totm nov 2023 SpaceX Discussion Thread

[Barzon]

And it would be incredibly cheap!

Say you want another ISS sized station... Just launch an old Starship and BAM!

[CatastrophicFailure]

1 hour ago, ThatGuyWithALongUsername said:

Y'know, now that you mention it, I'm surprised nobody's done the math on this- taking the albedo of stainless steel and calculating exactly how bright this could be, in ideal conditions (also, how far from Earth would it still be visible?). I'd do it myself if I knew how to and had enough time.

*pokes @cubinator resident math whiz*

[cubinator]

@CatastrophicFailure Heh, I'm glad you think so highly of me. I actually did dig around in some old papers but couldn't find a really solid reflectivity percentage for stainless steel, and I had some other stuff to do so I dropped it for a bit. Since you insist, I'll look around some more and try to math-whiz up some magnitude numbers for various distances for you.

I think it's around 90%, maybe a little over like 93% so I'll probably go with that.

Though this site says 49%: https://www.azahner.com/materials/stainless-steel

Edited March 24, 2019 by cubinator

[Barzon]

This info will be useful for @damonvv in the future.

[mikegarrison]

13 minutes ago, cubinator said:

@CatastrophicFailure Heh, I'm glad you think so highly of me. I actually did dig around in some old papers but couldn't find a really solid reflectivity percentage for stainless steel, and I had some other stuff to do so I dropped it for a bit. Since you insist, I'll look around some more and try to math-whiz up some magnitude numbers for various distances for you.

I think it's around 90%, maybe a little over like 93% so I'll probably go with that.

Though this site says 49%: https://www.azahner.com/materials/stainless-steel

Stainless steel is sort of dull gray unless it's all polished up. If it's polished, then it's shiny gray.

 

Edited March 24, 2019 by mikegarrison

[ThatGuyWithALongUsername]

14 minutes ago, mikegarrison said:

Stainless steel is sort of dull gray unless it's all polished up. If it's polished, then it's shiny gray.

 

Elon kinda gave the impression that it would in fact be polished, though.

[CatastrophicFailure]

14 minutes ago, ThatGuyWithALongUsername said:

Elon kinda gave the impression that it would in fact be polished, though.

Yup. That's where a good chunk of the heat rejection will come from, IIRC.

[cubinator]

Assuming a reflectivity of 49% in the visible light range for the stainless steel Starship hull, and a 55 m long orbiter, I'm going to wave my hands and say the part of the ship that's reflecting the sun directly towards you is nominally a strip about 0.5 m wide along the whole length of the ship. That's an area of about 27.5 m, we'll round down to 27 to be conservative.

So, what we want is the area that takes up in the sky. At a distance of 400 km (the height of the ISS) our Starship spans 0.004 degrees. 

Spoiler

As far as an area in the sky, since we said the sun-direct part of that reflection was 0.5  m wide, we'll cut that value in half for a degree-area of 0.0019 deg^2.

The sun's apparent magnitude is -26.74, and its area is pi * 0.5^2 deg^2 = .785 deg^2. We're dealing with small enough areas that I'm going to pretend the sky is flat for this.

So, we're dealing with an area of 0.00246 of the sun's area, reflecting 49% of its light. That means that .49 * .00246 = .0012 of the Sun's brightness is coming off the ship.

I will use the apparent magnitude equation m₂-m₁ = -2.50 log(B₂/B₁)

m2 = magnitude of the sun = -26.74

m1 = magnitude of Starship = ?

B2 = brightness of the Sun = 1

B1 = brightness of Starship relative to the sun = .0012

-26.74 - m1 = -2.5*log(828.8909)

-26.74 - m1 = -7.296

m1 = -19.44?

That seems very high. Although this was something of a best-case scenario like an Iridium flare but much bigger. If it does get this bright, it would likely only be for a fraction of a second.

I'll try for an area of just a spot instead of a whole line. 0.5 m by 0.5 m, we'll say. Using the methods above, we get an area of 0.0000358 deg^2, which is .0000456 of the Sun's area. In this case we get m1 = -15.89. Still very high. 

I doubt this is accurate.

[CatastrophicFailure]

52 minutes ago, cubinator said:

[math stuff]

Hmm. Could you maybe use the ISS as a baseline to check the numbers? IIRC that tops out around -4 in perfect conditions.

 

[magnemoe]

22 hours ago, Barzon Kerman said:

It could SSTO, then be used as a wetlab. and it could be done at the end of a SS's life?

Easier to just launch an couple of modules with an cargo versions who would be much easier. Rater use them as permanent bases / fuel depots on moon or mars. 
Or simply use them as tankers until they break. 
 

[CatastrophicFailure]

25 minutes ago, magnemoe said:

Or simply use them as tankers until they break. 

