totm nov 2023 SpaceX Discussion Thread

[StrandedonEarth]

18 minutes ago, Elthy said:

Yeah, the old plan of using the normal tanks as dewars for the header tanks sounded realy good. Maybe issues with structuraly supporting them under engine power when the main tanks are empty?

Well, simplicity for the Mk1/2 testbeds. Later versions may change. SpaceX is known for flexibility 

[sevenperforce]

If all that is being stuffed into the nose, I wonder if we see a move to a Skylon-style cargo bay/crew compartment rather than the pretty nose-mounted crew cabin.

Elon mentioned going carbon-neutral on Earth, ultimately. This could be a super-lucrative move if more nations transition to a carbon tax. Companies can buy negative carbon from Elon less expensively than paying a carbon tax.

[tater]

3 minutes ago, sevenperforce said:

If all that is being stuffed into the nose, I wonder if we see a move to a Skylon-style cargo bay/crew compartment rather than the pretty nose-mounted crew cabin.

The nose on these is not meant to haul anything, anyway, so why not put stuff where it's easy to work on?

[CatastrophicFailure]

Question is, are there two more?

 

Also, picks of the header tanks (via NSF)

[Elthy]

Those look very small, i kinda doubt enough fuel fits in there for landing. Maybe its a ballast tank to simulate a varrying payload...

[Ultimate Steve]

17 minutes ago, Elthy said:

Those look very small, i kinda doubt enough fuel fits in there for landing. Maybe its a ballast tank to simulate a varrying payload...

That looks to be approximately 1.5 perosns in diameter and 2 persons tall, so 3 meters by 4 meters ish. That gives a volume of about 28 cubic meters. Density of the fuel is not 1 but it should be close so let's assume it's 1. If starship dry mass is 100 tons, and it can hold 28 tons of fuel, and if isp is 350, then it has about 900m/s of Delta V, more than enough to land in Earth's atmosphere at least.

Also that assumes no cargo. Might be a problem landing on Mars (super thin atmo) with 100 tons of payload.

But for testing and orbital flights 900 should be fine.

Although that's an estimate, take it with a grain of salt.

 

[magnemoe]

1 hour ago, tater said:

The nose on these is not meant to haul anything, anyway, so why not put stuff where it's easy to work on?

True, only issue is that this makes the test and production version different, on the other hand the manned will have an heavier nose. Returning stuff to earth is also an relevant mission 

[tater]

Mk1 is I think for lower alt testing. It’ll be interesting to see what they do mk2.

 

 

Edited September 22, 2019 by tater

[ThatGuyWithALongUsername]

46 minutes ago, tater said:

Mk1 is I think for lower alt testing. It’ll be interesting to see what they do mk2.

I was under the impression that both Mk1 and Mk2 were for higher alt / reentry testing, and we know from FAA documents that this is what Mk1 is for, but SpaceX hasn't exactly been known for sticking to a strict plan with this thing so far, has it?

Guess we'll get updated with the presentation in a week, and then see for sure when these actually fly...

[tater]

I’ve assumed that they’d use FL for higher or more distant flights given the facilities.

[RedKraken]

2 hours ago, CatastrophicFailure said:

Question is, are there two more?

 

Also, picks of the header tanks (via NSF)

right size for ch4 header tank....12 tonnes?(~28m3 as per steve)

the lox header (pipes ?) have to hold 3.6 x 12 = 44t

total 56t

provides about 1000m/s on a 185t vehicle @ 380s.

[IncongruousGoat]

3 hours ago, sevenperforce said:

Elon mentioned going carbon-neutral on Earth, ultimately. This could be a super-lucrative move if more nations transition to a carbon tax. Companies can buy negative carbon from Elon less expensively than paying a carbon tax.

Probably not. If there's money to be made in carbon capture, you can bet that we'll see dedicated carbon capture companies set up, who will be far better equipped to do it in the most optimal manner possible. Methane production is almost certainly not the best way to do it, just because of storage concerns.

[sh1pman]

37 minutes ago, tater said:

I’ve assumed that they’d use FL for higher or more distant flights given the facilities.

Since they’re adding fins to Mk1, it’s safe to say that they’re going to test reentry with it. 

15 minutes ago, IncongruousGoat said:

Probably not. If there's money to be made in carbon capture, you can bet that we'll see dedicated carbon capture companies set up, who will be far better equipped to do it in the most optimal manner possible. Methane production is almost certainly not the best way to do it, just because of storage concerns.

 

(Although, capturing carbon with trees or other plants is still not the best approach)

Edited September 22, 2019 by sh1pman

[tater]

28 minutes ago, sh1pman said:

Since they’re adding fins to Mk1, it’s safe to say that they’re going to test reentry with it. 

No, test skydiver. 

Kills velocity at high AoA, then drops, uses fins to go back to nose high, lands.

[mikegarrison]

8 hours ago, tater said:

 

I would make a joke about "what's he smoking" here, but in the case of Elon we probably already know that.

