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Starship is still not understood

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Re: Starship is still not understood

#341

Post by ncmncm »

modeless wrote: post #323
ncmncm wrote: post #306

Earlier quoted context omitted.

Rapid turnaround is not demonstrated. Nor is it even, in fact, planned , in any real sense. I.e., to be clear, there is no evidence that they have any idea how to achieve rapid turnaround of a Starship orbiter. (Superheavy is a different story.) All this turns on the design of the heat-shield tiles. Those we have seen are basically the same as on the shuttle. Each individual tile needs a great deal of personal attent…

I don't think that the heat shield is the same as the shuttle's. The tiles are tougher and attached in a different way, and the underlying steel structure can stand more heat than the shuttle's aluminum could. Orbital refueling is required for HLS and depends on rapid turnaround so SpaceX and NASA must have some confidence that it can be achieved without a radical redesign.

There are differences from the shuttle's tiles, but they are much more similar than different.

Rapid turnaround seems at this stage more an aspiration than a designed feature. It was the same with the shuttle. In that case it was never achieved at all, even with decades of experience.

Turnaround and reuse are desirable, particularly to minimize cost, but not strictly necessary for a successful mission.

Re: Starship is still not understood

#342

Post by ncmncm »

qayxc wrote: post #333
ncmncm wrote: post #314

Earlier quoted context omitted.

Just incidentally, a RTG powered by strontium-90 works as well as or better than a plutonium-powered one. Strontium-90 builds up as a waste product in ordinary uranium fuel rods, and can be (pretty) cheaply extracted from what we call nuclear waste. I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry.…

> I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. You suspect wrong. In order to be a decent RTG power source, an isotope needs to have: 1. Good power density, since the current energy conversion efficiencies are quite low 2. Good half-life, since it is aimed to operate the spacecraft as long as…

Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed.

Overprovisioning is needed in every case, and excess power has to be dealt with the same way.

Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.

Re: Starship is still not understood

#343

Post by ncmncm »

panick21_ wrote: post #322
ncmncm wrote: post #309

Earlier quoted context omitted.

Once it becomes Somebody Else's Problem, and not NASA's, then NASA can quietly abandon it. Since nobody else will want to pay for even a single launch, it may then die a final death.

> Once it becomes Somebody Else's Problem, and not NASA's, then NASA can quietly abandon it No, what happens is that you create a seriously powerful group of private institutions that will go full force into lobbying congress to not kill the project. Even more so then now.

What you describe is the present condition.

Re: Starship is still not understood

#344

Post by brianwawok »

snowwrestler wrote: post #195
brianwawok wrote: post #158

Earlier quoted context omitted.

if computers taught me anything.. it's if you can do 1 thing really really well perfectly, it's not a ton of work to do it 10x or 100x in parallel. We don't have enough data to know reliability of say Falcon long term. Is it 99% successful? 99.9%? 99.999999%? Doing a ton of launches, you really want to work on the 9s, so 0-1 of your launches blow up, not 100+. But once you do, you just copy paste it baby and go to to…

There is a literally a book called “The Mythical Man Month” about the limits of parallelism in computer engineering. It’s arguably one of the fundamental texts of the discipline.

Right. I didn't say we hire 100 developers to code the project 100x faster.

I said let's build 100 parallel rocket pads with 100 parallel rockets.

It will indeed launch rockets 100x faster.

The term you are looking for is embarrassingly parallel.

Re: Starship is still not understood

#345

Post by zardo »

panick21_ wrote: post #321
zardo wrote: post #271

Earlier quoted context omitted.

> I really think this is overrated in how much of a technology risk this is. Specially if you have enough weight that you can invest in the solution. Refueling is a schedule risk, maybe the first design will fail to account for something and it will take a couple iterations to get it right. There is zero risk that the problem is unsolvable.

I would agree mostly. If you need to many iterations you might run out of money however. In general I think refueling is a far smaller problem then the heat shield.

The heat shield is such a massive component that it's conceivable that the first design is flawed, and resolving the flaw requires a dry mass increase that cuts into the payload and makes a Mars return unachievable.

Re: Starship is still not understood

#346

Post by snowwrestler »

brianwawok wrote: post #344
snowwrestler wrote: post #195

Earlier quoted context omitted.

There is a literally a book called “The Mythical Man Month” about the limits of parallelism in computer engineering. It’s arguably one of the fundamental texts of the discipline.

Right. I didn't say we hire 100 developers to code the project 100x faster. I said let's build 100 parallel rocket pads with 100 parallel rockets. It will indeed launch rockets 100x faster. The term you are looking for is embarrassingly parallel.

Only parts of the system are embarrassingly parallel. But you need the whole system to be available for every launch.

Poorly parallel parts of the system include regulatory clearance and accident investigations. There is only so much clear air space available each day. And if a rocket blows up (which they often do), they’ll all sit on the ground until there is understanding as to why. And then potentially sit even longer if they each require work to correct or mitigate flaws that are found.

Starships are still hand-built prototypes. And Musk has a history of underestimating the difficulty in scaling and automating physical manufacturing.

It’s way too early to be confident this can embarrassingly parallel. Parallelism is hard in general. It’s hard because often constraints aren’t obvious until you’re actually doing it.

Re: Starship is still not understood

#347

Post by marktangotango »

wildermuthn wrote: post #257
marktangotango wrote: post #248

Earlier quoted context omitted.

