Monday, October 30, 2017

ITS/BFR design updates

At the last IAC, Elon Musk presented the latest changes to the BFR.

Throughout this post, I will be referring to what was previously the ITS as the BFR, as that appears to be what SpaceX now refers to it as externally now. The interplanetary spaceship will be refered to as the BFS, and the booster will be called the BFB.

Some of the numbers were changed between the presentation and the PDF being posted. One of these changes was a factual error, about the internal cabin volume of the BFS, and the other was a change of the total BFB thrust, and the removal of the total BFR mass. For this post, I will mostly use the numbers from the presentation, as they are more complete, but I will note where the numbers differ or are wrong.

The reusable payload to LEO has been cut by about half, from 300t to 150t. The payload to Mars has gone from 450t to 150t, and the goal for the number of people per flight has gone from 100+ to 100.


The Raptor engine is further along in development than last time, and has been slightly downscaled to match the smaller BFR. This is because it is harder to make engines throttle to lower percentages of full thrust, which they would need to do to land safely, and still have redundancy.
"The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk" (source)

Elon Musk said in the presentation that the Isp had the potential to be increased by 5-10 seconds, and the chamber pressure by 50 bar, which would make the 2017 Raptor have the same stats as the 2016 one.

250bar vs 300bar
Deep throttle to 20%/ 20%


Exit diameter 2.4m/ expansion ratio 200
thrust 1,900kn/ 3,500kn
isp 375s/ 382s


exit dia 1.3m/ expansion ratio 40
Thrust (SL) 1,700kn/ (SL) 3,050kn
Isp (SL) 330s, (Vac) 356s/ Isp (SL) 334s, (Vac) ???


58m x 9m/ 77.5m x 12m

Mass is tricky, as the video of the presentation gives exact values, while the PDF of the slides give a different exact value for thrust, and no numbers for mass. For this post, I'll use the numbers described in the presentation. Dry and prop masses are based on ratios from the 2016 BFB.

Mass (dry, prop, wet, prop mass fraction[prop/wet])
275t, 6700t, 6975t, ~0.96
~126.75t, ~3,088.25t, 3215t, ~0.96

31/ 42
(Apparently the important scientific and fictional reasons weren't that important, however the PDF does not say how many engines it has, which means that it is likely changing.)

This is weird. The talk showed a thrust of 5400 tons, which is equal to 48040.79kn, which is how much thrust 28 and a quarter SL raptors produce ASL. It's possible that this is a mistake, as there was also a mistake in the interior volume of the BFS, but it's also possible that it was from a different version of raptor.
In the PDF, this was changed to 52,700kn, which is exactly what 31 SL raptors produce ASL, lending weight to the theory that this was a typo. I'm going to assume that it was a typo.

52,700kn/ 128mn (SL) 138mn (Vac)

11288.2m/s / ???
10463.8m/s / 10590.47m/s
(note: this seems odd, as the thrust and fuel mass are roughly double in the 2016 BFR, but remember that Delta-V is a function of propellant ratio and Isp, and I assume the propellant ratio to be the same. Basically, scaling up or down a spaceship design just changes how much extra payload affects Delta-V)


The spaceship's largest change, aside from size, is in shape. The 2016 IAC spaceship had a much more complex body shape, with 3 fins that blended into the overall shape of the ship. The 2017 ship is much simpler, with a simple cylindrical shape and a single set of delta wings. This was done to avoid building a "box in a box" (source)

The BFS now refuels via a connection at the end of the ship, rather than at the side.

48m x 9m/ 49.5m x 17m (max) 12m (base)

Using the masses from the interplanetary ship in the 2016 numbers.
(2017)(2016)(dry, prop, wet, prop mass fraction[prop/wet])
85t, 1,100t, 1,185, ~.93 / 150t, 1,950t, 2100t, ~0.93
(In the presentation, Elon Musk said that the current design has the dry mass as 75t, but mass growth would likely occur)

(All vac)
9689.63m/s / 9886.28 m/s

4 Vac, 2 SL/ 6 Vac, 3 SL

7,600kn/ 21,000kn

3,400kn/ 9,150kn

However, Elon Musk said here that a 3rd medium area Raptor was added to the BFS since the IAC. I don't know what form this would take, or how this would fit on the BFS.