SpaceX News

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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Tom Ligon
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Re: SpaceX News

Postby Tom Ligon » Wed Oct 18, 2017 2:10 pm


hanelyp
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Re: SpaceX News

Postby hanelyp » Thu Oct 19, 2017 2:07 am

What about the people who want to get out of their seats during the "weightless" phase of the suborbital hop?
The daylight is uncomfortably bright for eyes so long in the dark.

paperburn1
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Re: SpaceX News

Postby paperburn1 » Thu Oct 19, 2017 2:54 am

I am not a nuclear physicist, but play one on the internet.

Aero
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Re: SpaceX News

Postby Aero » Thu Oct 19, 2017 4:41 pm

What! How can a ballistic trajectory not be parabolic?
Aero

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Re: SpaceX News

Postby paperburn1 » Thu Oct 19, 2017 5:52 pm

He will not be using ballistic trajectories.
You can’t fly humans on that same kind of orbit used for ICBMs so right away the 30 minute point to point is out.
For one, the acceleration and the G-forces for both the launch and the reentry would kill people. it’s a lot more than the G-forces on an astronaut we see today going up into space and coming back down, 3 for the space shuttle. So this basically puts a ballistic shot ICBM style out for most passengers. There are a number of stressing environments that passengers would be subjected to during a rocket launch

Quasi-static load
random vibration
shock
thermal
acoustic


Quasi-static loads can be thought of as constant accelerations. They are a result of the bulk acceleration of the entire rocket as well as the contribution of any random vibration Several g's is typically in human rated spacecraft; above 10 is not uncommon in ICBMs which would be required for the 30 min flight up and down.

Random vibration
(rocket motor running long term.) These are often the most stressing environments because amplification at specific natural frequencies can result in the effective load at certain locations in the vehicle to be many tens of g's equivalent
Shock
Shock loads are a result of events like stage or fairing separation and is caused by things like explosive bolts. It results in a high-frequency ring through the system. Structures tend to be less susceptible to shock than RV or QS loads, but they can be murder on mechanisms, electronics, and passengers. This is one reason why Musk uses cold gas pistons to separate stages.
Acoustic is sound loading.
Passengers do not like load noises that sound like you rocket is coming apart at the seams. High speed high Gs passage though atmosphere = loud noises.
Thermal environments can be extreme because of heating on the fairing during ascent, which then radiates into the passenger compartment. same factors apply during descent.
Another problem with ballistic trajectory is radiation exposure in the vacuum of space.You just can't take the 10:30 to singapore if space weather will toast your gonads.
So they will be going slower turning and flying in the Thermosphere then turning and landing. More like a 90 to 120 minute ride to make all the forces manageable for the passengers and spacecraft..

Space is a Is a Harsh Mistress
Last edited by paperburn1 on Thu Oct 19, 2017 6:01 pm, edited 3 times in total.
I am not a nuclear physicist, but play one on the internet.

Maui
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Re: SpaceX News

Postby Maui » Thu Oct 19, 2017 5:52 pm

It probably wouldn't gain much time compared to the amount of fuel used, but I'm supposing BFR would have enough fuel capacity to keep at least low level thrust all the way to re-entry.

This wouldn't be as fun. I'm sure if this becomes a thing, for quite some time the "space adventure" aspect (including weightlessness) will be a significantly bigger selling point than the trip time.

EDIT: I was responding to Aero, not paperburn. Interesting points paperburn...

Betruger
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Re: SpaceX News

Postby Betruger » Thu Oct 19, 2017 10:12 pm

Is there really not enough margin in the thrust performance to blanket enough of those inconveniences down to acceptable levels? Besides continuous G's. Meaning sound deadening etc.

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Re: SpaceX News

Postby paperburn1 » Fri Oct 20, 2017 4:11 am

Not and keeping with the 30 minute port to port concept. Physics is a harsher mistress. :D
If he was willing to go slower I do think the passenger comfort issues could be solved. But people tend to freak out when a airplane in flight pulls a half of G or more. For reference autopilots are commonly set to "quarter-G" pitch changes limit. Technically a kid on a swing reaches two G at the bottom of the arc back all the way to zero at the top of the arc. I could see a class of people doing the ride for "The Experience" And if you went from say Vegas to Macau that would be a niche market that could be well served.
Disclaimer: most of these were napkin calculations. But they clearly show PTP rocket travel has issues and these do not even take into account of air traffic management.
I am not a nuclear physicist, but play one on the internet.

