All posts by Kent Nebergall

Kent Nebergall became involved with the Mars Society in 2004, when he won the Mars Society’s Kepler Prize to design the Mars Direct Earth Return Vehicle. He has given several convention track talks each year ever since, focusing on new inventions for space settlement, along with gaining a systematic and historical understanding of the human need and ability to do so. Kent is a business analyst in the Chicago suburbs, where he’s worked in over a dozen industries. Kent served as astronomer for Mars Desert Research Station (MDRS) Crew 32, commander of Crew 124, and has helped design and build the new MDRS science dome. He is on the Mars Society Steering Committee.

Construction Update

I recently updated my WordPress version, but apparently my original (third party) theme does not work with the latest version of WordPress, or some other such issue. I’m not sure.

For now, that means the images on this site are off. But at least the site loads, which is better than before when the whole thing was unusable. The articles seem to be mostly-fine. I’ll need to clean up the home page a bit though.

Sorry for the inconvenience.

Mars Colony Design Update

While it was an honor to get to the top ten finalists out of one hundred entries, I did not get into the top five winners.    I don't think this is the time or place to get into why the judging took place the way it did, simply because I wasn't there for it.  What I will do is defend my thesis and purpose in the decisions I made with the Eureka design, and elaborate on the answers I gave the judges at the time.   As far as making up your own mind, the top 25 semifinalists will be published in a book next month.  So future mars colonists will be able to review our work. 

Pu​rpose

There were a number of hard questions meant to give a more complete picture of the settlement in the contest criteria.  You had to come up with the structure, then an economic model, then a political model, aesthetics, and culture.   I went well beyond this into the very toughest challenges to space settlement where Mars is concerned and took them all head on.  Everyone else was running a marathon, but I was running an obstacle course for the duration of the marathon.  

Here's the thing.  I've been working on parts of the problem for over a decade.  I had focused on the worst issues I could find, because I know that there are not good answers in a lot of those spaces.  No one had a complete solution that was designed to be complete and unassailable to the critics and possible risks of a settlement on Mars.  I decided to try for that.  Even if it fell short, it would show what the hardest issues were left to solve so that I could focus on those.  But as it turned out, I answered all my challenges and offered root cause analysis back to the origins of civilization as to why those solutions were chosen. 

The Greenhouses

Robert Zubrin asked about the power requirements for the greenhouses, and has stated in the past that he felt greenhouses on mars should be naturally lit.  I did not have an answer for him off the bat because I had realized that power was not a forcing function last winter with my design.  The forcing function was surface area per person for food growth.  The best shape for a pressure vessel is a sphere.  A flat greenhouse, even under partial pressure, is a nightmare in terms of structural needs, materials needed for transport to Mars, and so on.  It is wide open to meteor strikes or possible attacks by rival powers.  There is the possibility that a key crop is too sensitive to radiation to be reliably used as food after being grown with such exposure.  I chose a heavily shielded stacked LED lit greenhouse because this gave easy temperature control, 24 hour daylight where appropriate, radiation protection to the harvesters, meteorite protection, and restricted exposure to epigenetic crop stress.  In the rotating ring, I also had 1G gardens where that was potentially appropriate.  We cannot just assume that all plants we want or need will grow in a Mars greenhouse, or that building them would be a good idea.  As for the power question, when the population hits 1200 people, the power demand is 275 megawatts.  At 40 MT per 2 megawatt reactor, this calls for larger reactors for better economies of scale. The concept called for every sixth starship to be carrying a nuclear power plant with a six megawatt output.  My forcing function was volume, followed by reactor transportation, followed by reactor cost.   Natural light would not reach the inside of a stacked plant bed like that used in an LED greenhouse.  With Mars frequent dust storms, you would need a reactor-fed LED light system anyway in addition to all your glass.  So why bother making more failure points?
Anyway, I had to get that off my chest. 

