Author Topic: BFS - without BFR eats the launch market.  (Read 42801 times)

Offline JamesH65

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Re: BFS - without BFR eats the launch market.
« Reply #40 on: 03/02/2018 02:50 PM »
>
P2P service is, I suspect, just Musk brainstorming possibilities. I doubt it will happen for decades if at all.
>

They've already added a third sea level Raptor to BFS for better engine-out and to enhance it for P2P. 

Reddit AMA after IAC2017

Quote
Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

Still don't see SSTO as being of any use. Gravity well is too deep.

Offline speedevil

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Re: BFS - without BFR eats the launch market.
« Reply #41 on: 03/02/2018 03:13 PM »
Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

It starts being economical with two sites.

If a route has enough traffic to fly with a developed network, it has enough traffic to fly alone.
Other routes don't make a specific route more or less economic.

(at least before very handwavy stuff not even mentioned in the presentation such as on-orbit stations)

Offline CuddlyRocket

Re: BFS - without BFR eats the launch market.
« Reply #42 on: 03/02/2018 04:54 PM »
Quote
Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

Have to remember that P2P includes the case where P=P, i.e. launch and landing points are the same! You could start with sub-orbital tourist flights and expand from there.

Offline envy887

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Re: BFS - without BFR eats the launch market.
« Reply #43 on: 03/02/2018 06:13 PM »
Quote
Btw, we modified the BFS design since IAC to add a third medium area ratio Raptor engine partly for that reason (lose only 1/3 thrust in engine out) and allow landings with higher payload mass for the Earth to Earth transport function.

Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

Have to remember that P2P includes the case where P=P, i.e. launch and landing points are the same! You could start with sub-orbital tourist flights and expand from there.

And they could start with uncrewed test flights (daily or even faster) for several months and hundreds of flights, to prove reasonable reliability without a LAS. If the turnaround is as quick as expected this won't cost much and the data gathered will be useful for further Raptor, BFS, and BFR improvements.

Offline JamesH65

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Re: BFS - without BFR eats the launch market.
« Reply #44 on: 03/03/2018 07:48 AM »
Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

It starts being economical with two sites.

If a route has enough traffic to fly with a developed network, it has enough traffic to fly alone.
Other routes don't make a specific route more or less economic.

(at least before very handwavy stuff not even mentioned in the presentation such as on-orbit stations)

Even two sites is going to cost multi $100m, I just cannot see it being worth doing for many many years.

Offline rockets4life97

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Re: BFS - without BFR eats the launch market.
« Reply #45 on: 03/03/2018 01:30 PM »
Even two sites is going to cost multi $100m, I just cannot see it being worth doing for many many years.

I expect there would be some willing investors.

Online Lar

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Re: BFS - without BFR eats the launch market.
« Reply #46 on: 03/03/2018 03:26 PM »
Comes back to cost. The cost of building, from scratch, enough launch/land sites for a BRS to make P2P flights even vaguely economical.

It starts being economical with two sites.

If a route has enough traffic to fly with a developed network, it has enough traffic to fly alone.
Other routes don't make a specific route more or less economic.
That's actually not quite true. At least not in the hub and spoke model. Not every person flying LGA-DTW actually wants to go to DTW. Some want to go to CVG. Not every person flying CVG-DTW actually wants to go to DTW, some want to go to LGA (or LAX or SEA or PDX or DEN or MSP or ...).

Southwest went against this model with some success but they are in the minority.

The existence of all those other routes out of DTW makes the CVG-DTW route economic.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline speedevil

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Re: BFS - without BFR eats the launch market.
« Reply #47 on: 03/03/2018 03:45 PM »
That's actually not quite true. At least not in the hub and spoke model. Not every person flying LGA-DTW actually wants to go to DTW. Some want to go to CVG. Not every person flying CVG-DTW actually wants to go to DTW, some want to go to LGA (or LAX or SEA or PDX or DEN or MSP or ...).

Sorry - unclear.
I mean - the implied network of lots of landing spots does not mean that there are not several-many single routes that are viable as a point-point service alone.

For example - London heathrow to JFK has about a couple of million passengers a year, and if you can convince a tenth of those to try your new service, that's a really good start. (pretty much daily bidirectional flights with one vehicle).

Online Lar

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Re: BFS - without BFR eats the launch market.
« Reply #48 on: 03/03/2018 04:10 PM »
That's actually not quite true. At least not in the hub and spoke model. Not every person flying LGA-DTW actually wants to go to DTW. Some want to go to CVG. Not every person flying CVG-DTW actually wants to go to DTW, some want to go to LGA (or LAX or SEA or PDX or DEN or MSP or ...).

Sorry - unclear.
I mean - the implied network of lots of landing spots does not mean that there are not several-many single routes that are viable as a point-point service alone.

