totm nov 2023 SpaceX Discussion Thread

[YNM]

19 hours ago, tater said:

From a purely business POV, Starship dev started partially because of Mars, which is NOT a business POV (no plausible business case for Mars)

I think that nearly all human interplanetary mission (including the Moon and the asteroids here as well) isn't that economical if we're excluding surface extraction. If surface extraction are possible (or at least pursued), then this means a self-sustaining outpost where time is  the only extra resource needed to keep beyond what was initially sunk in really. I know that's only break-even - only in the best scenario that (given that we mostly won't be able to get all the resources like we do on Earth) - but given the large effort needed that's a business case of it's own if there's enough interest.

While F9 and FH, along with Dragon, was merely a "solving business case" situation and is a very good stepping stone, SS and SH, if it works beyond what HLS wants, is a business scope expansion endeavor. Benefits of it's gigantic nature in other operating areas (LEO-MEO-HEO, Moon) is only second to what they're pursuing.

I guess the next question after HLS and Artemis (with the support of SS and SH) is whether we're greenlighting surface extraction on the Moon or other bodies, and how we will achieve that. Without it we're just getting ISS but at further distances away.

[kerbiloid]

Spoiler

 

 

[RCgothic]

1 hour ago, tater said:

Cislunar tugs have been talked about for many decades. SpaceX might actually be able to do them, IMO. I know they have not talked about it, but it makes a lot of sense.

Imagine SS with the curved nose gone some number of rings above the dome. The fairing is attached using the same hardware used to to dock 2 SS together at the tail—resulting in a tug that can dock nose to tail with another SS.

This allows a 2-stage SS in orbit. I'm unsure about the ability of such a tug to aerobrake, but it seems like it should potentially be possible, otherwise it has to re-circularize in LEO after putting the other SS into an eccentric HEO (shy of TLI) propulsively—luckily for an empty vehicle this is surprisingly cheap in props.

Such a vehicle could give a full SS with 100t cargo 2300 m/s and leave props for 100% propulsive recircularization. Now we have a SS headed for the Moon fully tanked, and it needs to provide only 900m/s for TLI (305t props) itself. That gives us a lunar SS with 895t of props remaining otw to the Moon (I'm using 100t for the dry mass, so if SS is less, then more props or more cargo).

Assume its a normal SS that can aerobrake or aerocaptuure. If the cargo was propellants, and this was a ballistic trajectory to NRHO, we can in fact deliver 995t to NRHO. Some is needed for the 450 ms burn—less than 15t from NRHO. So we have a tanker at the Moon with980t of propellants. If LSS is 85t, then that's ~9.4km/s dv. ~5.5km/s is needed for the RT to the surface from there. Returning LSS has residuals, so future resupply can sacrifice propellant for cargo, and none the less slowly build up the props in LSS to being filled.

Ooh! I really like this idea! And it gets even better with a 22m variant fairing Starship-tug filled to the cylindrical extent. ~2400m3 or ~2150t of propellant.

I'd probably keep some manner of fins and aerobrake back into LEO without landing. It could even go up with all RVacs as the intent is for it to never land.

[tater]

SS manufacturing is pretty inexpensive, and "space only" variants are lower in mass, and lower in complexity—all SS variants need solar, RCS, etc, but the atmospheric ones also need tiles, flaps, legs, etc).

It really does allow for some novel mission architectures.

[RCgothic]

A 2-stage starship fully fuelled could weigh more than a Saturn V on the pad. Imagine a Saturn *starting* from LEO. Starship can do that. Tanker flights are inexpensive.

And then they could be daisy chained. Stick a third on the back. 3 stages can send a fully fuelled starship over a km/s past earth's escape velocity and still recover both tugs to LEO propulsively.

Four stages? That's a thousand tons to Trans Jupiter Injection.

 

If they can sort out trans-shipping of cargo, there's really no limit to the payload a starship can take along (as long as it doesn't all have to land).

 

Edited April 19, 2021 by RCgothic

[tater]

 

[tater]

BTW, it looks like a "noseless" Starship acting as a tug could give a full SS ~2.3km/s and still put itself back in LEO.

Now we have a SS that has to do an additional ~2km/s for the Mars transfer orbit (total dv varies, lowest maybe ~3.5km/s). Our SS with the crew section and 10 tons of cargo (experiments and a rover?) masses 110t dry including that cargo. It has a dv of ~9.2km/s. We use 2 km/s to finish the MTO burn. SS now has 7.2 km/s. Landing and course corrections? A few hundred m/s? EDL is mostly aero, with propulsion at the end. Looks like ~6.8km/s is required for leaving the surface to an Earth transfer orbit. It's tight, but that seems to leave about 400 m/s to spare. Maybe stacked tankers as stages for more margin? The goal here is a Mars sortie... flags and footprints, plus proof of concept. Unlike direct it requires no ISRU, and could be done as an opposition class mission (much sorter mission duration)

[Elthy]

An almost full Starship likely wont survive reentry and landing, also boiloff would be hard to manage without the header tanks.

[kerbiloid]

Next step back to classics, after the wings bended up, the aluminium instead of steel, and the heatshield tiles, will be the understanding of the fact that a cylinder is a bad, bad shape for reentry, and developing a BOR/Spiral/Dreamchaser shape, or so.

They have been experimenting with cylinders, cones, partially cut cones in 1960s.
Didn't work for them, won't work for them.

[RCgothic]

SN15 slips to NET Wednesday:

 

And general production roundup:

 

 

[JoeSchmuckatelli]

5 hours ago, kerbiloid said:

a cylinder is a bad, bad shape for reentry

Dumb jarhead asks 'why?' 

