SpaceX News

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

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happyjack27
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Re: SpaceX News

Post by happyjack27 »

and i'm not buying the "LEO refueling is more efficient".

It seems to me like, for the same launch profile (thrust/mass ratio and inclination at any given elevation) and air resistance, needed fuel per unit mass to LEO would be linear.

So yeah, multiple launches would be linear, too. But if you make the ship bigger, well doesn't it's mass increase slower than it's volume? So wouldn't a bigger ship be more efficient?

Secondly, why not just add a pair of rockets on the side, like in a falcon heavy, wouldn't this produce a more efficient launch profile? (since you need more thrust at a lower elevation) And thus wouldn't this thus allow you to lift more mass per unit fuel? (and thus allow you to lift more fuel to LEO)?

Not to mention the additional fuel cost of rendezvous.

Sounds to me like the real reason they use a tanker to refuel is because they ran into engineering challenges with making a bigger rocket, and/or with a parallel 1st stage.

...or perhaps they figured they only needed 2x the fuel, not 3x the fuel. And this way they wouldn't have to build as many first stage rockets before the first launch. (2 or even 1 instead of 3) and they wouldn't have to have 3 landing pads that can handle these big rockets available simultaneously.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

IMO, it should be 1 parallel stage and 3 vertical stages. With the top 2 stages optimized for vaccum. (and the top stage being the cargo)

v
v
sss

furthermore, the top stage should be a "wet launch" - should have its fuel tanks pre-fitted with bare living quarters (be it just a ladder and a hatch), so that the empty tanks can be reused for crew or storage.

presumably the 2nd from top stage would reach orbit - unless you're skipping orbit and going straight to transfer - so you have 3 options, going from least fuel cost to most:
* throw it away
* wet launch it as well - turn it into a space habitat
* land and reuse

it's also questionable whether you should just combine the top 2 stages, as long as you're converting them into habitat or storage. provided you can lug the extra mass to mars, it saves you the mass (and cost) of the next set of engines, and gives you more shielded & pressurized space on mars. Though it'd be a lot taller...

hanelyp
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Re: SpaceX News

Post by hanelyp »

With LEO refueling you can fly a smaller launcher off a smaller pad more often. High flight rate is favored for economics and crew proficiency.
The daylight is uncomfortably bright for eyes so long in the dark.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

hanelyp wrote:With LEO refueling you can fly a smaller launcher off a smaller pad more often. High flight rate is favored for economics and crew proficiency.
Sounds like an infrastructure issue.

Build a larger pad (and whatever support you need for launch). Problem solved.

Higher flight rate maintained. Hell, useful flight rate increased now that you don't have to use so many flights on refueling.
Recurring costs reduced.

But then there's assembly of a larger rocket... seems to me could also be an infrastructure cost that can be amortized.

Sure, the bigger rocket, with a parallel first stage, is a higher upfront cost, and there's still a calculation of how that's going to be amortized.

But you do gain some efficiencies, such as more mass per unit time and per unit launch, and higher frequency of crewed flight. (or cargo flight).

happyjack27
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Re: SpaceX News

Post by happyjack27 »

also here's an idea: land it on it's side, instead of vertically. or split it in half and land two on the side. Giving it more contiguous floor space. rockets would then be spaced for symmetry. The wider base would also give you:
* move vectoring leverage, for better attitude control on landing
* better aerobreaking on mars entry, for reduced fuel (and thrust) need

note when you're in the vacuum of space, the shape of your ship doesn't matter - you don't need aerodynamics. so it's fine to use these rockets for hoffman transfer and what not.

...not sure what you'd do with the nose cone... extra fuel? originally i was thinking mars communication satellite - but then if you're unfolding, then you'd have to disconnect it, unless you split it in half, and thus have two comsats but you'd want to put them at opposite orbits then. so is suppose youd just use that space for extra fuel for the previous stage, or maybe an earth satellite.

Image

happyjack27
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Re: SpaceX News

Post by happyjack27 »

and for greenhouses, instead of using solar arrays, followed by led lights, you could use inflatable solar concentrators:

Image

This would increase energy efficiency while reducing mass / production cost.

