totm nov 2023 SpaceX Discussion Thread

[Xd the great]

31 minutes ago, Rakaydos said:

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. 

Useful for Mars flights.

[magnemoe]

1 hour ago, AVaughan said:

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).    

Makes some sense, however this makes reentry very sensitive to the fuel weight in the nose tanks. And no I don't think you can store some fuel in the main tanks for reentry. You also need to insulate the tanks not only for long stays in space but also from the reentry heat. 

[Rakaydos]

1 minute ago, magnemoe said:

Makes some sense, however this makes reentry very sensitive to the fuel weight in the nose tanks. And no I don't think you can store some fuel in the main tanks for reentry. You also need to insulate the tanks not only for long stays in space but also from the reentry heat. 

The payload volume already needs insulation from reentry heat. The "Hot structure" main tanks don't. So you actually need less insulation in the payload area than in the empty tanks.

As for fuel weight in the nose tanks, they are always full until you land.

[sevenperforce]

12 hours 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.

Well, "storage" concerns are sort of the trick here. If you're going to be using a bunch of energy to capture CO₂ and sequester it, fine, but you need a sequestration method that won't outgass or do any other stuff like that. A little more energy and you can crack the CO₂ into fuel, which you can sell, but selling the fuel means you're no longer capturing the CO₂. Carbon-neutral but not carbon-negative.

The difference for SpaceX is that a significant amount of the CO₂ they'd crack into methane would be potentially leaving the atmosphere altogether. Granted, some will fall back to Earth, but they can prove trivially that a great deal will not, and so for that portion that escapes, they are carbon-negative. So they can essentially have their carbon-cake and burn it too.

15 hours ago, CatastrophicFailure said:

My eyes may be deceiving me, but it looks like there could be a mounting point for forward canards on the left edge of that sucker.

[sevenperforce]

9 hours ago, tater said:

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.

Agreed. The outer mold line (OML) has a particular "default" center of pressure (CoP) and a baseline center of mass (CoM). You can adjust the CoM to a degree by putting in header tanks and other internals at various points. Your fins will have complete pitch authority as long as they are positioned opposite on the other side of the CoM from the default CoP. Fold them back, and the combined CoP moves above the CoM, causing pitch-up torque; fold them out, and the combined CoP moves below the CoM, causing pitch-down torque. Feather them halfway and you get a CoP aligned with your CoM and thus a stable entry.

So as long as the fins are large enough, you can manage. 

One reason to have four fins (fore and aft) rather than just two is that it will damp dihedral torques for roll control. If you are stable in pitch because your CoP is resting on your CoM, then feathering back one of the two aft fins to correct roll will lift the CoP above your CoM and induce pitch-up. You can correct by feathering the opposite fin forward, but then you have a roll moment that is very far from your CoM and will thus tend to induce yaw. Having the canard-fins will allow roll authority that keeps everything located on the CoM.

3 hours 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 likelihood of moving from the "chomper" payload model to a Shuttle-style cargo bay seems high, at least for the cargo variant.

1 hour ago, AVaughan said:

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).    

When I was trying to do builds to test the last version of Starship entry I kept having to put header tanks in the nose to trim the CoM, too.

[mikegarrison]

2 hours ago, sevenperforce said:

The difference for SpaceX is that a significant amount of the CO₂ they'd crack into methane would be potentially leaving the atmosphere altogether. Granted, some will fall back to Earth, but they can prove trivially that a great deal will not, and so for that portion that escapes, they are carbon-negative. So they can essentially have their carbon-cake and burn it too.

This sounds incorrect to me. At least for LEO. I suppose solar pressure will push some gas molecules away from the Earth, but most should fall back into the atmosphere, I think.

Are you aware of any studies on this?

[tater]

31 minutes ago, mikegarrison said:

This sounds incorrect to me. At least for LEO. I suppose solar pressure will push some gas molecules away from the Earth, but most should fall back into the atmosphere, I think.

Are you aware of any studies on this?

It would depend on the RMS velocity of the gas in LEO. Any with escape velocity have a decent chance of escape.