See, this is the thing about that. If a Starship is at end-of-life, that means it's no longer structurally sound. Engines, mechanicals, life support, that can all be replaced. Just like a ship or aircraft, it's being retired because the hull is no longer worth using. Not exactly a good foundation for a space station. Just sayin.

 

[Scotius]

Question is: would polished stainless steel lose its shine in space? Micrometeorite crapes, surface outgassing, sun rays erosion and other environmental factors should affect the albedo over time. But how long would it take to notice a difference?

[tater]

3 minutes ago, CatastrophicFailure said:

See, this is the thing about that. If a Starship is at end-of-life, that means it's no longer structurally sound. Engines, mechanicals, life support, that can all be replaced. Just like a ship or aircraft, it's being retired because the hull is no longer worth using. Not exactly a good foundation for a space station. Just sayin.

 

The most stress is EDL, however, so once in space, it might well have a longer life there than after its last EDL.

Edited March 24, 2019 by tater

[Piscator]

Would the polished-ness even matter at all?  Instead of a bright - but narrow - band of light, we would see a less brilliant reflection spread out over a greater area. The total albedo should be pretty much the same, I'd assume.

[Spaceception]

Just imagine looking up and seeing a whole fleet of these just before leaving LEO.

 

[Ultimate Steve]

Just now, Spaceception said:

Just imagine looking up and seeing a whole fleet of these just before leaving LEO.

 

You would not believe your eyes

If 10 million fireflies spaceships

Lit up the world sky

as I fell asleep

[mikegarrison]

8 minutes ago, Piscator said:

Would the polished-ness even matter at all?  Instead of a bright - but narrow - band of light, we would see a less brilliant reflection spread out over a greater area. The total albedo should be pretty much the same, I'd assume.

I think a mirror polish will reflect back more incoming light. But I suppose it's possible that it reflects back the same light, only less scattered. I really don't know.

[Cunjo Carl]

1 hour ago, cubinator said:

Assuming a reflectivity of 49% in the visible light range for the stainless steel Starship hull, and a 55 m long orbiter, I'm going to wave my hands and say the part of the ship that's reflecting the sun directly towards you is nominally a strip about 0.5 m wide along the whole length of the ship. That's an area of about 27.5 m, we'll round down to 27 to be conservative.

 

I suspect the Starship will orbit with it's engine side facing the sun though, like the BFR with its axolotl solar panels, right?
 

BFR Concept art from SpaceX

Axolotl picture from ThreeLeaves on Youtube

[mikegarrison]

29 minutes ago, CatastrophicFailure said:

See, this is the thing about that. If a Starship is at end-of-life, that means it's no longer structurally sound. Engines, mechanicals, life support, that can all be replaced. Just like a ship or aircraft, it's being retired because the hull is no longer worth using. Not exactly a good foundation for a space station. Just sayin.

You alluded to this, but I'll say it plainly. Most ships, airplanes, cars, etc. get retired not because they can't be fixed or maintained, but because it's more expensive to fix or maintain them than to just replace them.

[ThatGuyWithALongUsername]

21 minutes ago, Cunjo Carl said:

I suspect the Starship will orbit with it's engine side facing the sun though, like the BFR with its axolotl solar panels, right?

Well, for crewed ships at least, passengers will probably want a nice view of the planet. Maybe cargo ships would be oriented like this but not crewed ones? (Until preparing for reentry, of course)

Edited March 24, 2019 by ThatGuyWithALongUsername

[cubinator]

34 minutes ago, Cunjo Carl said:

I suspect the Starship will orbit with it's engine side facing the sun though, like the BFR with its axolotl solar panels, right?

During a months-long interplanetary coast, probably, but in orbit around Earth it will likely go through many different sun-relative orientations. I picked one that would probably be one of the brightest.

[magnemoe]

33 minutes ago, Cunjo Carl said:

 

I suspect the Starship will orbit with it's engine side facing the sun though, like the BFR with its axolotl solar panels, right?
 

BFR Concept art from SpaceX

Axolotl picture from ThreeLeaves on Youtube

Wonder if they changes the solar panel to fit in the top fin?
Also think they change the door to something more like the one on the shuttle, optionally hinged on one side. Reason is that its easier to load new cargo and to offload cargo on moon and mars. 
The above design is only better for stealing stuff in orbit Moonraker style  

[Bill Phil]

1 hour ago, Scotius said:

Question is: would polished stainless steel lose its shine in space? Micrometeorite crapes, surface outgassing, sun rays erosion and other environmental factors should affect the albedo over time. But how long would it take to notice a difference?

Yeah. Continuous micrometeoroid impacts are kind of like a coarse sand blasting in most cases. 