Conclusion

 The Mars Society is now saying they want to sponsor a million person Mars city proposal.  At the moment, I'm not considering making an entry.  I put off a number of projects to do the work I did on the Mars Settlement design.  I'm unwilling to keep putting off life goals for things like this after having sunk a very critical, pivotal year into this one.   The lasting good effects of having a bulletproof (literally) Mars settlement design are just beginning.  Eureka may be the winner in the long run after all. 









Yet Another BFS Set of Changes from SpaceX – Some Speculation

UPDATES (12/9/2018):

The basic layout of the ship will remain the same. So most of the speculation below is off.  However, some issues, such as where the solar arrays will be stowed, are still open for interpretation.

Material Change to Vehicle

The main change is that they are shifting from a very high tech composite to a “fairly heavy metal”.  The lightest metal routinely used in spacecraft engineering at scale is an aluminium/lithium alloy that was used for the Space Shuttle external tank.  An unlikely option is something with a thin layer of stainless steel, which would be necessary for a reusable vehicle using liquid hydrogen.  That said, bear in mind that the initial flights to Mars may need liquid hydrogen in reserve (rather than drilling for water) because the reserves of water would have yet to be proven on Mars.  This was actually a key design detail of Mars Direct, which was designed before glaciers were observed below the dust in middle latitudes.  These glaciers are ideal for providing propellant, but it would take a crew to set up a proper drilling rig and return the melted water to the surface for processing.  The first ships will not have crews, meaning that either they have to take a fairly hard risk and build the propellant plant on the first crewed mission or die on Mars, or they have to have a backup plan like a hydrogen tank.  A lot has to go right on the first two missions to get that infrastructure in place.  That said, there would be no shortage of volunteers.

Schedule Updates

They also announced that we will be seeing “cool pictures of the demo Starship that will fly suborbital hops in the coming (roughly) 4 weeks.”  Four weeks out from the announcement date would be January 5, 2019.

SpaceX applied for a license to test the “up and down hopper” version of Starship in Texas.  They will initially do short tests up to 500 meters/100 seconds.  They will then extend to 5 km flights up to 6 minutes.  They are also finalizing construction of the test facility/launch facility with a large tent, fuel tanks, antennas, and moving a lot of dirt around to make causeways across the soft sandy ground.

(Original Article below).

Recently, Elon Musk mentioned that the new spacecraft in development, known originally as Mars Colonial Transport, then Interplanetary Transport System, then BFS, is now being called simply “Starship”.  The booster is now also simplified from BFR to “Super Heavy”.  This makes some sense in that his satellite constellation is being called Starlink.  This seems to go back to calling the space-suited mannequin on the Falcon Heavy test launch “Starman” after the David Bowie song.  It wouldn’t be the first time – Dragon is named after Puff the Magic Dragon, and Falcon after the Millennium Falcon from Star Wars.  The two drone ships are named after sentient starships from a science fiction book series.  There are also lots of Hitchhikers Guide to the Galaxy references in his nomenclature.

He has also indicated that he recently-announced Hello Moon version of the Starship design is being changed yet again.  The new change would be counter-intuitive, and when asked, he simply said, “RADICAL CHANGE”.  He also said the new design is exciting and delightfully counter-intuitive.  Note that he also said the Hello Moon design was counter intuitive due to the front and rear fins rotating in terms of lateral drag but appearing superficially like forward control surfaces.

Keep in mind that Blue Origin has second mover advantage here.  Whatever SpaceX settles on for a design, Blue Origin can come up with something similar or better with New Armstrong.  Constantly changing the design in radical ways will tend to kill that second mover advantage for Blue Origin in terms of years of lead time.  If SpaceX had stuck with their original design, Blue Origin would be studying it extensively and have a three year lead time in coming up with something better.  As of now, they are just as lost as we are.  That may also be part of the purpose of the ever-changing design, and some of the Issues mentioned below.

Analysis of Issues

Here are the key issues I’ve noticed with Starship’s design…

No escape system – It is designed like an airliner, but with no escape system for ground evacuation or launch escape.

