Horizontal landing for the BFR on Earth. UPDATED, 12/15/2018

Copyright 2018 Robert Clark

 In a recent discussion of the BFR, Elon Musk has discussed giving the BFS upper/landing stage extended-size fins, to the extent they resembled wings, to help with the landing on Mars:

Elon Musk reveals updated design for future SpaceX Mars rocket.
By Loren Grush@lorengrush  Sep 17, 2018, 9:38pm EDT

https://www.theverge.com/2018/9/17/17871724/spacex-big-falcon-rocket-bfr-mars-design-elon-musk

 In fact in this video Musk alternately called them wings and fins:

 Primarily these would be for control during reentry on Mars. The final landing would be through a propulsive, vertical landing.

 The idea has been that full wings would be too heavy, as for the Space Shuttle for example. However the X-37 provides an example that shows a reentry stage can use short, stubby, and therefore lightweight wings, to do a full horizontal landing on Earth:

 Boeing has not given the breakdown of the masses of the structural components of the X-37. But the Skylon gives another example of an orbital stage doing a fully, horizontal landing with short stubby wings:

 The designers of the Skylon have given the weight of the carbon fiber wings as only 2% of the gross takeoff weight. Because the Skylon does a horizontal takeoff, the wings have to support the full gross takeoff weight. But for the BFS assuming it flies a non-lifting trajectory to orbit, the wings would only have to support the dry(empty) weight for when it returns from orbit. This would mean a much smaller weight for the wings.

 For example, if we applied this to the Falcon 9 upper stage, the gross takeoff mass is about 100 metric tons, but the dry weight only about 4 metric tons. The 2% of the dry mass would only be about 80 kg for the wings.

 It may even be possible to get even smaller weight for the wings. A remarkable new development is lightweight but strong materials is the isotruss:

Isotruss strength comparison

 From the graph it is nearly twice as good as carbon fiber in bending strength on a per weight basis. So a Skylon-type wing might only have to be 1% of the landed weight or only 40 kg for the Falcon 9 upper stage.

Such small-size wings still would not provide a fully horizontal landing in the thin atmosphere of Mars, but they would cut down the amount of propellant needed for the propulsive landing.

  Bob Clark

UPDATE, 12/15/2018:

 The latest update on the SpaceX BFR is they intend to use metals for the tanks and other structures:

https://www.nextbigfuture.com/2018/12/elon-could-go-with-metal-hot-structures-and-rely-on-spacex-thermal-protection-team.html

 Then SpaceX succeeding in this could give impetus to resurrecting the X-33/Venturestar with high strength metal structures rather than carbon composite.

 Additionally, I suggested above using the isotruss for its bending strength for the structures such as wings. It is notable that the isotruss obtains its strength from its unique geometry since while it is composed of carbon composite it is twice as strong as a standard carbon composite tube.

Isotruss Tower

280' tower installed in Spanish Fork, Utah

 This suggests that a metal isotruss also could be twice as strong as a standard metal tube, and should therefore be even stronger when made of the specialty high strength metals SpaceX is considering.

 Another intriguing possibility is suggested by this. A recent development is scifer steel wire:

HIGH-STRENGTH (5 GPA) STEEL WIRE - AN ATOM-PROBE STUDY
www.phase-trans.msm.cam.ac.uk/abstracts/scifer.html

 Like carbon fiber this is only available in wire form, not sheets. However, it has from two to three times greater strength than the high strength steels available in sheet form. So the scifer wire could still be used to form a steel isotruss, possibly doubling its bending strength.

  

Robert Clark

Pumping pressurized fluids to high altitude for the space tower and for fighting forest fires, Page 4: water packets solution.

Copyright 2018 Robert Clark

(patents pending)

 In the blog post Pumping pressurized fluids to high altitude for the space tower and for fighting forest fires, Page 2: high volume, high head, single pump solution, I speculated on laminar flow to achieve kilometers-long water streams without using piping. And in the blog post Pumping pressurized fluids to high altitude for the space tower and for fighting forest fires, Page 3: achieving ultimate laminar flow, I proposed some methods of maximizing the laminar flow effect. However, in the absence of experimental or theoretical evidence we should remain doubtful that even laminar flows could achieve kilometer-long water streams without piping.

 Then I'll propose an alternative solution to achieving the long-distance water flow. In the high volume, high head "mud pumps" discussed in the Page 2 blog post, the pumps necessarily had to have both high volume and high head because in their dredging, mining, drilling use they have to pump both mud and rocks in addition to just water.

 The problem with sending water streams long distance is that the water streams will disperse into droplets due to air drag under the high speeds or pressures needed to send them the long distances. However, rocks certainly can be sent such long distances even acting under air drag. So the idea would be to have the water contained as some percentage within mud or rock. Depending on the relative proportion of the mud/rock to the water, the individual packets would be sufficiently cohesive to be sent the required distance.

 Another possibility would be to send the water as frozen blocks or balls. There would be the question then of how to get the water rapidly frozen while being sent out at high pressure or speed. One possibility would be to first use snow making machines that can make snow using up to 107 gallons per minute of water. The water is sent out in the form of a fine mist so that it will rapidly freeze. You would also need a compartment of refrigerated air kept below freezing so that the water will rapidly freeze like in the Winter conditions on a ski slope:

 The snow is made in the form of small ice crystals though. The snow/ice would then have to be compressed into balls or cubes so it would have longer range when then sent to the high pressure pumps.

 Still another possibility is the water could be encased in a biodegradable plastic bag or shell before being sent to the high pressure pumps. 

 To minimize drag we might want to put the encased or frozen water into a tear drop shape. We might also want to give it fins and use spin stabilization for a straight-line flight kilometers long.

 A gliding approach to long range.

 To get even longer range, we could also shape the mud packs, ice packets, or plastic shells into aerodynamic lifting surfaces. Gliders for example can have lift/drag ratios in the range of 40 to 60.

 The glide ratio which is the ratio of the horizontal length traveled divided by the vertical drop is equal to the lift/drag ratio. 

 So in the originally proposed scenario in this series of blog posts of using a high volume fireboat such as the Warner Lawrence, we could first send the high volume of 38,000 gallons per minute to 400 feet. 

 Then encasing the water into a gliding aerodynamic shape with a ca. 60 to 1 lift drag ratio, we could cover 24,000 feet horizontally as it fell 400 feet.  

 This would have the advantage in that you would not need to convert this high volume pumping capacity into lower volume, but higher head.

 The disadvantage is the high lift/drag ratio shape of a sailplane would be highly subjected to winds. 

An ASH 31 glider with very high aspect ratio (AR=33.5) lift-to-drag ratio (L/D 56)

 So we likely would need some method of automated control surface maneuvering to ensure straight and true flight. One possibility might be the rolleron fins used on rockets:

A rolleron is a type of aileron used for rockets, placed at the trailing end of each fin, and used for passive stabilization against rotation.[1] Inherent to the rolleron is a metal wheel with notches along the circumference. On one side, the notches protrude into the airflow. During flight, this will spin the wheels up to a substantial speed. The wheels then act as gyroscopes. Any tendency of the rocket to rotate around its major axis will be counteracted by the rollerons: the gyroscopic precession acts to move the rolleron in the opposite direction to the rotation.[2]

https://en.wikipedia.org/wiki/Rolleron

   Bob Clark