[Cunjo Carl]

Edit: Made a minor oops, fixed in the bottom paragraph.

 

Because it was interesting, I took a little bit and thought through the math of the maxmimum magnitude of light reflected by starship in low orbit (like with the ISS). I thought it'd need an integral, but luckily it looks like not!

I reasoned that any shiny surface can be thought of as n-many flat mirrors. A distant flat mirror will either shine with the full intensity of the sun or not at all (we used tiny steel mirrors in search and rescue for signalling at ridiculous distances, it's an interesting property). Given this, the lux from starship will be the lux from the sun times the ratio of apparent solid angles.

The maximum brightness should occur (roughly) at sunset when the Starship is directly overhead with its nose pointing towards the sun and 45 degrees up from the horizon, and with the shiny dorsal side rotated to face down towards us. The light reflected will be from the horizontal axis of the sun right across the middle. Assuming this position, the brightness should cover a much larger area of Earth than say the iridium panels.

The part of starship that reflects to us will be the cylinder's entire length (guessing 40m at 45 degrees to us), and around the radius (4.5m) the same angle as the sun takes up in the sky (0.53 deg), making a strip about 4cm across. Apparent Reflective Area = 40m * sin(45deg) * 2 * 4.5m * sin(0.53deg / 2) . The distance will be the altitude of ISS, say, 409km. Let's call its reflectivity at 0.93 like @cubinator suggests. The greyish steel is carbon steel turned grey from FeO, Stainless is shiny!

The sun shines at 98000 lux and is 696Mm radius and 150Gm distance. It shines from the apparent area of a disc at pi*r^2

Starship Lux = 98000 * 0.93 * [ 40 * sin(45deg) * 2 * 4.5 * sin(0.53deg / 2) / 409E3^2 ] / (3.14*696E6^2 / 150E9^2)  = 0.0095 lx (at max)

Finally converting this is about a -8.8 magnitude, so the same as an iridium flare! (about -8 to -9) . If starship is pointing its engines at the sun though, it won't look nearly as impressive. Let's hope the SpaceX execs are the sort of people who like to put on a show on occasion

* Your mileage may vary, all this was calculated on a napkin. Also, I used an approximate formula for solid angle, but it's good enough for this case.

Edit: Eeek, Sorry all, I missed a factor of 2 on the angle of the Starship cylinder that would be reflecting to the viewer. It should be 2 * 4.5 * sin(0.53deg / 4) making a 2cm strip. Updating, that puts the brightness at .0048 lx, and -8.1 magnitude. Still bright enough to be a show, but not quite as impressive!

 

Edited March 25, 2019 by Cunjo Carl
Fixing an oops

[Rakaydos]

7 minutes ago, Cunjo Carl said:

Because it was interesting, I took a little bit and thought through the math of the maxmimum magnitude of light reflected by starship in low orbit (like with the ISS). I thought it'd need an integral, but luckily it looks like not!

I reasoned that any shiny surface can be thought of as n-many flat mirrors. A distant flat mirror will either shine with the full intensity of the sun or not at all (we would use tiny steel mirrors in search and rescue for signalling at ridiculous distances, it's an interesting property). Given this, the lux from starship will be the lux from the sun times the ratio of apparent solid angles.

The maximum brightness should occur at sunset when the Starship is directly overhead with its nose pointing towards the sun and 45 degrees up from the horizon. The light reflected will be from the horizontal axis of the sun right across the middle. Assuming this position, the brightness should cover a much larger area of Earth than say the iridium panels.

The part of starship that reflects to us will be the entire length (at 45 degrees to us), and around the radius the same angle as the sun takes up in the sky (a strip about 4cm across). Apparent Reflective Area = 40m * sin(45deg) * 9m * sin(0.53deg / 2) . The distance will be the altitude of ISS, say, 409km. Let's call its reflectivity at 0.93 like @cubinator suggests. The greyish steel is carbon steel turned grey from FeO, Stainless is shiny!

The sun shines at 98000 lux and is 696Mm radius and 150Gm distance.

Starship Lux = 98000 * 0.93 * [ 40 * sin(45deg) * 9 * sin(0.53deg / 2) / 409E3^2 ] / (3.14*696E6^2 / 150E9^2)  = 0.0095 lx (at max)

Finally converting this is about a -8.8 magnitude, so the same as an iridium flare! (about -8 to -9) . If starship is pointing its engines at the sun though, it won't look nearly as impressive. Let's hope the SpaceX execs are the sort of people who like to put on a show on occasion

* Your mileage may vary, all this was calculated on a napkin.

 

Interesting! With how bright the best case is, I wonder if reflection just off the curve of the nose would be visible at night. Also, how much dimmer would a moonlight reflection be?