For example - London heathrow to JFK has about a couple of million passengers a year, and if you can convince a tenth of those to try your new service, that's a really good start. (pretty much daily bidirectional flights with one vehicle).
That's fair, if both your endpoints are major population centers, a single segment service is a great place to start. (by the time CVG , for example, has BFR service , BFR/BFS P2P will be ubiquitous... CVG is inland, and is not THAT major of a market)

That said, I would wager that a goodly fraction of LHR JFK traffic continues onward at either one end or the other, or both. So part of the numbers (whether that is 10% or 50% is hard to predict in advance) might well depend on how fast can you get from the BFR site in the outer NY harbor to EWR, JFK, or LGA to transfer onward on conventional aircraft.  That's where a REALLY fast hovercraft or other water transport will pay dividends....

But this is all side talk. The main point that BFS P2P might come sooner than the doubters think is quite sound in my view.

It's fun to think about where you would site a flight site for inland locations... DTW ... Lake St Clair?  ORD ... Lake Michigan?   CVG has to be on land though, you won't be landing in the Ohio river....
« Last Edit: 03/03/2018 04:11 PM by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline sghill

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Re: BFS - without BFR eats the launch market.
« Reply #49 on: 03/03/2018 09:35 PM »
Reality check. BFR will not fly anywhere within earshot of major population centers, or even rural population centers. Plus even 100 reuses is too expensive a system for a passenger route.

Take one look at the political battles that rage around simple aircraft noise- especially in Europe- and then multiply that by thousands.

I used to watch Concorde fly out of Dulles. You could hear it rattle windows for miles. The only thing I've ever heard that was louder from a normal viewing distance was the space shuttle. That noise plus the $10k ticket prices that killed it.

Bring the thunder!

Offline alang

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Re: BFS - without BFR eats the launch market.
« Reply #50 on: 03/03/2018 10:39 PM »
Concorde used to start car alarms where I worked. Leave a car for two weeks and the battery could be drained.

Offline Robotbeat

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Re: BFS - without BFR eats the launch market.
« Reply #51 on: 03/03/2018 10:55 PM »
Who said the passenger version would only do 100 reuses?
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Online Lar

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Re: BFS - without BFR eats the launch market.
« Reply #52 on: 03/03/2018 11:04 PM »
We'll see... I think 100 reuses is a lowball number, by far.... (I'd bet serious coin on that) and the siren song of super fast transit to the other side of the world MIGHT overcome the NIMBYs.  (no bet)
« Last Edit: 03/03/2018 11:11 PM by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline envy887

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Re: BFS - without BFR eats the launch market.
« Reply #53 on: 03/03/2018 11:10 PM »
Reality check. BFR will not fly anywhere within earshot of major population centers

Nobody said it would. Launch sites will be connected by ferries/trains and later by loop/hyperloop.

Who said the passenger version would only do 100 reuses?

Nobody, AFAIK. I think by the time BFS is used for P2P instead of just suborbital or orbital tourism (which have much higher sustainable ticket prices), they will be used several times more than that without major refurb.
« Last Edit: 03/03/2018 11:15 PM by envy887 »

Offline guckyfan

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Re: BFS - without BFR eats the launch market.
« Reply #54 on: 03/04/2018 06:03 AM »
Who said the passenger version would only do 100 reuses?

I am sure at the 2016 IAC 100 reuses of the orbital vehicle was metioned. But that is out of the window with recent developments. At that reuse rate they would not even have to think of regular point to point services at airline like cost.

Offline sghill

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Re: BFS - without BFR eats the launch market.
« Reply #55 on: 03/04/2018 11:25 AM »
Reality check. BFR will not fly anywhere within earshot of major population centers

Nobody said it would. Launch sites will be connected by ferries/trains and later by loop/hyperloop.


Please elaborate to and from where.

Major population centers are nearly always near the coast, and when they aren't, they are surrounded by lesser populated areas. So unless you get super remote, like White Sands or the Gobi, you are going to have to have water-based operations far enough out that people can't see it or hear it (as evidenced by the fight over off shore wind farms).

At that point the time, cost, and hassle of transporting people back to their intended destinations had negated the speed advantage for all but the most distant destinations (that highly coveted Calgary to Abu Dhabi route).
« Last Edit: 03/05/2018 05:51 PM by sghill »
Bring the thunder!

Online RonM

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Re: BFS - without BFR eats the launch market.
« Reply #56 on: 03/04/2018 03:56 PM »
The best use of BFS as a suborbital passenger service would be to cross the Pacific and Atlantic oceans. Might add Arctic and India oceans too depending on overflight considerations.

The pads would be offshore for obvious reasons and would connect to transportation hubs via ferry. There will still be the hassle and time of loading/unloading baggage, security checks, customs, etc. just like any international airport. The only thing that saves time is the flight. So, suborbital flights only make sense for the longest flights.

Inland cities aren't going to work. An Atlanta to Paris flight would be nice, but where do the pads go? There's nowhere to put them.

Offline Robotbeat

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Re: BFS - without BFR eats the launch market.
« Reply #57 on: 03/04/2018 04:30 PM »
Who said the passenger version would only do 100 reuses?

