SpaceX News

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

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krenshala
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Re: SpaceX News

Post by krenshala »

Once out of the gravity well, your only considerations are fuel efficiency versus time and the dV needed to get where you want to be going. dV never changes for a given flight path (baring where the flight falls on the porkchop plot for the intercept, of course). Low Isp, high thrust engines are not fuel efficient, but they are time efficient, while high Isp, low thrust engines are not time efficient, but are instead fuel efficient. If time doesn't matter, you can do miliNewton or microNewton thrust levels (e.g., Ion propulsion) with very high Isp, and move even very large payloads ... in very large amounts of time, and assuming you stay clear of gravity wells. :D

paperburn1
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Re: SpaceX News

Post by paperburn1 »

Range fees and insurance can be as much as 30 percent of a launch cost. fixing that would be a huge money saver.
I am not a nuclear physicist, but play one on the internet.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

RERT wrote:Hmm... if the fuel cost from earth to LEO is ~$10000/lb, what is the cost from the Moon or Mars to LEO? Does that create a commercial opportunity to build a fuel station somewhere to service a very large demand for fuel in LEO? Is there enough carbon, oxygen and hydrogen on the Moon to make fuel there?
Elon's plan for Mars ISRU:

CO2 from the air + H2O from ice in the soil = oxygen + methane (aka rocket fuel)

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RERT
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Re: SpaceX News

Post by RERT »

Yes I saw that, but didn't hear mention of exporting fuel from Mars, just using it for return trips.

Anyway, in this context the premise is faulty - Musk's presentation shows $1.6M for each tanker trip to LEO with about 300T fuel, so the number is more like $2.4 per lb, rather than $10,000! (page 41)

D Tibbets
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Re: SpaceX News

Post by D Tibbets »

RERT wrote:Hmm... if the fuel cost from earth to LEO is ~$10000/lb, what is the cost from the Moon or Mars to LEO? Does that create a commercial opportunity to build a fuel station somewhere to service a very large demand for fuel in LEO? Is there enough carbon, oxygen and hydrogen on the Moon to make fuel there?
I've wondered about the Moon as a fuel source. Certainly the penalty for getting out of the gravity well is much less. But, just how much water is available, could it support the large scale use envisioned by Musk's Mars plans? And, is there carbon available in exploitable amounts

Another consideration is that using the Earth and Mars as fueling stations is that space ships can aerobrake upon arrival. This saves a tremendous amount of fuel. For example, if the transit velocity is about 60,000 miles/hr then aerobraking- either direct entry to landing or deceleration to orbital speeds eats up about 40-60,000 miles delta V on each leg of the journey. Decelerating to Lunar orbit or a Lagrange orbit does not have this advantage (though some fancy maneuvering that utilizes much aerobraking might minimize the difference).
Using filling stations in the outer Solar System works due to orbital mechanics. A trip with a elliptical path could reach the distance from the Sun at the apex of the elliptical orbit such that the space ship closely matches the target asteroid/moon results in little fuel cost match speed. Refueling could then allow for a new elliptical path with a delta V that allows for even greater distances from the Sun. The net result is energy to get there and do so more quickly or with more mass (I think) Trips back to the inner Solar system does not have this option, of at least that is my limited understanding. Unless, of course, you invoke aerobraking.

This assumes that the weight of the heat shields and associated equipment is much less than the weight of fuel to get the same delta V.

Dan Tibbets
To error is human... and I'm very human.

D Tibbets
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Re: SpaceX News

Post by D Tibbets »

Tom Ligon wrote:Please, all, take a moment to consider we're looking at the aspect of space travel that got Dr. Bussard interested in nuclear power for spaceflight.

The abysmal payload fractions to LEO, and far worse for Mars travel, was the whole reason he started looking at heating hydrogen in the '50s, and later to using more sophisticated strategies with the Polywell.

Yes, the use of in-situ resources for propellant is a huge boon. Which is a big reason Musk is interested in exploiting asteroids.

You guys catch this earlier this year: http://www.space.com/33185-earth-quasi- ... 6-ho3.html
Indeed, Dr Bussard's hopes for fusion power in the form of a dilluted fusion product or plasma rocket solves many problems. Good efficiencies at high thrust (like takeoff from Earth) and very good efficiencies for lower thrust space propulsion is an elegant solution. Even if the fusion power cannot be used directly for thrust, it could power space based engine like a VASMIR type engine that has enough thrust to push a large and heavy spaceship at reasonable accelerations. Even an extremely efficient ion rocket engine has limited use if the acceleration rate is so slow that a much less efficient chemical rocket can get you there as quickly because it gets up to adequate cruising speeds quickly.

One of Dr Bussard's papers may warrant another visit:
http://www.askmar.com/Fusion_files/QED% ... ulsion.pdf


Dan Tibbets
To error is human... and I'm very human.