Depending on the fuel cost of geosynchronous orbit, it may save fuel to deploy these in space, and then have that much less mass you have to land. you'd need very precise tracking, though. So essentially these would be like a concentrated solar power plant, except the mirrors would be in space. Drawback of this is you can't create them in-situ then. So wouldn't be able to grow capacity from mars. unless you launch from mars but what'd be the point of that? my understanding is the martian atmosphere is rather thin, so solar energy collection is about the same in orbit or landed. so why waste the fuel?

all things considered seems might as well land the solar concentrators. at least that way they're maintainable.

Using knowledge acquired from class and from the additional referenced literature, this paper addressed the key requirements for non-terrestrial PV systems. We noted how modern launch payload costs in excess of $10,000/kg dictate volume and mass requirements, in addition to efficiency, for such PV systems. I showed that, although alternate systems exist, inflatable membrane concentration systems offer excellent weight-to-area kg/m2 scaling (at 1-to-2 orders of magnitude better than PV modules) at decent efficiency – thus addressing space-based requirements

...

Inflatable membrane solar concentration technology has a bright future.
-- http://www.travisdeyle.com/files/public ... mPaper.pdf

Oh, I shouldn't appreciate that pun, but i do!

D Tibbets
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Re: SpaceX News

Post by D Tibbets »

The video shows the transport entering atmosphere in a lifting body attitude and then smoothly transitioning to vertical retro propulsion. More complicated shapes and even seperations means more complexity and weight. Remember that a very good heat shield must protect each component. This adds even more weight and failure points.

A linear arrangement of floors- like an airplane or Shuttle may allow for more deck space, but I am not sure. There are structural and load bearing issues.
There are no runway landings- wings are useless and landing has to be finalized with rocket thrust. In a vertical near cylindrical arrangement attitude control is probably easier, and importantly if one engine fails the remaining engines near the central axis can take over- easier design. With a long horizontal attitude this would be a problem with widely separated rockets engines. I suspecting clustering them near the center of gravity would not be a solution. It is relatively easy to correct for tilting with a tall vertical ship by vectoring the clustered engines. For a long horizontal ship attitude I suspect thrust vectoring from the center of gravity cluster of engines is more problematic.

I'm not sure if a larger vehicle is necessarily more efficient and cost effective. There is a trend that larger is better, but cost possibly scales exponentially. Dr Bussard used the old engineering principle that cost scales as the cube of the size. Reusability also completely changes the picture. A larger ship might be reusable also, but what are the tradoffs. Eg: a smaller ship might transport 400 tons to LEO per flight and be used 1000 times. A much larger ship might transfer 1,000 tons but only be reusable 200 times. Everything else being equal the smaller ship is more economical. Loss of a ship is also less devastating. ULA has shown plans for a Lunar mission with a much smaller payload, and a rocket that is perhaps 1/3 to 1/2 the weight of the Spacex rocket . It costs a lot and it is designed for a much easier mission.

I'm sure SpaceX worked the numbers- the engineering challenges, the feasability, the cost, the risks, and the size/ payload relationships, etc. and this is their best solution. Without orbital refueling by reusable rockets, and advanced structures the costs of getting the necessary mass to LEO is too high. The goal of ~ 200 thousand dollers per passenger is optimistically possible with this scheme. It would be interesting to see cost estimates for other schemes and size comparisons.

Also, keep in mind that fuel stations set up in various locations in the Solar System would be much more difficult with much larger ships- both the tankers and the payload carrying ships. And, the common baseline design for the various missions has large cost saving and time saving consequences.

I wonder if modestly smaller scales may be a better solution, not a larger.

Besides, once out of the Earths punishing gravity well, the huge booster is not needed. And having a spaceshi9p that looks somewhat like a classical Buck Rogers spaceship is cool!

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

Tom Ligon
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Re: SpaceX News

Post by Tom Ligon »

The loss of ship issue does bear on the chart above, showing the various rockets. Switching to the Merlin 2, for example, introduces a very un-Muskian property: It is doomed with an engine out because it just has one. One engine on a Falcon 1 was OK for testing the original Merlin, but the only reason for it now on a Falcon 9 is to test the first few Merlin 2s, or whatever replacement is used.

On the Falcon XX, as illustrated in the chart, 6 engines is also a problem for a crewed, or worse, passenger-carrying ship. Its engine-out capability is listed as limited. One can imagine Musk saw this chart and told his minions to design another engine, as this one was not meeting his needs. I suspect this chart is essentially pretty artwork salvaged from the rubbish bin.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

I don't think the engine count in the chart is accurate. In any case it isn't relevant to the size of the rocket. You can use more engines of a smaller size or vice versa.