Aero control does not just come down to "% of cross-sectional area" without regard to location. The control authority depends on the torque that you can apply to the vehicle, and that depends on the lever arm between the source of force and the center of mass. The farther away from the center of mass, the more torque that the same force can generate. This is why airplanes usually have control surfaces out on the wingtips and at the nose and/or tail.

So if he's using control surfaces close to the center of mass, they need to be be bigger. If they are farther from the center of mass, they can be smaller.

You can also get coupling between the different axes, which can also be affected by the location of the control surfaces.

None of this is to say that canard-like things in the front are necessary, per se, but the explanation he's giving doesn't make a lot of sense.

[tater]

I think he's talking about the setup they have, specifically. Meaning they can mess with the area, assuming the adjust both fins. He's saying within bounds of CM and CP He's answering a random tweet with 2 sentences. I imagine they've run simulations, and know what their options are.

[CatastrophicFailure]

Sunglasses required: 

Spoiler

 

[tater]

 

Already has engines!

 

 

[Wjolcz]

It already has engines!? I thought they weren't even on site yet! :0 This might actually be flight-ready when the presentation occurs. I am impressed.

[IncongruousGoat]

Aero covers! Looking at the video, the covers are just more sheets of stainless with some ribbing welded to the convex side, probably for support and to make them easier to weld to the fuselage.

EDIT: On the subject of the header tanks, I had a thought as to why they might be mounted in the nosecone. This prototype isn't going to fly with payload (it's not equipped to), which means that on ascent the center of mass is going to be substantially further back than it would be on an actual launch, which leads me to think that the header tanks were put in the nose (in part) to simulate the mass distribution you'd see when launching with payload. Admittedly, it doesn't work quite so well on descent, but still.

Edited September 23, 2019 by IncongruousGoat

[AVaughan]

2 hours ago, IncongruousGoat said:

On the subject of the header tanks, I had a thought as to why they might be mounted in the nosecone. This prototype isn't going to fly with payload (it's not equipped to), which means that on ascent the center of mass is going to be substantially further back than it would be on an actual launch, which leads me to think that the header tanks were put in the nose (in part) to simulate the mass distribution you'd see when launching with payload. Admittedly, it doesn't work quite so well on descent, but still.

If you assume that Starship will sometimes re-enter and land empty, and at least occasionally re-enter with a significant mass of cargo, then having fuel storage in both locations even on the production version should make it easier to control the centre of mass.  Indeed I think that is part of the reason for the addition of aft cargo storage (that way they can distribute some of the cargo mass aft of the COM, and not have it all forward of the COM).  Being able to store landing fuel either for and/or aft should give them more flexibility to design starship to re-enter, flip and land with varying cargo loads and distributions.

[magnemoe]

56 minutes ago, AVaughan said:

If you assume that Starship will sometimes re-enter and land empty, and at least occasionally re-enter with a significant mass of cargo, then having fuel storage in both locations even on the production version should make it easier to control the centre of mass.  Indeed I think that is part of the reason for the addition of aft cargo storage (that way they can distribute some of the cargo mass aft of the COM, and not have it all forward of the COM).  Being able to store landing fuel either for and/or aft should give them more flexibility to design starship to re-enter, flip and land with varying cargo loads and distributions.

The problem is that you kill 8 meter off your cargo hold unless you go for pancake tanks who negate much of the benefit of header tanks.
If they want to lift them up put them in the upper part of the oxygen tank. Not perfect as its easier to put things in methane than lox. 
This also resolves another thing I thought about. If the atmospheric engines was feed from the header tanks all the time. Now its clear they need valves to switch.  
We don't know that happens to rear storage, the new legs eats up quite a bit of it i think

 

[AVaughan]

26 minutes ago, magnemoe said:

The problem is that you kill 8 meter off your cargo hold

Use a small diameter liquid Oxygen tank right in the nose (as far forward as practical), then a larger diameter liquid methane tank right behind that.  That way you can use  space that is probably unsuitable for cargo, as it is likely to be forward of the cargo hatch.  Assuming Starship's total tank volume stays the same, then you aren't losing much available cargo space, just rearranging it.  (Your aft tanks will be smaller, since the total available tank volume is the same.  Also if they have room for 150 tons of passengers/supplies/life support, then I suspect they will have more than enough space for 150 tons of satellite(s) anyway).    

[Wjolcz]

What's the point of header tanks anyway? Are they like spare fuel just in case other tanks fail for some reason/run dry?

Edited September 23, 2019 by Wjolcz

[Rakaydos]

14 minutes ago, Wjolcz said:

What's the point of header tanks anyway? Are they like spare fuel just in case other tanks fail for some reason/run dry?

Specifically, they are SMALLER tanks that don't slosh because they are full, and have less surface area exposed for evaporation over the course of a flight. 

Also, being in the nose, the are useful for trimming reentry.

Edited September 23, 2019 by Rakaydos