Solar Panels – Most recent versions do not seem to have a place to stow solar panels.  The ones protruding from the initial design could have been stowed in the tri-form fin-like elements, but the past few ships show nothing but tanks in the rear sections. This implies some sort of lateral spine or (in the Hello Moon version) a place to put much smaller panels in the tail-fin.

Center of Gravity/Center of Pressure on Landing – The Falcon boosters can land tail-first in part because all the engine weight is in the tail.  They basically land like lawn darts.  A Starship will launch with a heavy nose, fuel tanks, and a heavy tail with the engines.  After using up the fuel supply from launch, there is still weight in the nose and tail, averaging out the center of gravity.  That said, the rear fins went from protrusions to a small delta control surface to a full pair of fins because the side-facing atmospheric entry would make it difficult with so much mass in the tail to keep the vehicle stable.

Unloading on the Moon/Mars – Depictions show a door opening out of the cargo bay side with a crane lowering components to the surface.  This would work with some elements, but may be difficult with large equipment.  It would also be very difficult with passengers in mass or any sort of emergency landing with a point-to-point flight on Earth.  Unloading like this with heavy gear may actually risk tipping the ship over, especially if it were landed on any slope toward the cargo door or an unsteady surface.

Engine Plume Hammering on Unprepared Surfaces – One key reason that NASA resorted to the extreme measure of landing the Curiosity rover using a skycrane was that engines big enough to land a one ton rover could easily drill the lander into the surface by way of the extreme supersonic exhaust plumes of the rocket engines being so close to the surface.  The Apollo LEM dealt with this by using a wide engine bell, low impulse propellants, and cutting the engine about a meter in the air and then dropping down to the surface.  Existing Mars landers typically use a cluster of tiny rockets in pods on the side to spread out the exhaust plume, and also use low impulse propellants.  SpaceX is using fairly high performance engines, and sea level ones in the case of the Hello Moon version, for landings.  On Earth, landing on a concrete pad solves the problem for the most part – the surface is strong enough to support the lander on a level surface, and also to take the force of engines blasting it with hypersonic, superheated propellant.  Unprepared surfaces would have to be very level bedrock outcroppings.

That Massive Window – This would be heavy, fragile, expensive, and deeply impractical from a structural prospective.  The only aircraft I’ve ever seen with a cockpit like this is the US B-36 bomber.  While the second version of Starship lacked this window, the Hello Moon version brought it back again.

Possible Options

So what does that mean?  SpaceX must be aware of all the issues listed above. Here are some speculations off the top of my head – some wild, some pragmatic.  The wilder ones are simply in response to how loaded that announcement was in terms of language.

Canard Changes?  Given the purpose of the canard wings on the Starship, it seems logical to change the arrangement from rear-swept to forward-swept to decrease drag and increase “reach” for the movement arm of the control surface.  If the lateral engine nacelles were used as noted above, a second set of downward-angled control surfaces could be on the back side.

Move the Solar Panels up to the Cargo Bay?  This has a lot of potential.  They wouldn’t be subject to the stress of being so close to the engines on take-off.  They would also be more accessible by the crew in the event of a deployment or stowing error. On the surface of the moon or Mars, they would be up and away from the dust and surface operations close to the vehicle. This wouldn’t be a radical change, but it’s got some practical benefits.  They could also be more practically scaled to the mission by being larger for deep space, or possibly replaced entirely with a low-enriched uranium reactor like KiloPower.

Detachable Payload/Crew Section?  This also has potential for Lunar and Mars operations.  One benefit of Mars Direct is that the Earth Return Vehicle (filled with propellant) is stored a safe distance from the crew habitat.  By recombining the two (the very first concept that lead to Mars Direct was a single vehicle like Starship), SpaceX adds this risk.