[kerbiloid]

20 minutes ago, JoeSchmuckatelli said:

Dumb jarhead asks 'why?' 

They have tried all simple shapes in the past and weren't happy with them.
Original hopes were that a simple cone can glide and fly like a plane at high speed, so finally they came to the specific shape.

They tried the cylinder even with race cars. With same result. 

Spoiler

 

Because air drag doesn't like such shapes, a lot of turbulences appear.
For a glider that's also low L/D which means high accelerations and narrow bottom with concentrated heating.
Flat bottoms have high L/D and dissipate the heat more effectively.

Bad, bad is the cylinder.

Edited April 20, 2021 by kerbiloid

[Elthy]

A cylinder is very good for designing propellant tanks. Which starship (and any ordinary rocket) is mostly, so it dictates the shape. As long as it survives reentry, its better this way at its main task.

[kerbiloid]

15 minutes ago, Elthy said:

As long as it survives reentry

 

[Hannu2]

I am sure they have simulated whole reentry thoroughly. Empty Starship has relatively low mass/area -ratio, which probably gives more possibilities than compact capsule with high mass per area.

[RCgothic]

Turbulence is irrelevant at hypersonic speeds. Basically punches a hole through the atmosphere with vacuum behind. If all you intend to do is aerobrake at high altitude and thrusters have enough control authority, it's no big deal.

It would be a problem for landing, but that's why all the variants that land have fins.

[kerbiloid]

Any known aerobraked cylinder?

Why did the shuttle have a lifting body bottom? Its fuselage was more or less cylindric.

57 minutes ago, Hannu2 said:

I am sure they have simulated whole reentry thoroughly.

And I believe that aerodynamics hasn't changed since 1960s.

[RCgothic]

The shuttle wasn't primarily a pressure vessel, it was free to deviate from a cylinder without excessive stress concentration or mass penalty for a bit of extra aerodynamic efficiency which it desperately needed (heck of a cross range requirement).

I also note that the top of the shuttle was a half cylinder, which isn't noticeably better than a cylinder for vortex shedding.

I also note that a blunt bottom pushes the shockwave away from the hull and blends into the wings, thereby limiting re-entry heating, so that shape may have other functions than aerodynamics.

Finally, almost any shape is a lifting shape if presented at an angle of attack, even a sphere if you spin it. It's just a question of how efficient, and efficiency doesn't matter very much if not very concerned about L/D.

Bricks do fly if they're going fast enough.

Edited April 20, 2021 by RCgothic

[zolotiyeruki]

36 minutes ago, kerbiloid said:

Any known aerobraked cylinder?

Why did the shuttle have a lifting body bottom? Its fuselage was more or less cylindric.

And I believe that aerodynamics hasn't changed since 1960s.

The shuttle had a lifting body and wings because 1) one of the requirements was for 1000 miles of crossrange capability, i.e. it could land anywhere within 1000 miles of its intended target, and 2) it needed to be rapidly reusable (which never happened), and a horizontal landing had the potential for minimizing how much refurbishment was required before relaunch.

[kerbiloid]

16 minutes ago, zolotiyeruki said:

The shuttle had a lifting body and wings because 1) one of the requirements was for 1000 miles of crossrange capability, i.e. it could land anywhere within 1000 miles of its intended target, and 2) it needed to be rapidly reusable (which never happened), and a horizontal landing had the potential for minimizing how much refurbishment was required before relaunch.

Another one, definitely no 1 000-2 000  kmcrossrange. Does it have the flat belly just because it can?

Spoiler

P.S.
Anything known which is gliding (i.e. not falling like a bomb) and not flat below?

Edited April 20, 2021 by kerbiloid

[RCgothic]

Again, not a pressure vessel so doesn't have to be cylindrical.

It's not that cylinders don't work, it's that these other shapes have additional benefits *if free to choose them.*

Pressure vessels that aren't cylinders incur enormous mass penalties.

So yes, it has a flat belly because it can. That doesn't mean non-flat bellies don't work. 

See Vostok and Voskhod.

Edited April 20, 2021 by RCgothic

[tater]

49 minutes ago, kerbiloid said:

Any known aerobraked cylinder?

Why did the shuttle have a lifting body bottom? Its fuselage was more or less cylindric.

And I believe that aerodynamics hasn't changed since 1960s.

You know who knows a lot about how plain old cylinders act when hitting the atmosphere at high velocities?

SpaceX.

F9 booster at ~2km/s, and they have telemetry, cameras, and instrumentation on F9 stage 2. We really have no idea how much data they have collected on S2 reentry, but they must have some idea how it orients, how long it survives, etc.

I'm confident that that aspect is well modeled by them. As for one without flaps aerobraking? I have no idea. Maybe they need flaps for that.

[RCgothic]

Wow, the LOX issue was towards the top end of my estimate. That's an extra 1.3 tonnes!

With the vent system not sure it still remains at liftoff, but boiling off a tonne of LOX surely isn't easy. That'd be worth over 100kg of additional payload.

[kerbiloid]

51 minutes ago, tater said:

F9 booster at ~2km/s

SR-71 is ~1 km/s and it's hot.
Re-entry speed 8 km/s.

They have a room to grow.

54 minutes ago, tater said:

that aspect is well modeled by them

But do they have a tested solution? Theories are good.

[Nothalogh]

3 hours ago, kerbiloid said:

Why did the shuttle have a lifting body bottom?

USAF requirement of 1000 mile cross range capability,  required for RTLS after less than one orbit in a polar inclination.

3 hours ago, RCgothic said:

Bricks do fly if they're going fast enough.

Shuttle was proof of that.