D Tibbets
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Re: SpaceX News

Post by D Tibbets »

RERT wrote:Yes I saw that, but didn't hear mention of exporting fuel from Mars, just using it for return trips.

Anyway, in this context the premise is faulty - Musk's presentation shows $1.6M for each tanker trip to LEO with about 300T fuel, so the number is more like $2.4 per lb, rather than $10,000! (page 41)
Perhaps the premise is somewhat optimistic, If the system can manage 1000 launches the capital cost may be reduced to perhaps a few million per flight, the fuel costs might be $1 per pound or about $20 million(?) per flight plus the ~ $600 thousand cost of the fuel payload. The range costs and the infrastructure costs must be minimal. Assume that the costs are about $30 million to deliver ~300 tons (metric tons?) to LEO. That is about $50 per pound of fuel delivered to LEO.

In an earlier post I used $100 per pound to LEO based on delivering a passenger to Mars in turn based on Musk's mention that the assigned mass per passenger was perhaps 2 tons and the cost target was about $200,000 per passenger. This seems very optimistic, but it still leaves some margin with the higher fuel costs. Of course the cost of the spaceship per flight, and many other factors has to be included.

Dan Tibbets
To error is human... and I'm very human.

Tom Ligon
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Re: SpaceX News

Post by Tom Ligon »

RWB estimated more like 50 tons per passenger for Mars. Think of a couple of moving vans.

Then again, you should have seen the junk he hauled out to San Diego! He was a packrat.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

i don't know about you, but from playing KSP i've learned you can save a lot of fuel by doing a well placed aerocapture before the aerobrake.

http://arc.aiaa.org/doi/abs/10.2514/1.1 ... alCode=jsr

krenshala
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Re: SpaceX News

Post by krenshala »

D Tibbets wrote:
RERT wrote:Hmm... if the fuel cost from earth to LEO is ~$10000/lb, what is the cost from the Moon or Mars to LEO? Does that create a commercial opportunity to build a fuel station somewhere to service a very large demand for fuel in LEO? Is there enough carbon, oxygen and hydrogen on the Moon to make fuel there?
I've wondered about the Moon as a fuel source. Certainly the penalty for getting out of the gravity well is much less. But, just how much water is available, could it support the large scale use envisioned by Musk's Mars plans? And, is there carbon available in exploitable amounts

Another consideration is that using the Earth and Mars as fueling stations is that space ships can aerobrake upon arrival. This saves a tremendous amount of fuel. For example, if the transit velocity is about 60,000 miles/hr then aerobraking- either direct entry to landing or deceleration to orbital speeds eats up about 40-60,000 miles delta V on each leg of the journey. Decelerating to Lunar orbit or a Lagrange orbit does not have this advantage (though some fancy maneuvering that utilizes much aerobraking might minimize the difference).
Assuming you were meaning 'm/s' instead of 'miles' in the paragraph above, your numbers are a bit off (if not, you misused Δv). the transfer between LEO and LMO is only 5710m/s (plus any plane change requirements -- up to ~300m/s of Δv). On the Earth-to-Mars trip, you can aerobrake and reduce that total by 1440m/s (4270m/s + plane change). On the Mars-to-Earth trip you can aerobrake and reduce the total by 3210m/s (2500m/s + plane change). These numbers are assuming Hohmann transfer orbits and periapsis burns for maximum Oberth Effect. Other tricks (gravity assists, etc) may reduce numbers further (I'm basing these numbers off a couple of different Δv maps).
D Tibbets wrote:Using filling stations in the outer Solar System works due to orbital mechanics. A trip with a elliptical path could reach the distance from the Sun at the apex of the elliptical orbit such that the space ship closely matches the target asteroid/moon results in little fuel cost match speed. Refueling could then allow for a new elliptical path with a delta V that allows for even greater distances from the Sun. The net result is energy to get there and do so more quickly or with more mass (I think) Trips back to the inner Solar system does not have this option, of at least that is my limited understanding. Unless, of course, you invoke aerobraking.

This assumes that the weight of the heat shields and associated equipment is much less than the weight of fuel to get the same delta V.
The Δv required for a given specific path never changes. What the ship can do, however, does vary based on wet- versus dry-mass (the Dreaded Rocket Equation). The rest of what you state I agree with. :D

RERT
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Re: SpaceX News

Post by RERT »

To round this out, wiki says only trace quantities of Carbon on the moon, so only liquid oxygen could come from there. (I guess unless some enterprising person can land a large carbonaceous chondrite there!). Seems like you need CH4+3O2 -> CO2+2H20, so 18 grams of methane for every 48 grams of oxygen - i.e. 73% of fuel mass is Oxygen, so the moon may still be useful.