Also, habitats can be left behind or used as space stations, particularly if they have their own solar power supply as mentioned above. Note that the current Starship design has more cubic meters of pressurized volume than the entire International Space Station.  It simply lacks docking ports and would need a much larger solar array. A detachable payload section would have both. At any rate, ISS replacement could simply be a crew module with a solar array, coupled to a power/propulsion/docking Service Module on a second flight.  The Service Module would also have a lot of potential working in series for space factories, solar propulsion systems, and fuel depots. It would basically eat the entire Blue Origin and ULA long term road map in one swift stroke.

This would also add some capacity for crew escape, even if it’s a passive detachment with parachutes or landing rockets.

Lateral Engines?  This is pretty interesting.  The Skylon design has engines on the side because it was derived from the HOTOL concept from a couple decades earlier.  HOTOL would have been a European space shuttle, and used a very heavy engine arrangement at the rear.  However, the center of gravity would have shifted too far back to be stable after fuel burn.  This is also a problem with Starship, and had lead to the ever-increasing wing size at the tail.  Having the small landing engines on nacelles off the side of the vehicle would deal with landing on unprepared surfaces.  It would also give an option for crew escape if coupled with the smaller propellant tanks used for landing in the original design. It may allow for cargo at the rear end if all the engines were put on pylons.  It would remove some of the need for the rear wing, which may be difficult to engineer due to the stress on the hinge and electric motors.  It would give a deployment platform for a crane and a counter-weight for laterally unloading payloads. It may have also led to the name change.  After all, nacelles are a design hallmark of the Enterprise and Firefly’s Serenity.  Elon also doesn’t design anything unless it looks better than Hollywood could design.  This could be a pretty elegant design when he gets through with it.  With such inspirations as Serenity and Enterprise, not to mention the Star Wars Y-Wing, that could be the inspiration for the name change as well.  It certainly would look like a starship in the popular imagination. The disadvantage would be drag on takeoff from Earth, but it would allow for retractable landing gear that could be scaled to the engine arrangement and destination. I can call this neither probable or improbable, but simply file it under, “I wouldn’t put it past them”.

Payload section at the back?  While this would solve the load/unload issue, it would make crew escape much worse. It would also make center of gravity far worse, as well as subjecting the crew/payload to intense vibration.  This is not entirely unlike the back of the Shuttle cargo bay being close to the engines, but it seems incredibly unlikely. It would also be relatively ugly on the interior, since putting windows that close to the compression between the tanks and the engines would be suicidal.  For these reasons, I’d say no.  Though a variant of it is already in place with the tail-end cargo pods for the Hello Moon version.  I cannot see them doing more than that.  Then again, if they put all the engines in nacelles and not just the landing ones, this may make more sense.  The cabin could simply drop down to the surface.

Split Payload Transformer?  Imagine a ship with a payload nose section that splits in two.  Each section could be lowered to the ground for easy access.  The side with the crew/passengers would also have an escape system.  It could also be split into more than two sections, provided they had some symmetry.  They could even be spun for low-level artificial gravity. This seems highly unlikely, because having multiple pressurized bays becomes incredibly heavy for no practical benefit.  I’d say unlikely for this reason.  Elon Musk loves platonic ideals, not complexity for its own sake.

Conclusion

This rather extensive and somewhat wild flight of imagination is the sort of thing one pictures when Elon Musk says, “radical change” and “delightfully counter-intuitive”.  I suspect it will solve some of the issues noted above, because they would need some dramatic reason to change the design at this stage.  Elon Musk likes idealized solutions with minimal draw-backs, and tends to iterate towards a democratized, ubiquitous product line rather than away from it.  I may have hit on some of it with the more likely options.

Knowing SpaceX, I’m looking forward to being proven wrong in some delightfully clever way.









What is Space Force?

Here is the short version.

There is a lot of concern about Russia and China working on hypersonic weaponry, primarily for anti-ship activities.  China probably has enough money and technology to make this work, and they have set up a series of artificial islands around the South China Sea that contain military bases.  The US has spent decades maintaining a vast collection of radar and space assets to track ICBMs and SLBMs anywhere in the world.  The common trait of these missiles is that they exit the atmosphere en route to their targets.  This makes them much easier to spot, and theoretically, makes them vulnerable to lasers or fast interceptor missiles.

