SpaceX test flight of Starship SN-5 [video] • Hacker News Board

Re: SpaceX test flight of Starship SN-5 [video]

#351

Post by jcims » Thu, Aug 06, 2020, 11:26 AM UTC

sandworm101 wrote: post #316

Robotbeat wrote: post #262

Earlier quoted context omitted.

It's not particularly hard. Just like with regular rocket upper stages that require multiple burns to complete the mission, you can use small ullage thrusters to settle the propellant and make sure liquid (instead of pressurant gas) is transferred from one tank to another. The physics is exactly the same as making sure the engine receives fuel. Just need the thrusters to be fired longer. SpaceX has done experiments w…

Yes but ullage thrusters would have to fire continuousky for the duration of the transfer. That is a significant burn time. If they are going to be draining the fuel using only the ullage thrust, it will take more than a slight nudge.

They would build a dedicated cargo section and fairing for this, all of that would be fairly straightforward to accommodate. I don’t know if SpaceX has acknowledged the idea but there was discussion of installing ’retro-rockets’ up in the fairing for the moon lander (to reduce exhaust blast on the regolith), could use the exact same technology.

If they were so inclined they cold even recover the helium from the tanks they are filling and use that to assist with pumping pressure.

Re: SpaceX test flight of Starship SN-5 [video]

#352

Post by fastball » Thu, Aug 06, 2020, 1:42 PM UTC

DannyB2 wrote: post #305

martythemaniak wrote: post #205

I really love the out-in-the-open development process. It doesn't cost SpaceX anything, yet they attract lots of internet media where people set up streams, discuss progress etc. The next year should be pretty exciting as they develop things further. Starship has a number of big innovations, which are: - the raptor engine. This was developed behind closed doors and is sort of finished. We won't see much of it, but it…

Lettuce not forget Raptor runs on Oxygen and Methane. Both of which might be produced on Mars.

I mean we know for a fact that there is already (in small amounts) methane and plenty of CO2 on Mars. Obviously if we find lots of H2O this all becomes even easier as we can just combine the abundant CO2 with the H2O to get loads of CH4 and O2.

Re: SpaceX test flight of Starship SN-5 [video]

#353

Post by credit_guy » Thu, Aug 06, 2020, 1:47 PM UTC

sobani wrote: post #346

credit_guy wrote: post #250

Earlier quoted context omitted.

Emissions calculation for Starship: - each starship burns 4600 tons of methalox fuel - the mixture ratio of the propellent is 78% O2 and 22% CH4 [1] - that means that each 100 tons of methalox results in 53.6 tons of CO2 and 2.5 tons of CH4 - generally in the climate science it is accepted that one ton of CH4 is equivalent to 25 tons of CO2 - overall, each 100 tons of propellent will produce 116 tons of CO2-equivalen…

> the propellent is O2 and CH4 [1] > that results in CO2 and CH4 The chemical reaction of O2 + CH4 -> CO2 + CH4 does not compute. More likely is the reaction 2 O2 + CH4 -> CO2 + 2 H2O. So no methane emissions, that's the actual fuel. Only CO2 (and water) emissions.

Sorry, I skipped some steps in my calculation.

The chemical reaction is indeed CH4 + 2 O2 -> CO2 + 2 H2O. So, for each 16 grams of CH4 you get 64 grams of O2, that's a stoichiometric mass ratio of 1:4.

The fuel ratio for the Raptor engine is 22:78, according to Elon Musk's tweet. So, for each 100 g of propellant mixture you get 19.5 g of CH4 to burn using 78 g of O2 and you are left with 2.5 g unburned CH4.

I simply neglected the H2O resultant from the reaction because, while H2O is a very potent greenhouse gas, it has a fast cycle in nature (rain, etc). Also, I have no idea how to account for it as a greenhouse gas.

Re: SpaceX test flight of Starship SN-5 [video]

#354

Post by Koshkin » Thu, Aug 06, 2020, 1:50 PM UTC

koalala wrote: post #347

nfoz wrote: post #182

Earlier quoted context omitted.

This will sound weird, but I have no idea why anyone cares about the abstract "survival of human species". I care very deeply about the lives of people actually living here right now. But if Earth is wiped out, what does it matter that some humans are alive somewhere? Except of course that I'm glad for them that they didn't get killed. But we're talking about generations from now anyway, and there are more direct way…

Because we are all genetically related, and humans, like most or all organisms, care about their genetic code surviving.

It may be not obvious (yet it should be if you think about it), but humans, in a very essential way, are not like most all organisms, and "genetic code surviving" is not our primary concern.

Re: SpaceX test flight of Starship SN-5 [video]

#355

Post by Koshkin » Thu, Aug 06, 2020, 1:56 PM UTC

hnrq wrote: post #201

sekai wrote: post #174

Earlier quoted context omitted.

Let's look at this way, humanity needs to become multiplanetary species to prevent extinction if some sort of cataclysm happens on Earth: nuclear war; Yellowstone erupts, etc. Compared to that, any other problem seems insignificant.

I hate this argument. There is no foreseeable disaster that would make Earth less habitable than any other solar system body. Even if every nuke we have was detonated, it would still be orders of magnitude easier to survive on what remains of Earth than to live in Mars. I'm not saying there are no existential risks at all, but being on other planets would not make us any more likely to survive to them than to simply…

The fact is, and it may sound like a paradox, that, for more than one reason, it is the humans themselves that is the biggest danger to their own existence in the long term.

