elite business jet or even business class commercial competition. Maybe there’s ultra high priority cargo. Maybe the zero G space Tourism experience and novelty would fill a lot of seats.




Offline john smith 19

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #50 on: 04/04/2018 06:39 am »
What’s the business case for P2P on earth?
The business case?

Time.

When there is now a non stop London/Sydney flight of 17 hrs and you can cut that to 45mins of flights (maybe 90 mins in total on the vehicle) and your longest time cost is the trip in and out of your destination city, and the ongoing absurdity that is security (hint. Check how fast things move at Reagan National, where most of the US lawmakers transit through, against a regular airport).

The idea of "Breakfast in London, Lunch in New York, dinner in London" was real during the Concorde era.

How much (and how many) people would pay for that to be extended to Shanghai, Beijing, San Francisco?

As for size I'll note that people though Concorde was the smallest  vehicle that was viable for this service (despite the French initially wanting to build it smaller). AIUI most "Concorde II" design studies have gone bigger thinking at least 300 passengers.
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Offline speedevil

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #51 on: 04/04/2018 02:31 pm »
I get that there may be a theoretical break even with large numbers of passengers at relatively low prices. Say 500 at $5000 business class each gets to $2.5M, about halfway to $5M. It’s just not clear that there is that much demand for those tickets. How much more other inconvenience will a passenger tolerate for the convenience of a much shorter travel time? Different departure and arrival cities? Different travel day? In a sense it’s a bet on hub and spoke for long distance which didn’t work out well for the A380.

There are a number of city pairs which have very, very high constant demand, and are many many hours away from each other.
In some cases - London/hong-kong, geography of one end means you're probably going to need to land 50 miles away from the city.
There is no particular reason to not have a service several times a day.

Also, you can't assume $5M, as for P2P to be real, the probability of loss has to be tiny, so you can amortise vehicles over many flights, making any assumption leading to $5M implausible.
The price of launch is intended to - for P2P - be in the $1M range, with the costs other than amortisation and fuel having to be in the $250K or under range.
However, several thousand flights a year is quite possible, meaning the profit even at these fares is not inconsiderable.

Offline docmordrid

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #52 on: 04/04/2018 06:20 pm »
>
As for size I'll note that people though Concorde was the smallest  vehicle that was viable for this service (despite the French initially wanting to build it smaller). AIUI most "Concorde II" design studies have gone bigger thinking at least 300 passengers.

AIUI Boom SST is 55.
DM

Offline guckyfan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #53 on: 04/05/2018 06:18 am »
At what altitude would the noise of BFR flying over inhabited land be acceptable? I think once BFR/BFS is certified for commercial passenger service it should also be certified to fly over land, except for the noise.

Offline envy887

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #54 on: 04/05/2018 01:05 pm »
At what altitude would the noise of BFR flying over inhabited land be acceptable? I think once BFR/BFS is certified for commercial passenger service it should also be certified to fly over land, except for the noise.
Overflight altitude is not so much a noise issue, launch and reentry are the loud parts. Only the area around the launch/landing site will have issues with noise.

Offline guckyfan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #55 on: 04/05/2018 02:29 pm »
At what altitude would the noise of BFR flying over inhabited land be acceptable? I think once BFR/BFS is certified for commercial passenger service it should also be certified to fly over land, except for the noise.
Overflight altitude is not so much a noise issue, launch and reentry are the loud parts. Only the area around the launch/landing site will have issues with noise.

I am thinking of the launch phase where the engines are running and they are already quite high.

Edit: Like launching from London and still suborbital over Scandinavia.
« Last Edit: 04/05/2018 02:31 pm by guckyfan »

Offline envy887

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #56 on: 04/05/2018 03:39 pm »
At what altitude would the noise of BFR flying over inhabited land be acceptable? I think once BFR/BFS is certified for commercial passenger service it should also be certified to fly over land, except for the noise.
Overflight altitude is not so much a noise issue, launch and reentry are the loud parts. Only the area around the launch/landing site will have issues with noise.

I am thinking of the launch phase where the engines are running and they are already quite high.

Edit: Like launching from London and still suborbital over Scandinavia.

BFR will typically leave the sensible atmosphere before it leaves the noise footprint of it's own launchpad. At that point, even if it's still suborbital, there is no real noise propogation to the surface.

The only noise issue is the sonic boom along the entry corridors for the space ship and the booster, and launch noise immediately around the launch site.

Offline guckyfan

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #57 on: 04/05/2018 04:56 pm »
Thanks. So basically as soon as BFR/BFS are certified safe, they can fly over land. Just have the launch platform far enough from the coast.

Offline Slarty1080

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #58 on: 04/05/2018 05:04 pm »
At what altitude would the noise of BFR flying over inhabited land be acceptable? I think once BFR/BFS is certified for commercial passenger service it should also be certified to fly over land, except for the noise.
Overflight altitude is not so much a noise issue, launch and reentry are the loud parts. Only the area around the launch/landing site will have issues with noise.

I am thinking of the launch phase where the engines are running and they are already quite high.

Edit: Like launching from London and still suborbital over Scandinavia.

BFR will typically leave the sensible atmosphere before it leaves the noise footprint of it's own launchpad. At that point, even if it's still suborbital, there is no real noise propogation to the surface.

The only noise issue is the sonic boom along the entry corridors for the space ship and the booster, and launch noise immediately around the launch site.

I thought a platform in the North Sea might have made a suitable launch site. It could probably be positioned sufficiently far away from land so as not to cause major problems during take-off, but the final stages of re-entry with sonic booms across Ireland and England would be a show stopper.
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Offline Lar

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Re: How BFR Earth-to-Earth Might Actually Get Started
« Reply #59 on: 04/06/2018 01:45 pm »

BFR will typically leave the sensible atmosphere before it leaves the noise footprint of it's own launchpad. At that point, even if it's still suborbital, there is no real noise propogation to the surface.

The only noise issue is the sonic boom along the entry corridors for the space ship and the booster, and launch noise immediately around the launch site.

I thought a platform in the North Sea might have made a suitable launch site. It could probably be positioned sufficiently far away from land so as not to cause major problems during take-off, but the final stages of re-entry with sonic booms across Ireland and England would be a show stopper.

If it's in the north sea, that might be ok because I would expect use of Great Circle routes, and most of those would come from, or go to, the north (why change inclination?)
« Last Edit: 04/06/2018 01:47 pm by Lar »
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