One thing is sure: Elon Musk dreams big. He has made billions with
PayPal and pioneered electric cars for the mass market with the Tesla
brand. His SpaceX rocket has made history several times, last but not
least with the first commercial manned mission to the ISS in 2020.
His supporters stand in awe whenever he announces another big dream he
intends to pursue. Making space flight affordable for people like you and
me, by pushing for 100% reusability is the aim of the BFR, the
Big Falcon Rocket, except the F is not for Falcon, says Musk with his
signature big smile.
The BFR is the vehicle for another big dream: going to Mars, building
colonies on the Red Planet. Concept art and high quality CGI can be
found with little effort. In one clip Elon contemplates another idea: If
I have a BFR that can go to Mars, why use it only for long haul? Why not
use it for city to city hopping? Think of it, at speeds up to
27,000km/h, you can get from anywhere to anywhere often in under 30
minutes. How would that work? Let's go to the source and find out. This
is the SpaceX promotional video:
The scene is set in Manhattan, New York City. 57 people are boarding a
ship destined to a swimming space port a few km south where the mighty
BFR stands erect, flanked by propellant tanks, next to a tower.
Passengers enter the rocket via a 30m long retractable arm. Then the
monster roars towards the sky. The spent first stage retrofires to be
reused. The shiny metal tube reenters over Shanghai and descends onto
another swimming space port with the skyline maybe a few km in the
background. Flight time: 39 minutes.
Wow! How impressive!
Or is it?
Sometimes ideas collide with reality and reality wins. Just because a
dream is big, doesn't make it feasible. Just because an idea is put
forward by Elon Musk doesn't make it come true in 2022. Just because someone is a
billionaire doesn't allow them to violate the laws of Physics. Or
engineering. Or economics (only for while). If the idea is put under scrutiny by
independent and down-to-earth, slide rule carrying boring physicists,
engineers, accountants and safety regulators, who ignore for a moment
whose idea they are looking at, will the idea survive a reality check?
Will it crumble? Is the verdict "Take my money!" or "Nope, not gonna
happen."? City hopping with the BFR, thumbs up or down, that is the
question I want to shed some light on. Why? Because talking rockets is
fun! And maybe to help people make sound investment decisions. We all
know the saying about fools and their money.
What are the arguments in the way of the proposed mode of transport? How
hard would obstacles be to overcome? Which problems could be solved by
throwing money at them? Which can't? Which problems just need more time?
Which are hopeless and why? Which problems in rocketry are intrinsically
hard to solve? Is it allowed to call a spade a spade or is criticizing
the visionary a case of Lèse-majesté that should be punished with a
tweet storm? The deck of persuasion is stacked heavily in favor of those
who can spend big bucks for a marketing department capable of ordering
snazzy CGI clips. So I find it fair game if critics don't beat around
the bush and use strong words to describe an idea. Here's one of them.
To paraphrase and occasionally expand a bit on what is to see:
- A fueled BFR is comparable to a 10kT bomb, that's half a Little Boy,
which is why it is usually done last after astronauts are
strapped in with BFLB (Big Frigging Leather Belts), and takes hours.
Lying there on your back and immobilized was a nuisance to Neil, Mike and Buzz
but might ruin the desired passenger experience so important to marketing
departments.
- The advertised time does not account for getting to the pad, security screening,
technical issues that may arise during countdown. An honest calculation of
travel time should take this into account. Droplets of snake oil?
- Launch sites are far away from cities for a reason: safety and noise (see
bomb above).
People living near airports hate the noise. Would the mayor of New York City
want a space port near the Statue of Liberty with noise levels exceeding
Saturn V launches? Possibly several times a day? Sonic booms for incoming
BFRs? I doubt any mayor (or voters) would welcome a busy space port.
- Landing conditions at the destinations must be ok, or you can't start.
This can make the travel time unpredictable. This is a constant theme in
space flight. How long did it take before they lit Big Al's candle?
Launch scrubs are not uncommon, either. Anybody got delay statistics for
Mercury, Gemini, Apollo, Space Shuttle, SpaceX? Can we expect the BFR
to be different?
The aforementioned issues could likely be ignored or accepted. People
might be ok with some discomfort for the trip of a lifetime. I certainly
would. Regulators might require space ports to be 50km away from cities
of more than N inhabitants. We accept getting to an airport may take 2 hours, plus
a 2 hour margin for international travel. At both ends (baggage claim?
customs?). When it comes to space ports, well, there's a fueled rocket
waiting for me, I'm fine if the actual travel time door to door is 20
times that of the snake oil droplets in a friggin promo CGI. We've heard
a lot of sales pitches before, haven't we? Who would trust them anyway?
But other issues are not so easy to accept, overcome, or ignore. They
are more fundamental. Earth's gravity well is devilishly deep and hard
to climb. No amount of money and engineering will change that fact.
Not even dreams of a 150 bn wunderkind.
- I'm neither an economist, nor an accountant. Neither is Elon.
Flying with rockets is massively expensive. It takes several engineering
disciplines to their limits. Materials science, thermodynamics,
fluid dynamics, combustion to name a few.
Costs are: development, fuel, ground infrastructure,
hardware refurbishing, enough personnel and backups for each of these.
People don't work for free.
Even with a 100% reusable rocket there are high operating costs.
What is the path to economic viability? Knowing Elon's nonchalance with
numbers and deadlines I would call any of his economic calculations
industrial grade snake oil unless confirmed by an independent double
blind team of economists and accountants, swearing under penalty of perjury
that the numbers match with reality within a factor of two.
- The success rate of rockets is... not good. About 1 in 100 fail, which
the field considers a successful track record. Wouldn't we consider
airplanes with a 1:100 failure rate suicidal? Would we want rockets to
have the same reliability as aircraft (about 1 in 10.000.000 flights)?
This would require engineering to improve reliability by a factor of
100.000. How much has rocket reliability improved over the past 50
years, despite rocket engineers trying hard? How many engines are
there on a single BFR? About 40. It takes only one to catastrophically
fail for rapid unscheduled disassembly. Are there safety devices for
passengers to survive obviously a major malfunction(*)? How do these
drive mass and cost? The rocket equation is a harsh mistress, as all
of physics can be. The reality is: shit happens. Maybe that welder
welding that fuel pipe had a bad day and the weld is substandard. Near
staging you suddenly experience engine-rich exhaust. Maybe an engineer
used an old i386 with an FDIV bug to calculate the number of bolts
needed to keep SN9 upright in the bay and not look like a drunk
clinging to a street light (**). Quality is a bitch and it is hard to
change people and develop effective quality control processes.
Redundancy only gets you so far without running into the damn mass
constraints of the rocket equation. Lack of quality in rocketry is not
merely an oops. An oops can quickly avalanche to a boom, and funeral
music. Rocketry is hard. Space is hard. Both will stay that way.
Which is why BFR city hopping is an exercise in snake oil salesmanship
that would make Trump blush. A publicity stunt to throw a bone to the
cultists. NYC to Shanghai, thank you, I take the cheaper airplane flight
that gets me there alive. If it takes twice as long so what, I'm jet lagged
either way.
(*) Obviously a major malfunction
https://www.americaspace.com/2016/01/30/obviously-a-major-malfunction-30-years-since-the-loss-of-challenger-part-1/(**) The leaning tower of Bocachica
https://www.teslarati.com/spacex-starship-prototype-sn9-damaged/