Re: SpaceX test flight of Starship SN-5 [video]

#356

Post by Koshkin » Thu, Aug 06, 2020, 1:59 PM UTC

stallmanite wrote: post #330

reddog wrote: post #191

Earlier quoted context omitted.

If there was a full scale nuclear war. And then the next week the Yellowstone super volcano erupted. And then the week after that an asteroid as big as the one that killed the dinosaurs hit Eurasia, earth would still be wildly more habitable that Mars.

And after a collision like the one that created the moon? How habitable during a gamma ray burst?

... and wha' if Earth jus' 'splodes one day... (takes another sip)

Re: SpaceX test flight of Starship SN-5 [video]

#357

Post by Robotbeat » Thu, Aug 06, 2020, 2:06 PM UTC

sandworm101 wrote: post #316

Robotbeat wrote: post #262

Earlier quoted context omitted.

It's not particularly hard. Just like with regular rocket upper stages that require multiple burns to complete the mission, you can use small ullage thrusters to settle the propellant and make sure liquid (instead of pressurant gas) is transferred from one tank to another. The physics is exactly the same as making sure the engine receives fuel. Just need the thrusters to be fired longer. SpaceX has done experiments w…

Yes but ullage thrusters would have to fire continuousky for the duration of the transfer. That is a significant burn time. If they are going to be draining the fuel using only the ullage thrust, it will take more than a slight nudge.

They would be using a pressure differential to transfer the fuel, not relying on just ullage thrust.

You'd be surprised at how low of an acceleration is required for propellant settling and transfer. On the order of 0.0001-0.01m/s^2. At a 310s Isp for the ullage thruster, that's between 0.01kg and 1kg of propellant usage per second. Even if it takes an hour to transfer fuel (on the ground, fuel is transfered in about 20 minutes and in flight, all the propellant is transfered from the tanks to the engines in ~8 minutes), we're only talking about maybe 1% of the propellant needing to be used for ullage thrust and perhaps as low as 0.01%.

ULA has published a paper on the topic: https://www.ulalaunch.com/docs/default-source/extended-durat...

Re: SpaceX test flight of Starship SN-5 [video]

#358

Post by nickik » Thu, Aug 06, 2020, 4:08 PM UTC

ben_w wrote: post #349

nickik wrote: post #312

Earlier quoted context omitted.

How are tunnels pointless in a future of self driving cars, the opposite is true, tunnels make even more sense when cars drive themselves.

Self driving cars done right will be able to do full speed while literally , not metaphorically, touching bumpers. Traffic jams with human drivers happen spontaneously when the cars get close enough that overcompensation turns into a negative spiral.

So in the city of the future you gone have 1000s of self driving cars going full speed threw the city? It makes much more sense to open up the city for people and ban cars for the most part. Move the transportation under ground.

Re: SpaceX test flight of Starship SN-5 [video]

#359

Post by Someone » Thu, Aug 06, 2020, 4:30 PM UTC

njarboe wrote: post #253

napier wrote: post #247

Earlier quoted context omitted.

> any upcoming engine that can compete with it. There's the criminally underfunded SABRE engine, an a air-hydrogen mix breathing-rocket hybrid design from Reaction Engines that rated to reach speeds of Mach 25 (roughly 35,000 miles per hour). As modelled, it's more efficient than any other current or proposed chemical propulsion technology, and the proposed single-stage runway launchable system would be capable of de…

SpaceX needs $kg to orbit to be much lower for their Mars plans to be possible. It is planning on their starship and superheavy rocket combo to bring down current launch costs by a factor of around 100 in the long term. A factor of two improvement in costs is not interesting to them.

Most of the time, you get an improvement by a factor of 100 not by two of a factor of 10, but by hundreds of a factor close to 1.

I think they would be very happy to get a factor of 2 in one step. It would only 7 such steps to get that factor of 100.

Having said that, they aren’t aiming for the most modern rocket technology as that implies “less well tested”, and will be less reliable. They’re building T-34s, not Tiger tanks.

Re: SpaceX test flight of Starship SN-5 [video]

#360

Post by rotexo » Thu, Aug 06, 2020, 4:41 PM UTC

Robotbeat wrote: post #262

sandworm101 wrote: post #245

Earlier quoted context omitted.

>> - in-orbit refueling. Starship is big and heavy and basically can't get anywhere unless it's re-fueled in orbit. This is one of those "let's see" things that SpaceX has talked about but never actually explained. Their current rocket tech offers no hint as to how this is might happen. They aren't running internal bladders in their tanks. So the physical act of moving fuel from one tank to another is an open questio…

It's not particularly hard. Just like with regular rocket upper stages that require multiple burns to complete the mission, you can use small ullage thrusters to settle the propellant and make sure liquid (instead of pressurant gas) is transferred from one tank to another. The physics is exactly the same as making sure the engine receives fuel. Just need the thrusters to be fired longer. SpaceX has done experiments w…

Out of curiosity, which of the engineering problems posed by the Starship/Superheavy system are the biggest potential showstoppers? My instinct is the belly flop/heat shield required to make Starship reusable, but I know nothing about rocketry.