totm nov 2023 SpaceX Discussion Thread

[tater]

30 minutes ago, insert_name said:

Are those aluminum grid find on the core?

Yeah. The side boosters need to move the CP back (on return the top is rear) because of the nose cones vs interstage and require the larger Ti fins.

The 2 RTLS landings are going to be epic.

Edited December 20, 2017 by tater

[sevenperforce]

6 minutes ago, tater said:

Yeah. The side boosters need to move the CP back (on return the top is rear) because of the nose cones vs interstate and require the larger Ti fins.

The 2 RTLS landings are going to be epic.

Ti fins also give the returning boosters more control authority and a higher glide ratio, meaning they don't have to reserve as much propellant for boostback because they can glide crossrange. The core doesn't need  crossrange capability, so aluminum fins are fine. Of course, the aluminum fins on the core will get torched since it's a high-energy entry, but what else are they good for?

[tater]

Dang autocorrect hasn’t learned “interstage,” yet.

[Guest]

27 minutes ago, tater said:

The 2 RTLS landings are going to be epic.

I'm honestly curious if they added collision avoidance routines to the guidance software and, if so, how much of a problem that's going to be for landing. Alternatively, how they plan to handle the dual landing. I'd stagger them, personally, but Space-X probably has better ideas. Titanium grid fins might allow for greater separation during flyback.

[_Augustus_]

Those FH pics are beautiful.

[DVDRW]

Probably using GPS to avoid collision.

[Guest]

Just now, DVDRW said:

Probably using GPS to avoid collision.

Any chance of that cutting out like broadcasts do?

[tater]

I would imagine they’d loft one a little to separate them in time. A fraction of a second is all that is required.

[CatastrophicFailure]

11 minutes ago, sevenperforce said:

The core doesn't need  crossrange capability, so aluminum fins are fine. Of course, the aluminum fins on the core will get torched since it's a high-energy entry, but what else are they good for?

Probably unlikely this particular core will ever see re-use, anyway. I expect it’ll get an extra thorough teardown so they can go over everything with a fine tooth comb, then maybe Get gate guard duty at Boca or something. 

 

4 minutes ago, tater said:

Dang autocorrect hasn’t learned “interstage,” yet.

It’s apparently started randomly capitalizing words, too. 

[tater]

Horizontal sep is what, 300m? Easy enough to put a km between them in alt.

[magnemoe]

15 minutes ago, regex said:

I'm honestly curious if they added collision avoidance routines to the guidance software and, if so, how much of a problem that's going to be for landing. Alternatively, how they plan to handle the dual landing. I'd stagger them, personally, but Space-X probably has better ideas. Titanium grid fins might allow for greater separation during flyback.

Simplest is to have one have an slightly longer boostback burn, might angle for an higher Ap,  Having some sort of avoidance system would be too complex and having one move a bit after than the other would work.
Now after separation they would be some hundred meters apart and the two pads are also seperated by 200 meter I think but that would add an risk. 

[KSK]

5 hours ago, sevenperforce said:

Someone, ahem, me.

Poring over the image of the octaweb mating at the bottom. Looks like clasps at the nearest attachment points, plus two pneumatic pushers on each side. Also looks like those pneumatic pushers will rotate up after the separation.

This is really quite complicated.

Yep - I think if anything really hammers home the complexity and difficulty of real-world rocketry it's those pushers and the struts at the top. They don't look like very much at all considering that they'll need to be keeping 5.5 million pounds of thrust flying in sufficiently close formation and then separating it all nice and cleanly.

[tater]

Yeah, vertical separation is trivial as it’s just timing. 

[Nightfury]

34 minutes ago, regex said:

Any chance of that cutting out like broadcasts do?

They maybe could use some distance tracking system in the two stages themselves

If they are flying only ~ 300m - 1km next to each other the system might work (I dont have much knowledge about how signals are behaving in reentey)

[PB666]

23 hours ago, IncongruousGoat said:

Boca Chica question- launching from BC, it looks like the only available launch corridor is directly east, over the Gulf of Mexico. Which begs the question-what the heck is SpaceX expecting to launch from that site? They can't reach the ISS, they can't reach polar or sun-synchronous orbit. They can reach GTO, but that's only a part of the launch market and not worth the investment of building a whole private launch complex.