Gobi desert doesn't have 1/3 earth gravity.

What are the economic benefits of low gravity? If so, would the moon be more economically valuable than Mars? I’m genuinely curious about the economics of doing more in space than putting up satellites.

For a spacefaring civilization reliant upon chemical rockets, less gravity is better. Point being Mars is a better place for such a civilization, not earth.

Re: Starship is still not understood

#348

Post by qayxc »

ncmncm wrote: post #342
qayxc wrote: post #333

Earlier quoted context omitted.

> I suspect the use of plutonium for RTGs in spacecraft is mainly because people already had a pipeline for extracting and concentrating plutonium for the weapons industry. You suspect wrong. In order to be a decent RTG power source, an isotope needs to have: 1. Good power density, since the current energy conversion efficiencies are quite low 2. Good half-life, since it is aimed to operate the spacecraft as long as…

Strontium-90 has better power-density per unit mass than Pu, albeit with larger volume. Half-life is comparable. Safety is better: less gamma, no neutrons, no shielding needed. Overprovisioning is needed in every case, and excess power has to be dealt with the same way. Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.

> Strontium-90 has better power-density per unit mass than Pu

The power density of Sr-90 fuel is 15% lower by mass than Pu-238 - 0.46kW/kg thermal compared to 0.54kW/kg thermal. You must have been looking at the wrong figures. Since the Sr-90 fuel rod doesn't get as hot as a Pu-238 fuel rod of the same size, energy conversion efficiency drops significantly and you need up to 100% more Sr-90 fuel (and consequently more shielding as well) to get the same power.

> Half-life is comparable.

A factor of 3x is not really comparable (87.7 vs 28.7 years).

> Safety is better: less gamma, no neutrons, no shielding needed.

Where did you get that from? Strontium-90 and its daughter product Yttrium-90 emit high-energy beta particles and thus give off high energy bremsstrahlung which requires heavy shielding. The BUP-500 battery (the largest RTG ever built, designed to provide 500W electric after 5 years; 1.8m³ in size and 3.6 metric tons in weight) used a tungsten alloy for shielding; its predecessors used depleted uranium...

Pu-238 on the other hand is pretty much exclusively an alpha emitter and has no short-lived daughter products (U-234 has a half-life of >200ka).

> Overprovisioning is needed in every case, and excess power has to be dealt with the same way.

Again, where did you get that from? A space mission is designed to a certain power spec and if you need your primary instruments for a 15 year mission, you plan your power such that after 15 years all primary instruments can still be powered. With Pu-238 that'd be about 75%-80% of the initial RTG capacity (some losses due to development time and efficiency losses from thermocouples are included).

Due to its properties, Sr-90 would require about 3x as much fuel just to make up for its shorter half-life in addition to the up to 100% increase in required fuel mass from its lower temperature.

Since it's a beta emitter, the additional radiation requires heavy shielding as well. So no, there's a pretty substantial difference there.

> Low cost to orbit means more fuel / more ∆V is practical to provide, thus much shorter transit times, translating to less up-front decay.

That's not how interplanetary missions work. Chemical rockets are incapable of providing enough delta-V to substantially shorten transit times to the outer solar system. If you want to do anything other than fly-by missions, more chemical fuel isn't going to help with that - you'd need nuclear or electric propulsion [1].

[1] https://www.esa.int/gsp/ACT/doc/PRO/ACT-RPR-PRO-ISTS2004-Plu...

Re: Starship is still not understood

#349

Post by garmaine »

Dylan16807 wrote: post #235
qayxc wrote: post #182

Earlier quoted context omitted.

> Based on what, your gut feel? Based on the fact that every orbital launch requires airspace restrictions, local road closures, blocked patches of sea and launch permissions. The regulatory framework alone is incapable of dealing with that (keep in mind that every orbital rocket is basically an ICBM), let alone the fact that you'd have to spread launches over multiple sites therefore rendering considerable patches o…

> Based on the fact that every orbital launch requires airspace restrictions, local road closures, blocked patches of sea and launch permissions. So set up a [semi]-permanent airspace restriction somewhere inland, remote enough that there are no local roads to worry about, and don't block any patches of sea. And do the permission in bulk, obviously. > major explosions (worst case) per launch campaign Failure rate is…

It can’t be inland, because there’d be a huge stretch of flyover restrictions. It’d be at sea on top of mobile floating platforms. Like the two oil rigs SpaceX bought last year…

Re: Starship is still not understood

#350

Post by garmaine »

moralestapia wrote: post #308
Gravityloss wrote: post #80

Earlier quoted context omitted.

100 tons of payload. Rockets are typically about 2% payload so 5000 tons fully fuelled. Fuel and oxidizer combine to H2O, OH, CO and CO2. So, not all the exhaust product is CO2. But some of the CO released is combined with atmospheric oxygen to CO2, so that increases the mass of CO2 besides direct propellant mass. We could as first guess say that it's about 5000 tons of CO2 per launch. So 1000 launches would be 5 mil…

Except it's not 10,000 launches, it's 170,000 (see other comments here). So that's 850M tons, about a 10% increase (on top of the 2nd most polluting country in the world), so yeah, not happening. I would personally rally against the govt. (and everything that allowed that to happen) if Elon gets a free pass on that.

The plan is to use atmospheric processing to produce methane and LOX on-site using renewable power. So it’d be net-zero carbon emissions.
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