So about that mini BFR Elon Musk tweeted about


#1

So Elon Musk on Wednesday tweeted:

Elons%20Tweet

After people started asking questions about it, he also voluntered: “aiming for orbital flight by June (2019)” and “won’t land propulsively (…). Ultra light heat shield and high mach control surfaces are what we can’t test well without orbital entry. I think we have a handle on propulsive landings.”

This has led to some further discussions and speculations about what exactly Elon Musk has in mind specifically; amongst others it was discussed by Scott Manley (here:https://www.youtube.com/watch?v=adSY9nek1Hg ) and Tim Dodd, the Everyday Astronaut (here: https://www.youtube.com/watch?v=CSCmC7p0Ud0&t=3279s at 28:00). I can really recommend those two discussions.

Here’s an illustration of how that might look like by Twitter user @itagakikeisuke1:

I also put my two cents in, in the comments of the two above mentioned videos and here’s what my toughts were (for reference, Scott was suggesting that the mini BFR Ship might be more of a Falcon 9 second stage with fins and a heat shield rather than a bonafide miniature BFR Ship):


#2


BFR (six to one)
Length 55 m (180 ft)
Diameter 9 m (30 ft)

Mini BFR
Length 16 m (72ft)
Diameter 3.7 m (12 ft)

Would fit on a truck, without its fins.
Fairing hinge mechanism
Only on rocket :rocket: motor rather than 7


#3

…it’s only a model. Can they really get accurate data from it? Not a fluid dynamics expert but wouldn’t a small scale model like this perform significantly different from a full scale one?


#4

They test scale models in wind tunnels all the time. The math scales at the same rate as the model.


#5

So it’s kinda predictable… cool.
I knew about airplanes, cars and wind tunnels. I just did not think the same would apply on ludicrous speeds on orbital reentry.

  1. Variable atmosphere density
  2. Different size of actuating surfaces
  3. ???
  4. …vortices or something?

#6

Atmospheric Reentry Materials and Structures Evaluation Facility (Arc-Jet) does testing of thermal protection systems (TPS) in hypervelocity wind tunnels:


#7

I do question carbon fiber with enamel paint is enough, especially when scaled down to 5mms thickness. Compression plasma are extreme. Moon :crescent_moon: return mission could be twice the speed as an orbital mission. Twice the plasma.


#8

Actually, energy rises with the square so for a doubling of speed it would be 4 times the energy.
KE=0.5m(v^2)

Specifically, for the moon vs LEO it’s:
(12500 m/s entry velocity from moon ) / (8000 m/s orbit velocity in LEO) = 1.5625
1.5625^2 = 2.44 times the kinetic energy

Also, PICA is not so much a paint as an infusion of resin that inter mixes with the carbon fiber structural mesh of the heat shield. The “phenolic resin” then is vaporized by the heat of reentry and forms a layer of opaque gas around the area from which it vaporizes. This gas is what stops energy from the glowing hot compressed atmosphere from reaching the surface of the ship’s hull. Here’s some awesome pictures of the “Stardust” heatshield’s carbon matrix being depleted of its resin (Stardust entered from interplanetary orbit, which is an entry velocity of more than 12500 m/s):


That snowy stuff in slice 30 is the untouched resin. Note also that about 6 mm of heatshield is completely gone (the “char surface” is “slice 0”, which is 6 mm below where the top of the heatshield used to be. Thus, the first 6 mm have been burned off in the atmosphere of Earth and are unavailable for examination. There’s a lot of untouched material because PICA was a new material back then, and they purposefully over did it on the thickness.


Stackpoole M., Sepka S., Cozmuta I., and Kontinos D. Post-Flight Evaluation of Stardust Sample Return Capsule Forebody Heatshield Material. AIAA-2008-1202, 46th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada. 2008. ( https://www.researchgate.net/profile/Ioana_Cozmuta/publication/27541469_Post-Flight_Evaluation_of_Stardust_PICA_Forebody_Heatshield_Material/links/58e6afad0f7e9b092771258c/Post-Flight-Evaluation-of-Stardust-PICA-Forebody-Heatshield-Material.pdf )