This is a statement whose foundational logic is in error. The ISS is at an inclination of 51.6 degrees and Cape Canaveral is at 28.6N Boca chica is at ~26'N. During every orbit the ISS crosses 28.6'N and 25'N once as it travels north and once as it travels south. At a semi-major axis of 404km in altitude (a = 6776000 m, v = 7669.76 m/s, P = 5550 sec, f = 31/day) the ISS, on average, crosses either launch site and any launch site between 56.0'N and 56.0'S passes within 5.8  degrees along the east west (or north as one approaches 51.6 degrees north). The reality however is the 51.6 - 28.6 =  23' or 51.6 - 26' = 25.6 means that the distance in absolute degrees is ~ 2.26' at CC and ~2.59' at BC. Given the earth is 40,000 km in diameter this is a distance from ISS course (DfIC) of 251 km for CC and 288 km for BC. However if one looked at windows every 2nd day or 4th day the DfIC would be half or a quarter. But lets just use the daily value. Lets say that 251 km puts the two vessels at 0.351' and 0.412' orbital inclination to each other. To correct this in orbit (the worst case scenarios) one would need a burn dV of 46.9 for CC 55.72 m/s for BC, a difference of only  8.73 dV higher for Boca Chica.

However, the western launch from Boca chica has a higher contribution from Earth's rotation. 40,000,000/86400 = 462.6 meters per second on any east west departure at the equator. For CC its 406 m/s and for BC its 415 m/s, since both intercept trajectories will be traveling ENE or ESE to intercept there are cosine losses 373.3 for CC and 374.3 (and yes I am aware of the east launch restriction, but also don't forget due east is only restricted to 320 km downwind). Consequently we can calculate the dV required to reach orbit for any average day if we stipulate that drag and gravity losses equate to 1200 dV. Boca chica launch site is about 8 miles from the Mexican border so the maximum variation is  2.92' South which means at 200 miles it would only need to turn 23' South.

SQRT(2 * (7669.7^2/2 + u/6371000 - u/6776000) ) + 1500 = 8142.3 + 1500 = 9642.3 dv. For CC it would be that + 46.9 - 373.4 =  9315.4 m/s for BC it would be 9323.72  a difference of only  8.32 dV. Remembering that we have to make an orbital inclination burn anyway of 0.412' orbital inclination burn, but we also use more momentum from the Earth. One could after traveling 183 miles eastbound heading south at 25.6 degrees, the horizontal orbital velocity at that point would be 3000 m/s so that the turn would cost 1296 m/s however the cost could be lowered if and agreement could be reached with Mexico allowing it to change course over the commercial zone. However in choosing an east to west launch one gets back 30 dV of the cost. The other alternative is to simply catch the ISS on the rise, since the first stage would be captured in the gulf of Mexico, the only down range risk is the failure of the second stage (which it will either fail at firing or quickly be out of risk for the continental US.). Space Law applies to any vessel above 100 km; however, and this has major importance on such a course change. Because the ISS is so high now, 404 km, one can choose a vertical course steeper, and essentially fly over Mexico's internationally recognized airspace of 100km with a surface velocity of 600 m/s (and 415 = 1015) Since. this will occur pretty close offshore, you could launch to the east get about 300 or so horizontal velocity and turn to 120' the cost of the 30' change would be a loss of 200 dV and (increased because of the loss of some of earths rotation that contributed with east to west take-off). In the grand scheme of things that would only be an increase in Boca-chica versus Canaveral of  2.2%.

Ok that is cleared up. The same is also true for GSO orbits, since they are so high, there is no general benefit to try to cling to the Earth, However, Boca Chica is better launch point for Equitorial GSO than any launch site in the continental US. However because Space X has a barge technology they can simply barge a rocket from the Port of Brownsville, haul it to a launch site and launch it, this could be done from any port that has access to the rocket assembly technology.

Finally, the Sun, the sun (tropic of cancer) approaches Boca-chica from the south, Once June 21st this orbit 3 33'86'' from the Boca Chica site at Midday, it can be also reached at Midnight on December the 21st. Both can be accessed from a due east flight and this allows for launches to other targets outside the solar system. During spring it is as great as 25.8 degrees to the south (or north if launched at midnight). Boca chica is a better launch site for orbits that travel out of the planetary system, it captures more of the Earths momentum (9 dV more and needs to make a smaller dV change to achieve a decent orbit for escape from our system, however because of the dynamics of the Earth moon system this needs to be precisely calculated.

Boca chica is not suitable for polar orbits, those are best launched from Siberia or Alaska.

