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Talk-Polywell.org • SpaceX News - Page 128
Page 128 of 153

Re: SpaceX News

Posted: Mon Oct 23, 2017 11:06 pm
by TDPerk
As far as I am aware, Musk stated at the latest IAC conference the anticipated lift off G was 0.3 net, 1.3 thrust to weight ratio. He did not say it would keep that ratio throughout the boost. I can see advantages to burning one engine at min throttle throughout the flight until the 180 flip around.

Re: SpaceX News

Posted: Tue Oct 24, 2017 1:21 am
by Skipjack

Re: SpaceX News

Posted: Tue Oct 24, 2017 9:13 am
by paperburn1
There seem to be some confusion in the interwebs on the size of the engines. One figure that stands out in my mind is from Spacex. 157 tones to LEO. ., that is bigger than Apollo (135 tons) for the smaller version that is currently being proposed.Looks like I am going to have to write a strongly worded letter to Elon to clear this and other things up. :D
One engine at 100 percent is 900 tons just below the 1100 ton fuel capacity.

Re: SpaceX News

Posted: Tue Oct 24, 2017 11:27 am
by ladajo

Re: SpaceX News

Posted: Tue Oct 24, 2017 11:45 pm
by TDPerk

Re: SpaceX News

Posted: Tue Oct 24, 2017 11:46 pm
by TDPerk

Re: SpaceX News

Posted: Wed Oct 25, 2017 12:23 am
by Maui
There are will put riders under 3G for some sustained amount of time.

Re: SpaceX News

Posted: Wed Oct 25, 2017 7:27 am
by paperburn1
Computing this can not be done.
not because of technical knowhow but lack of hard data.
Elon has changed the design of the upper stage just last week, AGAIN!! :evil: :evil:
Ok just a brief rundown of the changes in the past 6 months.
Booster went from 42 to 31 raptor style engines.
Of those engines the thrust has changed by being cut in half.
The second stage/ passenger compartment has changed as well. From 9 raptors engines to 4 raptor engines size and 2 smaller engines now to 4 Smaller Raptor engines and 3 small engines unknown thrust (I have a good idea based on goal requirements)
Why you may ask?
Well it all hinges on two drivers. The first is that all parts are to be used on mars and earth to make a multiuse rocket that has and high ratio of part interchangeability (90 percent plus). Second the main driver of launch costs are facility's and infrastructure at this point. By reducing rocket size and payload size he can continue using LC-39A and other like facility's with no large investment. Size change allows it to be used to launch the Orion pressure vessel with modifications.
Changes in the upper stage allow both mars and earth landing with a redundant engine/(s) in case of problems. (Just like the FAA requiring three engines for overwater flying until air body/engines can be certified by a safety record to allow less. :wink: ) Three engines also allow less granularity to be used while maintaining those thrust levels required to land on both planets.
So all I can do is a very crude analyses based off of other rocket data on the flight ranges. (subject to change at any moment)
So by holding a steady Gz load and keeping below Vne /Vno your looking at about 15 percent more fuel usage.
Maximum of minimum range of the upper stage would be 2000 +- 200 miles and using the booster with fly back capability would be 18000 +-1800 miles. This assumes a land with 40 tons of cargo/passengers.
He is also making a assumption that landing the second stage back on to the launching platform is due able to reduce maintenance and turnaround time. Something else I found out by using AREO braking you can get rid of a outstanding 10000 M/S of speed at 1.25 Gz something the space shuttle routinely did.
Disclaimer: This is all based on guesses because I do not have the real numbers and am not going to try until it is settled on what we have.

Re: SpaceX News

Posted: Thu Oct 26, 2017 6:14 pm
by ladajo
So why don't we do the back of the napkin, as I have been suggesting. Let's do it with what we know, and the goal of minimizing g stress on pax.
Sooo, since 1.3g was let go, we can run the fuel mass required for escape (up boost only), with an estimated total vehicle mass of 100T (dry), and limiting accel to 2g, with 31 raptors. All we need is the fuel mass flow rate for a raptor at say about 50%, and assume a constant throttle setting.

This would tell us the minimum rough on the ground fuel mass to get to escape velocity (11,126m/s).
Then we could consider if that is a reasonable number volume and mass wise.

I suspect, it may not be reasonable, and require a silly sized booster to hold it.

Also, the point of aero braking is considered, as the BFR does have winglets. In addition, we would not consider the g effect on pax for an end to end maneuver to support aero brake into a tail first landing approach. All I was thinking to consider was the required size of a booster to minimize g for pax to an acceptable leisure/business traveller level. I didn't even consider the distance aspect (sub-orbital arcs, etc.).

Re: SpaceX News

Posted: Thu Oct 26, 2017 9:20 pm
by krenshala
delta v = v_e * ln( wet mass / dry mass )

If we assume the lower (sea level) Isp of 330s for the raptors, and a 100 metric ton dry mass, then you would need 3020 metric tons of fuel to result in a delta v of 11126.31 m/s. If you change the Isp to the vacuum value (375s) and ignore the presence of atmosphere, the same masses would get you 12643.54 m/s of delta v. Assuming atmospheric drag and the changing Isp values as it climbs mostly cancel out, the actual delta v is probably just under 12km/s for a 3120 metric ton (fully fueled) rocket that uses a single stage to orbit. That ignores any extra drag/gravity losses caused by using a lower thrust value than max during ascent.

Re: SpaceX News

Posted: Thu Oct 26, 2017 10:53 pm
by hanelyp
One problem with a lower G launch is increased gravity losses, increasing the delta-V the engines need to produce.

Re: SpaceX News

Posted: Fri Oct 27, 2017 12:37 am
by TDPerk

Re: SpaceX News

Posted: Fri Oct 27, 2017 2:59 am
by TheRadicalModerate

Re: SpaceX News

Posted: Fri Oct 27, 2017 3:22 pm
by paperburn1
But remember it has to fill 4 different jobs with the engine selection. Sub-orbital point to point, orbital point to point, LEO at least to space station range with 173 ton payload and MARS.
At least according to grand plan using only two different types of engine.
Thanks for the number runs. Much appreciated as currently I do not have much spare time on my hands.

Re: SpaceX News

Posted: Fri Oct 27, 2017 5:16 pm
by Maui