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.  

Low-Thrust Breakthrough Propulsion Round-Up

Next Big Future has produced a number of articles recently on breakthrough propulsion.  Here is a summary of the current state of the art on these speculative systems.

Low-Thrust Propulsion systems are too weak to be used to launch from the surface of a planet, but have extreme efficiency and high exhaust velocities.  Basically, a space probe like Dawn had the propulsive force equivalent two sheets of paper held on your hand.  Yet it could thrust for years with this system.  Over the course of eight years, it thrust over 2000 days, with a change in velocity of 24,000 mph/ 38,000 kph.  That's more than it took to launch it into interplanetary space to begin with, and while using less than 937 pounds/425 kg of propellant on board.  Ion drive takes a slowly-released gas, gives it an electrostatic charge, and then accelerates it in a magnetic field and neutralizes the charge as it exits the engine.  Newer systems, such as the speculative EM-Drive, promise to work with no on-board propellant. 

While creating full-antimatter is very, very difficult and energy-intensive, making simple positrons (antimatter electrons) is fairly simple.  A group is working on a system that can fit on a cubesat for a demonstrator.  Rather than make full anti-matter atoms and trying to store them and use them later in an engine, a positon system is basically an ion drive system like Dawn uses, but using positrons made on the ship itself instead of ionized gas.  Positrons are naturally produced by radioactive materials.  So a sample of a radioisotope is placed on board, and the positrons it emits are cooled enough to allow them to be directed and accelerated in a magnetic field like in a conventional ion drive.  The resulting particles exit the vehicle at ten percent the speed of light.  If idealized, these systems could get to Mars in weeks, Pluto in months, and Alpha Centauri in 40 years.   The developer, Positron Dynamics, plans to launch a cubesat demonstrator somewhere between the middle of this year and next year.  The near term application of these motors would be as a more-efficient substitute for ion drive in positioning communications satellites.  In the longer term, they would be very useful for asteroid mining.  

To make a complex system a bit easier to understand, a small fuel pellet is compressed by a magnetic field and bombarded with particles.  It undergoes nuclear fission and the plasma is further compressed by the magnetic field.  The combination of compression and the heat, self-generated plasma magnetic field, and particle mix of the tiny fission explosion can trigger a fusion explosion, similar to the way an atomic bomb can trigger a thermonuclear (fusion) bomb.   As with the Positron example, this could theoretically get a vessel to Mars in a month.  It would generate exhaust ISP(the efficiency of a rocket)  of 30,000 seconds.   By contrast, a typical chemical rocket has an ISP under 450 seconds. 

This is similar to  EM drive, in that it depends on A) very tiny effects that can barely be observed and B) processes that defy conventional physics, but appear to be explained by some aspects of science.  When I say barely observable, we are talking 2 to 12 micronewtons.  A tiny bottle-rocket has 3 newtons of average thrust, or a million times as much as these experimental engines.  That said, they hope to get up to 10-20 millinewtons in a decade or two.  That said, a microthrust, propellantless system has no theoretical output limit, because there is no propellant to be used up.  This is why those who propose systems like this often speak of starships. 

Conclusions

The positron system seems quite logical, and can be demonstrated soon.  It has limits, though, because it relies on radioactive decay of highly radioactive materials.  This can be scaled down into tiny systems.   The PuFF system will require a lot of expensive research to get worked out, over a fairly long time period.  That said, it has a lot of similarities to Project Daedalus, and could be used for a simple starship probe eventually.  Such a system is at least a decade away, and would be quite large.  Mach Effect is a lot like EM Drive, but while the effects are more explainable, they seem to be even less powerful.   To power a vehicle with any velocity, it also has to be scaled up quite a bit.  
With new systems that can provide fission and possibly fusion electrical power coming online very shortly, systems that require lots of electricity (advanced ion drive, these propellantless systems, and so on) may be arriving sooner rather than later.  Expect to see something interesting in this range within five to ten years.