The arguments about the size of the craft are only relevant if you are changing the size of the craft. If instead you are adding two crafts of te same size in parallel on the first stage, such as in the falcon heavy, those considerations don't apply.

New ones do, of course, such as structural integrity of the parallel connection (thrust grows with the square of the diameter, and mass grows that tones height, but surface area between the parallel stages grow at diameter time height). Then there's parallel stage separation, but I don't imagine this would scale any worse.

New ones do, but considerations about re-entry and landing don't, except for the middle rocket which is going to be higher and faster so have to withstand greater re-entry forces.

One could argue that it already withstood those forces on exit, but:
1) re entry acceleration is due to gravity - varying up to 1G. Exit acceleration might be slower.
2) this assumes not tilting the rocket to take advantage of surface area for aerobraking.

Though the first one doesn't seem reasonable, especially given the thrust to mass ratio is going to be higher with so much less fuel.

And the second one - it seems you'd have plenty of opportunity to adjust your tilt below threshold.

So both of these considerations seem moot.

Skipjack
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Re: SpaceX News

Post by Skipjack »

Tom Ligon wrote:Perhaps the reason they don't expect to recover the tanker booster is that it IS an SSTO.
Of course they will recover the tanker and the booster.

happyjack27
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Re: SpaceX News

Post by happyjack27 »

Skipjack wrote:
Tom Ligon wrote:Perhaps the reason they don't expect to recover the tanker booster is that it IS an SSTO.
Of course they will recover the tanker and the booster.
Someone mentioned they wouldn't recover the tanker.

I looked it up. I'm seeing them saying that they will recover the tanker.

This makes sense with their economic strategy - that the cost of recovery - if you can make it reliable enough - is less than the cost of bio losing a new one. Especially given that fuel is about 3% of the total launch cost.

Tom Ligon
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Re: SpaceX News

Post by Tom Ligon »

happyjack27 wrote:
Though the first one doesn't seem reasonable, especially given the thrust to mass ratio is going to be higher with so much less fuel.

And the second one - it seems you'd have plenty of opportunity to adjust your tilt below threshold.

So both of these considerations seem moot.
Well, keep working on that line of thinking. The deal here is that a really big tank, when empty, ought to have a crappy ballistic coefficient, particularly if it can stand getting a little sideways. Picture what a soda bottle will do, empty versus full. It should be draggy as all getout.

I worked on something like this with a couple of SIGMA buddies a couple of years back. We were looking at emergency re-entry systems that could go into an ejection seat. NASA has been working up IRVE, an inflatable system for Mars atmosphere entry. What we came up with was basically IRVE filled with a quick expanding and setting polyisocyanurate foam. I demonstrated the capability of this stuff to almost handle re-entry on its own, and I think definitely with something like Nextel fabric over it (IRVE material). The trick is to make a lightweight low ballistic coefficient structure.

Which, as I just said, is what that tank would be, empty.

Basically, you want something that responds to a little air like a beach ball and not a javelin. If you can slow down at high altitude in thin air, its is a lot easier than a plunge into thick atmosphere at high mach.

A couple of inches of foam does a remarkable job, and it does not weigh much. I was not thinking along those lines because you don't want to add a lot of mass to the vehicle, and what they've done so far is use fuel instead of heat shielding. But maybe the big rocket can work it in to the balance. This is a multi-variable optimization.

D Tibbets
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Re: SpaceX News

Post by D Tibbets »

That three percent of launch costs is the fuel is true for traditional rockets. Reuse, especially high reusability with little turnaround costs elevates the fuel costs per mission as a percentage, though admittedly the fuel cost remains the same, but all other costs go down in relation. In commercial aircraft, fuel costs dominate.

Another point is that fuel costs may be as low as three percent of a traditional launch cost, but delivering fuel for use in LEO or beyond is not a part of the launch cost, it is payload and is very expensive, perhaps ~ 10,000 dollars per pound. If reuse, economy of scale and other technologies can reduce this payload delivery cost to perhaps 100 dollars per pound the fuel costs on station in LEO is correspondingly less but never as cheap as at Earth's surface.

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

RERT
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Re: SpaceX News

Post by RERT »

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?

Tom Ligon
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Re: SpaceX News

Post by Tom Ligon »

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

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