The problem with hypersonic missiles is that they travel too fast for conventional aircraft interception, but too low for conventional ballistic missile interception.  They would arrive with almost no warning from radar, designed to see things traveling at high altitude or low speed.  Michael Griffin has stated that the only way to even see these hypersonic missiles is with a set of radar systems in medium to low Earth orbit, similar to the networks that provide GPS navigation or high speed satellite phone conversations globally.  To intercept these missiles, the vehicles may or may not be space based.  In fact, they probably would not be.

There is a second factor of what appear to be satellites from both Russia and China that can approach other satellites and manipulate them.  They could be placing mines or jamming devices on board to be triggered in the event of a war.  They could simply snip wires or crush components.  Again, with legacy cold war satellites costing vast sums of money, there is very little defense against this form of attack.  There must instead be either too many satellites to intercept, or a defense system to prevent those satellites from being sabotaged.

Existing government agencies are relatively moribund.  They are no longer able to field brand new systems quickly without enormous waste in terms of time and money.  NASA was very efficient up until it fulfilled its initial purpose of the moon landing.  After that, it became another contractor game of how much money could be spent on how little work how slowly without the whole organization being shut down.

The US Military already spends more than NASA on space, and has done so for decades.  This is mostly with the Air Force, but also includes such things as navy weather satellites, the National Reconnaissance Office, and so on. Interestingly, these spy satellites will remain separate from the new Space Force, which means that 90 percent of the assets being transferred to  this organization will be from the Air Force.

Where Russia is concerned, there is some concern that this is a set-up. When Reagan announced the Strategic Defense Initiative, it was intended to leverage the US economy to end the Cold War without a fight.  The US spent less than five percent of GDP on military efforts, whereas the USSR spent roughly 25 percent.  The quickest way to win the war was to propose a plan to build a system that would invalidate the investments of the USSR that would not in itself be offensive.  The Soviets had two choices – ignore it and fall behind permanently, or respond to it and go broke.  SDI was only a $150 million research program at first.  This is less than a cheap Mars probe mission. Yet it broke the stalemate of the Cold War.

Fast forward to 2018.  Russia announces a series of wonder-weapons that could overwhelm existing US defenses.  They include stealth fighters – but those have been largely cancelled because they were being built in cooperation with India, who have since pulled out of the program.  They included an orbital ICBM that could be launched over the South Pole and hit the US from the side that lacks the DEW radar arrays for early warning.  The problem here is that the launches can be seen from space (assuming our satellites are not sabotaged), and the actual weapons have been around since the 1970’s.  A third item was a robotic submarine that would detonate a super-nuclear weapon under the ocean to wipe out all human life from the US East coast.  This seems like a Dead Hand weapon, however, where the attackers assume the Russian mainland has already been deeply attacked.  The fourth item were these hypersonic missiles, but those appear to be oversold. Test missiles have under performed.  Another effort involved a nuclear propelled missile, but this seems to be a re-hash of a cancelled US program called Project Pluto from the 1960’s that was too dangerous to ever be tested, let alone used. In short, these appear to be Russian efforts to assume that the US government under Trump has forgotten what happened under Reagan and would fall for the same trick to spend itself into oblivion.  It seems a bit ham handed as a strategy, and seems more likely to backfire into more boycotts and sanctions.

The real problem, then, is the Chinese effort.  China has launched many tests of hypersonic missiles in an effort to neutralize the US carrier fleet.  While Chinese fighters also seem to be oversold, the missile programs, ship programs, and artificial islands are quite real, as is the territorial ambitions in the South China Sea.

I suspect the Space Force will result in a fast, SpaceX-like effort to build radar satellites en masse, along with an effort to identify and disable any attempts to approach or intercept those satellites.  That may involve blinding, jamming, or conversely intercepting the interceptors with radio frequency attacks. A modern radar satellite could certainly manage such an attack in terms of power and perception, if it were designed to do so.