I am sure at the 2016 IAC 100 reuses of the orbital vehicle was metioned. But that is out of the window with recent developments. At that reuse rate they would not even have to think of regular point to point services at airline like cost.
Right. That was a WAG for the tanker version of IAC in 2016. The airline-like operations was for the 2017 BFR.
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Offline Robotbeat

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Re: BFS - without BFR eats the launch market.
« Reply #58 on: 03/04/2018 04:33 PM »
The whole point about doing flights from a sea platform is you can do it far from anyone's "back yard" (the BY in NIMBY). Also, you can serially produce the sea platforms from one location.
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Offline hkultala

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Re: BFS - without BFR eats the launch market.
« Reply #59 on: 03/05/2018 07:50 AM »
Lets use some numbers to end these fanboy speculations:

Assumed numbers

Lets assume the following numbers:
Raptor SL thrust: 1.7 MN at sea level, 1.833 MN at vacuum
Raptor SL isp 330 s at sea level. 356s at vacuum

Raptor vac thrust 1.62 MN at sea level, 1.93 MN at vacuum
Raptor vac isp 375s at vacuum, 320s at sea level

Thrust of 3 SL raptors: 5.1 MJ at liftoff
Thrust of 4 vacuum raptors running underexpanded at sea level: 6.48 MJ

Total thrust 11.58 MN = 1180 tonnes.

Note: these numbers already contain the optimistic assumption that vacuum raptor could be used in sea level without damaging it.


Empty BFR

Now consider BFS without any cargo, 85 tonnes of craft + 1100 tonnes of propellant.

Weight of BFS + full tanks: 1185 tonnes.

T/W with tanks full: 0.996

Cannot even take off with tanks full with nominial thrust. (*)

So, lets not have our tanks full. Leave out 100 tonnes of propellants from the tanks:

weight of BFS+tanks 1085 tonnes, T/W 1.088 , initial acceleration 0.86 m/s^2

Now the craft is capable of taking off, but initially >90% of the thrust(and so also energy of the used fuel) is spent fighting gravity, not accelerating ther craft.

Average isp during liftoff is 4*320 + 3*330 /7 ~ 324s.



The craft is using ~3.6 tonnes of propellant every second.

After 28 seconds it weights 100 tonnes less, T/W is now 1.20.

Average T/W during these first 28 seconds is about 1.143, making average acceleration 1.4 m/s^2.

So, during thje first 28 seconds it has accelerated about 40 m/s, risen about 560 meters and expended 100 tonnes of propellant.

Then the T/W is starting to be close to the T/W of normal rockets during liftoff, but we are still going to lose MORE to gravity losses than many other rockets because it's still slightly worse.


Now we have 900 tonnes of propellant left in our tanks, and we are heading upwards at 40m/s velocity.

The atmosphere is now getting thinner and T/W is getting better, but we still have to keep using our SL raptors for a long time to have reasonable T/W.

So, lets say that for the next 140 seconds we still keep using the SL raptors and the isp is somewhere between SL and vacuum isp and closer to the vacuum isp, average 352 seconds.

Those next 400 tonnes of propellant give us impulse of 1914 m/s.

Then, after that we can use just vacuum raptors with the 375s isp to burn the last 500 tonnes: 7089 m/s.

So, total impulse of ~9 km/s. This looks to be enough to reach LEO.

And this is consistent with Elon's comments that BFS might probably be able to reach orbit alone.




BFS with 20 landing fuel and 10-tonne payload

But what about having 10 tonnes of payload and need to left 20 tonnes of propellant unused for landing:

Use same weights etc. but just rise with 10 tonnes less fuel and don't use last 20 tonnes of fuel.

The last phase of the burn (that was calculated with full vacuum raptor isp) is now from 585-> 115 tonnes instead of 585-> 85 tonnes, other values stay the same.

The last phase of the burn now gives only 5978m/s. The total delta-v is only 7.93 m/s.

No way that is even near of reaching orbit after all gravity losses still happening after the first 28 seconds.


So, even if BFS could barely do SSTO without landing fuel and without payloads, doing it with landing fuel and even a very small payload IS MUCH MORE DIFFICULT and not realistic.

Possible changes to the craft

And replacing any of the vacuum raptors with SL ones  with ratio of 1:1 don't make these numbers much better. Slight increase in T/W would give reduced gravity losses, but that would also mean worse average isp for the middle flight.

(and replacing them or some of them with higher number of SL engines mean huge changes to the craft, the whole thrust structure would have to be totally redesigned and the craft structurally strenghened for the higher loads etc)


Also, having higher isp for the vacuum raptors, like 382s instead of 375s would only give about 150 m/s more delta-v, about 8.1 km/s after the first 28s gravity losses. Still not enough.


notes

(*) Yes, it might have an emergency thrust higher that this, but emergency thrust is for emergencies only, not for routine flights with reuse.

« Last Edit: 03/05/2018 11:21 AM by hkultala »

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