SpaceX News

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

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TDPerk
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Re: SpaceX News

Postby TDPerk » Mon Oct 23, 2017 11:06 pm

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.
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Skipjack
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Re: SpaceX News

Postby Skipjack » Tue Oct 24, 2017 1:21 am

TDPerk wrote:I can see advantages to burning one engine at min throttle throughout the flight until the 180 flip around.

That would drastically reduce the Isp...

paperburn1
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Re: SpaceX News

Postby paperburn1 » Tue Oct 24, 2017 9:13 am

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.
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ladajo
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Re: SpaceX News

Postby ladajo » Tue Oct 24, 2017 11:27 am

TDPerk wrote: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.


Most bigger rockets at launch are 1.15 to 1.3(ish)g's on initial boost. As the fuel mass burns away, it goes up accordingly.

The point to this excursion is to explore the penalty in launch mass if you want to limit g force to something reasonable for commercial passengers.
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TDPerk
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Re: SpaceX News

Postby TDPerk » Tue Oct 24, 2017 11:45 pm

Skipjack wrote:
TDPerk wrote:I can see advantages to burning one engine at min throttle throughout the flight until the 180 flip around.

That would drastically reduce the Isp...


Won't matter really, for one engine burning; one of so many will use little fuel even if on for a long period. The advantages of being under some thrust may well not outweigh the cost.
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TDPerk
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Re: SpaceX News

Postby TDPerk » Tue Oct 24, 2017 11:46 pm

ladajo wrote: if you want to limit g force to something reasonable for commercial passengers.


I do not presume that limit is 1.3 g. I expect most people will tolerate 3g very well.
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Re: SpaceX News

Postby Maui » Wed Oct 25, 2017 12:23 am

There are amusement park rides will put riders under 3G for some sustained amount of time.

paperburn1
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Re: SpaceX News

Postby paperburn1 » Wed Oct 25, 2017 7:27 am

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

ladajo
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Re: SpaceX News

Postby ladajo » Thu Oct 26, 2017 6:14 pm

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.).
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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Re: SpaceX News

Postby krenshala » Thu Oct 26, 2017 9:20 pm

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.

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Re: SpaceX News

Postby hanelyp » Thu Oct 26, 2017 10:53 pm

One problem with a lower G launch is increased gravity losses, increasing the delta-V the engines need to produce.
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Re: SpaceX News

Postby TDPerk » Fri Oct 27, 2017 12:37 am

hanelyp wrote:One problem with a lower G launch is increased gravity losses, increasing the delta-V the engines need to produce.


Eh, start at 1.3, ramp over 10 seconds to 2.5. Fuel is cheap.
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TheRadicalModerate
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Re: SpaceX News

Postby TheRadicalModerate » Fri Oct 27, 2017 2:59 am

ladajo wrote: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.


Since nothing about rockets makes sense until you run the delta-v numbers, here's the back of a slightly bigger napkin:

The BFR breaks down as follows:

BF Booster:
Dry mass: 160 tonnes.
Prop mass: 2905 t.
Max Sea level thrust: 1700 kN * 31 = 52.7 MN
SL Isp = 330 s, so exhaust velocity = 3234 m/s
Max Vac thrust: 1830 kN (SWAG) * 31 = 56.7 MN
Vac Isp = 356 s = 3489 m/s

BF Spaceship:
Dry mass 85 t.
Prop mass 1100 t.
Assuming only vac engines at going to orbit Max Vac thrust = 4 * 1900 kN = 7.6 MN
Vac Isp = 375 s = 3675 m/s

Let's assume about 10 tonnes of payload (i.e. 100 passengers at about 100 kg/passenger--enough for an average person and about 15 kg of luggage).
That makes gross liftoff mass = 4260 t.

T/W at launch is 1.36. Acceleration = T/W - 1 = 0.36 gee.
Acceleration (T/W doesn't make much sense now...) at BFB burnout = 4.3 gee. Throttle down to 45% to hit 2 gee.
Acceleration at second stage start = 0.6 gee (T/W doesn't make much sense now...)
Acceleration at second stage burnout = 8.2 gee. Throttle down to 24% to hit 2 gee.

Delta-v of BFR stack at 10 tonnes of payload:
BFB delta-v (assume the average of the SL and vac Isps) = 343 * 9.8 * ln (4260/1355) = 4210 m/s
BFS delta-v = 375 * 9.8 * ln (1195 / 95) = 9305 m/s
Total delta-v = 13,515 m/s

Note: delta-v doesn't really change if you throttle stuff down--especially if you're shutting down whole engines to do it. However, the big deal here is gravity drag, which is the loss of delta-v caused by the vehicle falling back toward Earth at 9.8 m/s^2 the whole time it's thrusting up. Typically, launch to LEO requires about 9200 m/s of delta-v, to achieve an orbital velocity of about 7800 m/s. Aerodynamic drag usually doesn't lose more than 200 m/s of delta-v, so "normal" gravity drag is about 1200 m/s. That number will get higher as T/W goes down, because the rocket has to spend more time thrusting up (and therefore also falling in the gravity field).

You've got an extra 4315 m/s to fool around with here. If you need to throttle back to 45% to keep things under 2 gee during BFB boost, and to 24% during the BFS burn, it's going to be fine. And things are even better if all you're doing is a point-to-point suborbital.

The BFS without the BFB is really close to being a viable suborbital point-to-point system. You'd have to launch on all 7 engines, and they'd probably have to be some kind of intermediate between the SL and vac expansion ratios (rather than 4 vac and 3 SL). Figure an average Isp = 355 s = 3479 m/s exhaust velocity and 1875 kN per engine = 13.1 MN

Liftoff T/W = 1.12
Delta-v = 3479 * ln (1195 / 95) = 8809 m/s.
Figure you'll need 500 m/s of delta-v to land, and you're down to 8309 m/s.
That ought to be good enough for a lot of suborbital destinations.

But now you really need to worry about gravity drag, because you're launching at low T/W. Throttling back is probably OK above 4 gee, but not much before then.

On the other hand, the big problem I've had with this point-to-point stuff is that launch on top of a 31-engine BFB and 2905 tonnes of methalox with no possibility of a launch escape system simply isn't going to (ahem) fly. So going the SSTO route ought to be very, very attractive.

If I were SpaceX, I'd be thinking about going with a special 9-engine BFS in an ocatweb. If I did the math right, you could fit a 1.9 m diameter engine (halfway between the current SL and Vac Raptors) in a 9 m vehicle with no problem. Assuming you're now at a dry mass of 89 t:

Thrust = 9 * 1870 kN = 16.8 MN
Liftoff T/W = 1.44
Delta-v = 3479 * ln ( 1199 / 99) = 8277 m/s with the landing reserve.

Whether that saves you more in gravity drag than it loses you in delta-v loss from the extra dry mass would require some serious simulation.

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Re: SpaceX News

Postby paperburn1 » Fri Oct 27, 2017 3:22 pm

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.
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Re: SpaceX News

Postby Maui » Fri Oct 27, 2017 5:16 pm

TDPerk wrote:
hanelyp wrote:One problem with a lower G launch is increased gravity losses, increasing the delta-V the engines need to produce.


Eh, start at 1.3, ramp over 10 seconds to 2.5. Fuel is cheap.

Sure, but that's not the question. The question is does BFR have the fuel capacity to do so.


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