A secondary benefit of such a system of satellites is that any sort of Malaysian airliner disappearance or stealthy 9/11 style attack with corporate or commercial jets would be largely nullified, because basically anything flying, anywhere in the world, would be tracked in real time.









Speaking in August at the 20th Mars Society Conference

I will be speaking at the August 23-26 Mars Society Conference at the Pasadena Convention Center.   This year will be another wonderful, informative convention with a new emphasis on VR projects we are currently getting ready to deliver for use at MDRS and eventually for Mars itself. We also have another university competition to design a heavy lander for Mars.

If you can’t attend the convention, I release the slides to this web site along with links to YouTube when The Mars Society puts those together a couple months later.

Planetary Protection and Settlement

On Thursday in the Science Track room at 1:00 PM, I’ll be speaking about finding ways for planetary settlement to work if and when life is found on another world.  This is actually highly likely given the amount of bacteria ejected into space from meteors colliding with Earth in the early days of the solar system after bacteria had already formed.  The issue is how to find a happy medium where we inhabit other worlds but we keep the existing biomes and our own habitats from interacting accidentally.

SpaceX, Methodologies, and Predictions

On Friday, I’ll be speaking at 2:30 in Room 214 on SpaceX and how the NewSpace companies are accelerating so quickly over their Legacy Space counterparts.  I’ll also forecast how some of these efforts will turn out based on past performance and current conditions.  Want a realistic estimate on when and where you will be able to fly around the Earth in a BFR for a family vacation?

 









A Little Mars Society/Robotics Update

Occasionally, I bump into people who are both interesting and who are interested some of the stuff I’ve described in conversation.  So some of these posts are basically for an audience of one in inspiration, but are probably of enough interest that anyone else who likes this site would enjoy them as well.  So anyway, Mike from the fly in breakfast this morning – this is for you.

I’m the Steering Committee Chair of The Mars Society. Our convention is coming up in Pasadena this August.  My abstracts for this coming year will involve reverse engineering how SpaceX does so much R&D so fast, and why Elon Musk is so “bad” at making predictions about when his rockets will be ready for launch.  The other talk is more of a handbook for preserving science while settling Mars and other places that may have native life, essentially by putting up a “dingo fence” at the microbial level between what humans do and what anything local may do.

You will soon be able to visit Mars Desert Research Station in VR if you have the headset, as well as explore roughly a square mile around the habitat.  You can do something similar right now with 360 views of the various facilities that are online for free just using your web browser.

The Mars Society runs an annual event called The University Rover Challenge.  Teams from universities all over the world compete with robots they have built to do various tasks in the Utah desert around MDRS. These competitors have gone from simple remote control carts to elaborate rovers worthy of any space agency.  Interestingly, Poland’s universities have a tendency to win this thing regularly.

Above is my “Hero shot” from my first trip to MDRS, over 14 years ago now.  Those are the first generation, custom made suits we had that were designed to be like the Apollo moon suits.  Yes, it really looks that much like Mars out there.  The geology is analogous as well, which is why we do so much Mars research out there for not only NASA, but many universities and other organizations.









SpaceX Starlink Update

The future fleet of over a thousand satellites seems to be on track.  The two prototype satellites in orbit are functioning very well. The satellites, named TinTin A and B, are able to communicate at high bandwidth with a latency of 25 ms.  That’s fast enough for online video games.

The satellite division has 4500 employees in itself.  They are building their own ground stations as well.

They are planning a tenfold decrease in the cost of satellites as part of this effort.  Furthermore, these satellites are ten times faster than conventional geostationary or mid-level satellite solutions.  So the net effect is a thousand-fold increase in capability for a given price.









Slight Update on BFR

SpaceX president Gwynne Shotwell recently did a TED talk that included a slide showing three designs for the BFR.  The original from 2016 is on the far right of this image, and is the largest and oldest concept.  In 2017, the design was revised to be smaller and far easier to build.  This version can also do airline service anywhere in the world near an ocean in less than 45 minutes. 