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Re: SpaceX News

Postby krenshala » Fri Oct 20, 2017 12:12 pm

Even if flight time is bumped up to 1 or 2 hours to mitigate G stresses on the passengers, that would still be significantly better than aircraft for flights traveling more than 1/3 of the way around the globe, and possibly for some shorter distances.

ladajo
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Re: SpaceX News

Postby ladajo » Fri Oct 20, 2017 1:53 pm

To really get a feel for it, one needs to integrate the accelleration profiles over the entire flight, to include boost and touchdown.
Personally, my in the head estimates believe that the pax are going to have to eat G's no matter what during boost and touchdown. The arc is another matter. The fuel required to slow down the boost cycle or touchdown cycle to 1/2 G or less is well beyond anything realistic.
Boost is what it is and modeled currently for max efficiency for desired flight profile. Touchdown, in the SpaceX realm, is a profile that uses atmospheric braking (minimal) until a final touchdown burn which sheds a large percentage of velocity in less than 30 seconds.
An enterprising individual could easily use the launch videos to get an estimate on the full flight profile G loading of a Falcon 9 RTLS booster. The masses are pretty much known, as are the velocities and speeds (the videos give them).
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)
What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

TDPerk
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Re: SpaceX News

Postby TDPerk » Fri Oct 20, 2017 3:51 pm

Several distinctions between what is advantageous and required of an ICBM and and BFR doing Earthly P2P -- The ICBM must boost as quickly as possible to minimize it's detectable plume. This and the fact the BFR has many engines and they can be throttled (and not all burning continually) resolves the fact the takeoff and landing gee forces for the P2P BFR profile are already known to be 1.3g. Because there is no particular advantage to the BFR to boost as fast as possible, it may well keep one engine burning at minimum throttle the whole time, flipping around midflight, and thereby achieve the total dV needed for a 30minute'ish trajectory without a 10g boost followed by a coast*. All problems of noise and vibration would be proportional to the throttle setting total at the time, and thereby also minimized. The P2P BFR only needs about half the thrust of one intended for orbit, PAX and gear are much lighter than 150tn to LEO.

*Also helps keep your drink in your glass.
molon labe
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ladajo
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Re: SpaceX News

Postby ladajo » Fri Oct 20, 2017 4:29 pm

The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)

What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

ladajo
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Re: SpaceX News

Postby ladajo » Fri Oct 20, 2017 4:53 pm

As an after thought, at 1.2g it would take about 16 minutes to get to 11,186 m/s.
How much fuel (launch mass & volume) would that take?
For reference, the space shuttle used to take a little over 8 minutes to reach 11,186m/s. And that was with an increasing g force over the flight profile, up to a max of 3g. The first peak was just before SRB separation, and then a drop back to about 1 g, with a build to 3g on the SMEs over the following 360 seconds (or so).
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)

What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)

paperburn1
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Re: SpaceX News

Postby paperburn1 » Fri Oct 20, 2017 6:51 pm

Now you have done it I'm going to have to pull up the old Jane's book and take a look. My numbers could be off as I was doing most of its calculation from old memories and swag. Which are still talking about burning 2+ metrics tons a second at liftoff and then a constant throttle down rate and at some point a flip maneuver then landing(.based on falcon 9 numbers) The whole time keeping your G at 1.3(where did 1.3 come from?) and then there is that fuel usage increased from burning at 1.3 g, rockets pull high G not because it looks cool but it is the most efficient way to orbit fuel wise. The gravity tax is the lost speed due to gravity at 9.8 ish meter per second. we really do not have enough facts to figure this out.


But let's segue over to the what I consider the real reason he is building the BFR. It's been confirmed that NASA is now going to do the return to the moon and Mars using the Orion configuration. The falcon nine and the falcon nine heavy are not compatible with the Orion nine pressure vessel and that is the foundation on which all of the systems and subsystems are going to be built into for this program and future ones. I think the BFR diameter will be compatible with the Orion and this puts musk back in the game for the Mars and moon launch business. But Elon is being crafty and doesn't want Lockheed Martin to figure out is going to be playing in their backyard again. I don't know this for a fact but it's but I would do.
I am not a nuclear physicist, but play one on the internet.

ladajo
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Re: SpaceX News

Postby ladajo » Fri Oct 20, 2017 9:43 pm

Agreed, however I think there is enough info to take a shot at it.
The number I posted above for 16 minutes is with a constant 1.2g.
That can be unpacked with a trial boost mass(fuel+vehicle) / thrust / mass shedding rate (fuel mass burn rate) for the BFR.
We have an idea for the BFR configuration, and engines, and can then probably make a guess as to fuel required.
The starting point is how much fuel is needed to burn for 16 minutes on the up. First number for a feel is the BFR burn rate.
Second number would be running a mission profile curve for the burn, ie. would BFR throttle back to hold 1.3g as it sheds mass? How much fuel would this save with the integration of the decreasing fuel burn as she gains speed?
That would then give a good idea of what the on the ground mass should be to get up at least.
Then we would need to add the transit and landing burn requirements, and subsequently readdress the boost requirement to include this mass.
At the end of the day, it is about pushing a changing mass at a constant g. Which is unlike how we fly now, which mostly look to be the most efficient push to 11,186m/s.
So the starting point would be the 14 minutes and 38 seconds needed at a constant 1.3g to get to 11,186m/s.
We know the numbers for the SpaceX engines regarding push and burn, it should be enough to back of the napkin the initial boost mass for fuel, with an estimated BFR vehicle mass.

https://spaceflightsystems.grc.nasa.gov ... ktpow.html

http://spaceflight101.com/spacex-launch ... s-designs/
The development of atomic power, though it could confer unimaginable blessings on mankind, is something that is dreaded by the owners of coal mines and oil wells. (Hazlitt)

What I want to do is to look up C. . . . I call him the Forgotten Man. (Sumner)


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