Talk-Polywell.org

I'm thinking that jerk, change in acceleration with respect to time, is a very important factor in passenger comfort. Ease into it and 2G shouldn't be shocking to a passenger. Slam into or out of the acceleration too hard and you invite injuries to unprepared passengers.

Granted, 2Gs may not be the most comfortable, but neither is sitting for hours in an airliner passenger seat.

Note: but some bad data into the thought stream (Thanks Ladajo don't know what i was thinking) ICBM start at about 3 G . but the basic problem still remains the same, will the BFR hold enough fuel to take off and land within a acceptable margin for passenger comfort. Lets throw out depressed trajectories even and see if we can hold enough fuel to make it at 1.3 G.

"Elon Musk’s Boring Company is digging a 10-mile tunnel in Maryland"

Seriously, Musk's outfit for digging the first Hyperloop tunnel is "The Boring Company". Which meshes perfectly with how boring his rocket landings have (thankfully) gotten. Now, do the Tunnel Boring Machines have cute names? "Honey, I Lost Track Of Time"? "Buried In Work"?

https://arstechnica.com/cars/2017/10/el ... -maryland/

paperburn1 wrote:Note: but some bad data into the thought stream (Thanks Ladajo don't know what i was thinking) ICBM start at about 3 G . but the basic problem still remains the same, will the BFR hold enough fuel to take off and land within a acceptable margin for passenger comfort. Lets throw out depressed trajectories even and see if we can hold enough fuel to make it at 1.3 G.

1.3g is 12.749m/s^2 acceleration.
Dry mass for BFT was estimated at 85 tons.
Thus you would need 243,618 lbf thrust for dry mass.
A Raptor engine, as currently conceived is capable of 147,123 lbf (654.44kN) with a throttle range of 20% to 100%. Thus low throttle would be about 29,424 lbf.

Next step is to look up Raptor burn rates.

100% Estimates found here (at end of page):
http://spaceflight101.com/spacex-launch ... s-designs/

Burn rate/s * 60sec/min * 16 min = Fuel mass to 11,126 m/s.

Someone have at it... I need to go to a meeting.

1090 metric tons of fuel in the BFR Best guess 2 metric tons a second at liftoff engines variable down to 20 percent or turned off. They have changed their design.
“Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance,” he wrote. “Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don’t need the high area ratio, deep space Raptor engines.”

That is similar to what SpaceX President Gwynne Shotwell said at the Oct. 5 meeting of the National Space Council, when asked about development of the BFR system. “That system is being designed also to do Earth hops, and those are some of the first tests that you’ll actually see with the Falcon spaceship,” she said.

The spaceship itself, Musk said, is capable of reaching orbit without the assistance of a booster, but with only a “low payload” that he did not specify. “Earth is the wrong planet for single stage to orbit. No problemo on Mars.”

Musk also addressed changes in the Raptor engine, the large engine powered by methane and liquid oxygen that will power both the booster and the spaceship elements of the BFR system. The original Raptor design, announced last year, was capable of producing more than 680,000 pounds-force of thrust. The revised design announced at last month’s IAC produces only about 380,000 pounds-force of thrust.

“The engine thrust dropped roughly in proportion to the vehicle mass reduction from the first IAC talk,” Musk wrote when asked about that reduction in thrust. The reduction in thrust also allows for the use of multiple engines, giving the vehicle an engine-out capability for landings.

Spacex could lower the margin on each launch if there is elasticity in launch where they create a larger launch market with lower prices. Spacex will be able to adjust their prices based upon the demand they see at lower prices.




Even with somewhat lower re-usability and more costs on the maintenance, Spacex should be able to comfortably get launch costs for a BFR to $30-40 million which would be $200-300 per pound.

Tom Ligon wrote:"Elon Musk’s Boring Company is digging a 10-mile tunnel in Maryland"

Seriously, Musk's outfit for digging the first Hyperloop tunnel is "The Boring Company". Which meshes perfectly with how boring his rocket landings have (thankfully) gotten. Now, do the Tunnel Boring Machines have cute names? "Honey, I Lost Track Of Time"? "Buried In Work"?

https://arstechnica.com/cars/2017/10/el ... -maryland/

First one was Waiting For Godot. IIRC they acquired a second one recently.

One of these tunnels needs to suddenly wind up being a Verne gun. Preferably nuclear powered.

You could give spacecraft a boost, but please, run the numbers before you try orbital velocity from Earth's surface! Mach 25 at sea level is a bad idea. For that matter, mach 25 at 20 miles up is still a bad idea.

The idea would work a treat on the moon, though. Recovering energy by flying into a tube could even work, and the success SpaceX has been having landing on a small target suggests it is not totally outrageous.

Some of the numbers given show that for the price the big deal isn't mach 25, it's bothering the neighbors.

There's sci fi dancing in the back of my head about a surprise immigration using such concepts, but no good storylines have come out of it yet.

kunkmiester wrote:Some of the numbers given show that for the price the big deal isn't mach 25, it's bothering the neighbors.

Yah, dang NIMBYs will probably complain about the shock wave leveling houses.

I've been wondering about Hyperloop's tunnels, too. The article suggests they think 2 tunnel diameters (28 ft) is deep enough. I doubt it. For anything larger than a single family home, sub basements, foundations, and pilings are likely to run that deep.

The hyperloop will not be able to reach the intended speeds if there are any significant turns. They can't pick their way between large buildings, they just have to point to the destination and run, with only the most gentle broad, sweeping turns. They may need to go deeper.

If the top of the tunnel is at 28 ft, the tunnel is likely to be in the water table on the Eastern Seaboard.

I suspect they'll have to go way deeper. And right of way will be a problem if trying to pass under property where the owner has rights to the center of the planet. They'll have more lawyers than engineers until that is dealt with. Eminent Domain is bound to be needed.

At least the tunnels should not make significant vibration.

once again I think this tunnel thing is just about mars and living there. learn here tunnel there.

You may be very right about that. Especially given how geologically stable Mars appears to be compared to Earth.

On another note, I have not yet had time given work commitments last week, over the weekend, and this week to finish the BFR launch mass idiot check for minimizing G force to 1.3. The last piece is running the m=f/a calc and then summing the dry and wet masses. If someone wants to do that great. I just don't have the time right now. I think the dry 85T for BFR is close. Maybe for argument's sake it could be 100T. We just need to establish the fuel burned at 1.3g constant push, and not worry about throttling / etc. 1.3g is about 12.5m/s^2. We know how much push each engine can deliver, and thus just need to carry that forward.

Designation Raptor Vacuum
Type Full-Flow Staged Combustion
Propellant Feed Multi-Stage Turbopump
Oxidizer Sub-Cooled Liquid Oxygen
Fuel Sub-Cooled Liquid Methane
Thrust (Vacuum) 3,500 Kilonewtons
Specific Impulse (Vac) 382 Seconds
Chamber Pressure 300 bar
Throttle Range 20 – 100%
Ignition Spark
Re-Start Capability Yes
Area Ratio 200
Mixture Ratio 3.8
Flow Rate (Calc.) 931.2kg/sec
LOX Flow Rate 737.2kg/sec
LCH4 Flow Rate 194kg/sec

Read more at http://spaceflight101.com/spx/spacex-ra ... f1sShbL.99

931.2kg/sec.
One engine for 16 minutes at 100% = 893,952kg fuel (1,970,826.8 lbs)

Next please... go to run again...