What is noteworthy is the middle version.  This is simply dated 2018, and shows a length almost exactly half-way between the original 12 meter diameter vehicle and the revised 9 meter diameter. This seems to be due to refinement of the aerodynamic design of the nose and wing surfaces.  The wing roots are much longer along the side of the fuselage, although that seems to be partially due to the slightly different angle in the renderings. 

Also included this time are four small landing legs, which protrude past the booster BFR stage.  They simply do not appear in last year's version.  In the original, there are three retractable legs in fairings, two of which are part of the entry shield.  

New BFR for 2018

New BFR for 2018

Legs and Engine Concerns

The engines in the original design protruded past the back of the fuselage in the original design.  In the 2017 revision, the engines are completely covered by the aft fuselage skirt.  It is possible that the extended landing legs allow for the engines to protrude slightly again.  It may also be a consolidation to landing on Mars or other unprepared surfaces like the moon.  There are two great fears in landing on unprepared surfaces with rockets, such as the moon and Mars.  One is that rocks can be kicked up by the exhaust and damage the engine bell.  This did happen with one of the Apollo landings.  While Apollo used a different engine to return to space, the BFS is stuck with the same exposed engines for ascent.  The second factor is that these engines are basically blow-torches that put out exhaust at hypersonic speeds.  As such, they can take the dirt of the moon or Mars and essentially stir it like liquid while landing.  After landing, the unsettled dirt can settle around the foot-pads, essentially burying them under the surface.  This is one of the reasons the Curiosity lander used a jet-pack on a tether to lower the rover to the surface - to keep those engines as far away from the ground as possible.   While less of an issue for lunar landing due to lower thrust requirements, it may be an issue for Mars.  In both cases, getting landing pads built will be a very early priority to avoid throwing rocks at whatever you had landed prior to that point. Early landings may avoid the issue by landing in craters. 

The four leg configuration also means another slight design difference from last year.  Since the legs protrude, the back-to-back configuration for one vehicle to refuel another on orbit will have to be done at a slight angle to allow the legs of each vehicle to slide past each other.  

Capacity Issues

The configuration changes imply they have stretched the propellant tanks slightly in the new design.  This would be good, as the reduction in diameter between 2016's twelve meter diameter and 2017's nine meter diameter, all else being equivalent, reduces both the propellant and pressurized crew compartment volumes by 70 percent.  The revised design slides form 2017 show only a 43 percent reduction in propellant load by mass, however. The revised design is much longer than a 25 percent reduction in length, and the propellant tanks appear to be more efficiently designed in terms of capacity than the 2016 design.  A recent photo analysis of the machine used to make the fuel tanks of both stages shows that the interior diameter is slightly over nine meters, instead of the exterior as originally expected.  Due to economies of scale, every little addition to the tank volume dimensions has a substantial impact on payload.   The 2017 design also mentioned 40 crew cabins, as opposed to the original crew of 100 people.  While you may well have two to a cabin, the substantial reduction in volume would make things cozier.  

Next Steps

SpaceX is also leasing an older large building with an adjoining dock in Los Angeles, California to build the BFR.  Tooling is already being delivered to the site. (Don't get too excited about the photo that appears to show the nose fabrication machine - that's actually a Boeing 787 machine that would be very similar to it.) 

While Elon Musk wants BFR going to Mars robotically in 2022 and with a crew in 2024, Gwynne sees humans on mars by 2028 as a "for sure" outcome.  Near term projects mostly involve short vertical hops, with longer term work on flying up, then using the engines and remaining fuel to slam into the atmosphere to simulate reentry from deep space.  This is very critical.  Vehicles arriving at these speeds heat up with the cube of velocity, so little changes in speed can make the entry fatal. This, in turn, dictates not only the flight time to and from Mars, but the payload capacity allowed for both flights.