Biggest problem with Boca Chica. The launch site they are building on is the flood plain of the Rio Grande river, its not particularly stable soil (I know this from very unpleasant personal experience). They will have to spend alot of time stabilizing the soil because of the large amount of sulfides and other unstable organic materials that have made the sand spongey. It is hard to imagine the amount of sediment at the site, but just consider the length of the rio grande and the terrain it has cut through.

 

 

[tater]

They’re starting from known, separated positions, and self-landing to the separated pads. They don’t need to know what the other booster is doing.

The KISS solution would be to have them programmed with slightly variant boost back burns such that they enter a fraction of a second apart, which at their speeds would translate into a very safe distance.

[Guest]

2 minutes ago, tater said:

The KISS solution would be to have them programmed with slightly variant boost back burns such that they enter a fraction of a second apart, which at their speeds would translate into a very safe distance.

Which was already postulated, and is made even easier due to the stated benefits of titanium grid fins.

That's likely exactly how it's going to go down.

Edited December 20, 2017 by regex

[sevenperforce]

I think the initial separation kick will be during the boostback burn; from there on, they'll be gliding back toward each other.

[IncongruousGoat]

2 minutes ago, PB666 said:

<snip>

I think you missed the point of my comment. I know it's physically possible to launch to the ISS and to polar and sun-synchronous orbits from Boca Chica. The problem is the launch corridor (i.e. the possible debris zone) is in a problematic location for a lot of potential launch azimuths. Remember, even SpaceX's rockets are not all that reliable, and neither the U.S. nor Mexico want dangerous rockets flying over their territory and important seaways. It's not an issue of physics and engineering so much as an issue of politics and regulations.

[PB666]

4 hours ago, IncongruousGoat said:

I think you missed the point of my comment. I know it's physically possible to launch to the ISS and to polar and sun-synchronous orbits from Boca Chica. The problem is the launch corridor (i.e. the possible debris zone) is in a problematic location for a lot of potential launch azimuths. Remember, even SpaceX's rockets are not all that reliable, and neither the U.S. nor Mexico want dangerous rockets flying over their territory and important seaways. It's not an issue of physics and engineering so much as an issue of politics and regulations.

Are we talking SpaceX rockets that have been launched in the last year or the last 5 years. In the last year launches have been extremely reliable. In fact their overall debris status is better than the space shuttle over the last year. Most of the first stages have been returned to a secured location. For the space shuttle, not counting in flight disasters, but two booster cores coming back to earth. In the last year has space X dumped a single space craft part into the water under the launch corridor.? Second thing is, Boca Chica is relatively stark terrain there is not alot of ship traffic crossing the border. Most of the ship traffic I have seen are boats weighting north of the N.  Port Isabel Jetty. There is a Sea Way there but its about 5 miles north of the launch site

Finally, when they start launching, i'm gonna take my 4x4 down there and camp on the beach. The more the merrier.

[tater]

 

[CatastrophicFailure]

Wow. 400 engines. That’s a lot of data points. And even more when a good chunk (most?) of those engines get examined after a flight. So with the above discussions in mind, it begs the question: how reliable is “reliable enough” for a rocket, in terms of overflying areas with X population? Airplanes are absolutely far more reliable, but there’s a lot more of them flitting about, too. Which is more risky in the long term, one rocket a day flying overhead or a couple dozen airliners?

If SpaceX’s P2P BFR is ever going to come about, that’s a question that will need to be addressed. 

[RedKraken]

Block 5 Merlin 1D

 

[GoSlash27]

I'm curious as to why they chose the exact same 2nd stage engine for the Heavy as they use in the 9. An engine that circs 25 tonnes is also capable of 70 tonnes? If that's the case, isn't that engine kind of overkill for the Falcon 9?

Best,
-Slashy

[Lukaszenko]

11 minutes ago, GoSlash27 said:

I'm curious as to why they chose the exact same 2nd stage engine for the Heavy as they use in the 9. An engine that circs 25 tonnes is also capable of 70 tonnes? If that's the case, isn't that engine kind of overkill for the Falcon 9?

Best,
-Slashy

So that they don't have to develop/ tool/ build/ support a completely different engine for what is used in 10% (or, in this case, 3%) of the rocket.

Commonality of parts is a big theme at Space X, because it creates huge cost and headache savings. It's at the cost of a bit of payload, but apparently that is something they can spare.