Let's see... the exhaust velocity of raptor is ~3.7km/s, and burns are generally tangential---but the wrong way---so that would in fact make them no longer have orbital velocity. Yeah, it seems like the exhaust products in LEO have a pretty strong chance of heading home.

 

 

 

Max Q abort mid November.

"The application includes a sub-orbital first stage, and a simulated orbital second stage."

https://apps.fcc.gov/oetcf/els/reports/STA_Print.cfm?mode=current&application_seq=95229&RequestTimeout=1000

Edited September 23, 2019 by tater

[CatastrophicFailure]

9 hours ago, IncongruousGoat said:

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.

Ah, but one point is getting missed here:

Starship will always launch on Superheavy, negating any COM/COL issues on ascent (by the time it stages, atmo is no longer a concern). The test mules, however, have to launch without it, so have an unusual need for more mass in the nose for aerodynamic stability. 

So putting the header tanks where they are is only necessary for the test program, and as a bonus makes access easier. 

[Terwin]

37 minutes ago, mikegarrison said:

This sounds incorrect to me. At least for LEO. I suppose solar pressure will push some gas molecules away from the Earth, but most should fall back into the atmosphere, I think.

Are you aware of any studies on this?

Highest Raptor ISP I have seen: 380s

calculated exhaust velocity = 380s * 9.8m/s^2 = 3,724 m/s  

Earth escape velocity: 11.2 km/s

Low Earth Orbit: 7.8km/s

So unless I have the wrong formula for converting ISP to exhaust velocity, only the first ~300 m/s of the deorbit burn will have exhaust that is going fast enough to escape.

[kerbiloid]

6 minutes ago, Terwin said:

only the first ~300 m/s of the deorbit burn

Of total 250?

[Terwin]

1 minute ago, kerbiloid said:

Of total 250?

The deorbit burn is the only one I know of where you are thrusting against your orbital velocity, so with an exhaust velocity of 3700m/s, you can have up to 300m/s of exhaust with the velocity to potentially escape earth if your deorbit burn is that long.

A shorter deorbit burn will of course mean less carbon-dioxide that leaves the gravitational influence of earth.

Also, the deorbit burn is usually after almost all mass has left the vessel(either burned fuel or released satellites), so this would probably represent a relatively trivial amount of co2.

 

[CatastrophicFailure]

ItsHappening.gif

 

[sevenperforce]

33 minutes ago, tater said:

58 minutes ago, mikegarrison said:

This sounds incorrect to me. At least for LEO. I suppose solar pressure will push some gas molecules away from the Earth, but most should fall back into the atmosphere, I think.

Are you aware of any studies on this?

It would depend on the RMS velocity of the gas in LEO. Any with escape velocity have a decent chance of escape.

Let's see... the exhaust velocity of raptor is ~3.7km/s, and burns are generally tangential---but the wrong way---so that would in fact make them no longer have orbital velocity. Yeah, it seems like the exhaust products in LEO have a pretty strong chance of heading home.

 

13 minutes ago, Terwin said:

Earth escape velocity: 11.2 km/s

Low Earth Orbit: 7.8km/s

So unless I have the wrong formula for converting ISP to exhaust velocity, only the first ~300 m/s of the deorbit burn will have exhaust that is going fast enough to escape.

You don't need to hit earth escape velocity; just getting into or near orbit should be enough. I suspect, though I am not sure, that solar wind will be sufficient to disperse any CO₂ molecules which reach a sufficient altitude. Whether that altitude is 80 km, 100 km, or 500 km is not known, but I'm sure they'll do the math if they need to.

And of course any propellant used BLEO will count as negative carbon.

27 minutes ago, CatastrophicFailure said:

Starship will always launch on Superheavy, negating any COM/COL issues on ascent (by the time it stages, atmo is no longer a concern). The test mules, however, have to launch without it, so have an unusual need for more mass in the nose for aerodynamic stability. 

So putting the header tanks where they are is only necessary for the test program, and as a bonus makes access easier. 

Single-stage P2P seems very possible. And they are primarily testing skydiver mode here, after all, where CoM/CoP is everything.