Also from Wiki on the Δv's: they say Earth to LEO about 9+m/s, Moon to LEO about 3- m/s, so better than 3 times. As near as I can tell, the spaceX numbers say $1.1m in propellant cost for the booster for a launch to LEO from Earth. So if 3X is a good number, fuel from the moon would have circa $750K cost advantage per launch. He is talking about having to launch minimum 5000 trips to Mars, 25,000 tanker flights over 100 years, say 250/year, of which about 180 launch Lox ( if he could separate them ). So the plant on the moon generates $135M/year in value, might make it worth considering if you can build it for less than $500 million. Capacity would have to be at least 1250T/day, about 14kg/second. You would need a deposit of 50 MT of water...

Again, wiki says 50kwh/kg to electrolyse Hydrogen from water, which means 6.25kwh/kg of Oxygen. 14kg/second thus means 315MW of power.

I guess with a ship that can take 450T payload to Mars, it's possible, but it doesn't have me reaching for my wallet to fund it.

On the other hand, sending oxygen and Methane separately to LEO sounds very attractive, if most of the flights are carrying only oxygen and so don't make such a spectacular 'bang' when sabotaged by aliens....

happyjack27
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Re: SpaceX News

Post by happyjack27 »

RERT wrote:To round this out, wiki says only trace quantities of Carbon on the moon,
Mars, OTOH, has an atmosphere 96% CO2. ( https://en.wikipedia.org/wiki/Atmosphere_of_Mars )
Also from Wiki on the Δv's: they say Earth to LEO about 9+m/s, Moon to LEO about 3- m/s, so better than 3 times.
A few things:
1. you need to do round trip, so 9*2 vs 3*2 = 18 vs 6, still 3 to 1, but bigger numbers. this is relevant for point #3, below.
2. on earth you can do aerobraking, saving a lot of fuel on return. not so much on mars, and not at all on the moon. If aerobraking can handle most of your return deltav, that makes it more like a 3 to 2 ratio.
3. fuel cost per deltav is not linear - fuel has mass. so your fuel requirement is going to grow a lot faster than your deltav requirement. a deltav saving of say 3 to 1 is a fuel saving of much more than that. exponentially. https://en.wikipedia.org/wiki/Tsiolkovs ... t_equation
As near as I can tell, the spaceX numbers say $1.1m in propellant cost for the booster for a launch to LEO from Earth. So if 3X is a good number, fuel from the moon would have circa $750K cost advantage per launch.
you're assuming fuel production costs are the same on the moon as on earth. power production, resource collection, and conversion efficiency could vary dramatically. not to mention the cost of the initial infrastructure.

Tom Ligon
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Re: SpaceX News

Post by Tom Ligon »

Yes, Mars has an atmosphere that is largely CO2, but not much atmosphere. And using solar to convert it to methane is plagued by not having much solar, either.

I'm not suggesting it can't be done. It can. But dang, wouldn't it be nice to have compact fusion reactors? Just go for the reaction mass, and forget making methane.

To me, the element lacking on the Moon is nitrogen. There's carbon in asteroids, so certainly there is some on the moon. Lacking nitrogen is a serious problem for life.

We're getting into the reasons I'm a belter. Any of these Mars rockets can exploit asteroid belt resources easier than you can do Mars landings. Mars is the popular target, but I predict it will be bypassed for targets that are easier to exploit and offer more return. Mars as a place to live has a LOT of negatives, all of which can be gotten around by well-engineered space habitats in a region with the right materials.

paperburn1
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Re: SpaceX News

Post by paperburn1 »

I have to agree,mars as a long term goal is great but there are many better things that could be exploited on the short term in the belt. And at a higher profit margin. And in the end it will be all about the money. Manifest destiny via capitalism. That could change if we had a cheap way to lift to LEO would like a space elevator concept in place (right after we get fusion cooking)
I am not a nuclear physicist, but play one on the internet.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

According to wikipedia's delta-v map, if you disregard the return fuel needed for the earth based booster and tanker (assuming significant aerobrake), compared to a LEO rendezvous from the moon, it's actually a bit worse deltav (10 vs 12.8):

LEO
----
enter 10+0
exit 2.5+0.7=3.2
refuel 1.6+0.7+4.1(aero)=6.4 *2 = 12.8

But if instead you rendezvous at geosynchronous transfer orbit (i don't know if this is even possible), for a small 2.5 delta-v initial investment, you can get a nice payoff from refueling from the moon (costing you only 6.4 delta-v a piece):

GTO
---
enter 10+2.5 (AERO)
exit 0.7
refuel 1.6+1.6=3.2 *2 = 6.4

From there, you'll need 7.7 deltav to land on mars. (whereas from LEO, you'd need 7.7+2.5=10.2)

So let's say you only had enough to get to LEO, then if it takes 3-5 tankers to get enough delta-v to mars (10.2), then it'd only take 1 to get to GTO (2.5). And then you can do the rest from the moon. (bearing in mind the tanker will need 3.2 deltav for its return trip, and can't save any by aerobraking)

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