On Monday, October 23, 2017 at 2:30:22 PM UTC+13, Sylvia Else wrote:
On 23/10/2017 1:37 AM, Robert Clark wrote:
The SpaceX BFR tanker can serve as a reusable SSTO by switching to a
winged, horizontal landing mode:
On a quick scan through, it looks to me as if he's ignored the mass of
the landing gear, which in a typical aircraft is 10% of the empty weight.
A non-air breathing SSTO ends up with a horribly small payload fraction,
which means in turn that only a modest increase in vehicle mass can wipe
out the payload completely.
Sylvia.
With VTOL using rockets, and with an advanced 'active' landing platform, no landing gear is needed. Merely hold down clamps.
Catching Balls with Robots
https://www.youtube.com/watch?v=R6pPwP3s7s4
Which I discussed here;
Active Landing Pads - catch rockets
https://vimeo.com/37102557
BFR Mass Budget:
BFR system consists of 85 t second stage with 1,250 t propellant capacity - delivering 150 t propellant for transfer to a BFR spaceship - after burning 1,100 t. 8 flights of a freighter refill the 1,200 t capacity BFR spaceship.
85 t / 1250 t = 6.8% structure fraction.
Way more than needed.
We have 9.75 t of propellant to bring the 85 t structure to a soft landing from 0.34 km/sec terminal velocity.
10% of the 95 t structure + propellant is 9.5 t - set that aside for landing gear.
Tank fraction is 3.5% of the original capacity - 43.75 t.
7 Raptor engines at 1 t each is 7 t. That's 50.75 t sub-total. 9.5 t landing gear 60.25 t sub-total.
Add 9.75 t propellant - and that's 70.00 t.
Add 15 t thermal protection and other hardware - and consumables - and you have your 85.00 t budget.
* * *
https://www.youtube.com/watch?v=tdUX3ypDVwI
Specifications
Complete................ BFR booster........... BFR spaceship
LEO Payload........... reusable: 150 t (330,000 lb)
................................ expendable: 250 t (550,000 lb)
Return Payload....... 50 t (110,000 lb)
Diameter................. 9 m (30 ft)
Length................... 106 m (348 ft)....... 58 m (190 ft) 48 m (157 ft)
Maximum mass...... 4,400 t (9,700,000 lb) 1,335 t (2,943,000 lb)
Engines.................. 31× SL Raptors 3× SL + 4× vacuum Raptors[37]
Thrust.................... 52.7 MN (11,800,000 lbf) 12.7 MN (1,300 tf; 2,900,000 lbf) total
Propellant Capacity 1,100 t (2,400,000 lb): 240 t CH4 + 860 t O2
Empty weight......... 85 t (187,000 lb)