 

[tater]

33 minutes ago, Terwin said:

Highest Raptor ISP I have seen: 380s

calculated exhaust velocity = 380s * 9.8m/s^2 = 3,724 m/s  

Earth escape velocity: 11.2 km/s

Low Earth Orbit: 7.8km/s

So unless I have the wrong formula for converting ISP to exhaust velocity, only the first ~300 m/s of the deorbit burn will have exhaust that is going fast enough to escape.

Yeah, but the leaving orbit burn (fully tanked SS, I assume, heading out from Earth) has the spacecraft going to escape, and the exhaust going the other way 7.8km/s - 3.7km/s. So the exhaust is in fact deorbited.

 

13 minutes ago, sevenperforce said:

You don't need to hit earth escape velocity; just getting into or near orbit should be enough. I suspect, though I am not sure, that solar wind will be sufficient to disperse any CO₂ molecules which reach a sufficient altitude. Whether that altitude is 80 km, 100 km, or 500 km is not known, but I'm sure they'll do the math if they need to.

And of course any propellant used BLEO will count as negative carbon.

Maybe legally, but it seems like a fully tanked up SS doing a TLI burn (for example) will in fact be deorbiting the exhaust products (the component that loses orbital velocity due to the burn, anyway). Give the fact that an orbit is half day, half night, solar influences should be about 50/50 each way, and the solar wind is more complicated due to magnetic field lines. The bolus of exhaust products from a TLI/TMI burn will be in a short time frame, though, and will have roughly half orbital velocity stripped.

Edited September 23, 2019 by tater

[sh1pman]

4 minutes ago, tater said:

deorbit burn (fully tanked SS, I assume, heading out from Earth)

Deorbit burn is when empty SS is heading back to Earth from LEO. Then its exhaust will escape the Earth’s gravity well.

[sevenperforce]

There has been a lot of back and forth about what the lower control surfaces ought to be called. Wings? Flaps? Flaperons? Feathers? 

But accuracy is key. So why not call them what they really are? Plasma Deflector Shields.

[Barzon]

Y E S

[tater]

5 minutes ago, sh1pman said:

Deorbit burn is when empty SS is heading back to Earth from LEO. Then its exhaust will escape the Earth’s gravity well.

Yeah, this increases the RMS velocity.

2 minutes ago, sevenperforce said:

There has been a lot of back and forth about what the lower control surfaces ought to be called. Wings? Flaps? Flaperons? Feathers? 

But accuracy is key. So why not call them what they really are? Plasma Deflector Shields.

Like this video posted at NSF?

 

[magnemoe]

40 minutes ago, CatastrophicFailure said:

Ah, but one point is getting missed here:

Starship will always launch on Superheavy, negating any COM/COL issues on ascent (by the time it stages, atmo is no longer a concern). The test mules, however, have to launch without it, so have an unusual need for more mass in the nose for aerodynamic stability. 

So putting the header tanks where they are is only necessary for the test program, and as a bonus makes access easier. 

Excellent point, note that it can also only launch half full of fuel because it can not use the vacuum engines here and will not get them until later. 
This will again change COM backward. 
Now landing you can compensate for COM with aerodynamic, during accent, well they should have kept their 3 symmetrical fins for that. 

[mikegarrison]

Anyway, I think the idea that any significant portion of the rocket exhaust completely leaves the atmosphere forever is incorrect. Even on a deep space mission, much of the total energy of the mission is spent inside the atmosphere.

[sevenperforce]

They are currently doing actuation tests on one of the fins.

[tater]

6 minutes ago, sevenperforce said:

They are currently doing actuation tests on one of the fins.

Or is it just really windy?

[sevenperforce]

27 minutes ago, tater said:

Or is it just really windy?

I'd wager that if something is moving then it is intended to be moving but I could be wrong.

[tater]

2 minutes ago, sevenperforce said:

I'd wager that if something is moving then it is intended to be moving but I could be wrong.

You'd think it would be locked down otherwise, yeah. The sound is really windy, and you can see indications it's decently windy (the line hanging from the crane).

[sevenperforce]

Higher-res pictures have made it clear that we are looking at the windward side of Starship Mk1.

Also, we now have a very good indication of where the forward flaps will be located: