Wiki Super Heavy/Starship - General Development Discussion

[Ben W]

In this shot from Tim Dodd's Starbase tour you can see the relatively beefy attachment points used for lifting Starship:

View attachment 710913

Given the discussion is around providing artificial gravity, we are only talking 1G loads for starship without a full fuel load, so those would indee seem to be capable.

My earlier musing about lateral acceleration had more to do with the challenge of getting the rotating evenly around a central axis evenly in a stable "orbit". It seems to me you'd have to:

1) Park the Starships nose-to-nose
2) Attach tethers
3) Use thrusters to back away from each other, fully extending the tether
4) Use side thrusters on each ship to begin moving radially around a central point

#3 would be tricky... you'd have to accelerate then stop relatively precisely to avoind any snap-back one you get to the end of the tether, or yanking the other ship around

#4 seems even trickier: your acceleration is around an imaginary central axis with a flexible attachment point. Each ship would have to accelerate in a very synchronized manner, and with almost exactly identical acceleration in order to keep the tether straight and keep the orbital axis in the center of the 2 ships. It seems like even small variations in mass/thrust or timing between the two ships would set off oscillations that would be difficult to control.

#3 should be solvable by building some flex into the tether, so the tension doesn't have a step-function; imagine using "bungee cords" at either end to give the system some play. Or with an adaptive friction mechanism to simulate the same thing. Or, instead of having the Starships get up close nose-to-nose, they could start at the final desired distance apart, and the tethers could be extended from each ship and connected using autonomous "space drones" or some such.

#4 I think is not a problem either. The ships don't have to apply thrust symmetrically; a single ship could do the entire job. (That's because there's no requirement that the center of mass of both ships stay fixed. This would also allow the spin-up process to double as a course-correction maneuver.) Any nonzero angular momentum will keep the tether taut; all the ships need to do is keep themselves pointed at each other, which can be done entirely with gyros/flywheels, and use gradual sideways thrust (from either ship) to adjust the angular momentum.

 

[Ben W]

number 4
it could be more than a bit tricky loading up the empty starships after you get the 2 spinning,
initial thought seems to me that loading fuel/passengers/cargo would slow angular momentum or if fuel sloshes around, induces unwanted oscillations, a bunch a passengers moving en mass to look out windows.

I think the idea would be to wait until the starships are fully loaded, before the spinning starts. Docking to a spinning configuration like this would be incredibly difficult, even at a very low spin rate.

 

[winfield100]

I think the idea would be to wait until the starships are fully loaded, before the spinning starts. Docking to a spinning configuration like this would be incredibly difficult, even at a very low spin rate.

an interesting idea, tail to tail, 240m long, crew at the tips, slow spin for ~.2 g, Mars g is 0.38 earth g so not enfeebled

How SpaceX Will Create Artificial Gravity on the Journey to Mars

Watching Scott Kelly struggle to walk after a year in space should be a sobering reminder to those making plans to travel to Mars. In…

medium.com

there has been a bunch of ideas for artificial g over the years
(weekends and retirement (7 saturdays in a week) are great for thinking)

 

[Ben W]

an interesting idea, tail to tail, 240m long, crew at the tips, slow spin for ~.2 g, Mars g is 0.38 earth g so not enfeebled

Nice thoughts! Starship is 50m long, so it may be too short for meaningful artificial gravity in tail-to-tail configuration. (Starship + Super Heavy is 120m.) At 2rpm spin, gravity in the crew compartment (~10m from the nose) would be about 0.15g. Having the spacecraft rigidly connected would help with some of the stability issues, and this sort of docking will probably be required for propellant transfer (unless an external depot is used for all transfers), but I still think a nose-to-nose tether-based approach for artificial gravity is probably ideal, mostly because the coriolis forces are less problematic; a tethered system can scale up closer to 1G for long-duration trips. My guess is that the health effects of reduced gravity will turn out to be roughly proportional to G forces; e.g. 0.5G would incur half the bone loss, etc. Much research still to do on this, but of course it could only be done in an environment where intermediate G-forces (such as 0.7G) can be sustained! So tether-based artificial gravity would almost certainly be needed for such research, even if it turns out to not be optimal for deep-space travel.

 

[bxr140]

The launch loads are compressive, and distributed through the thrust puck.

The entire vehicle structure endures massive loading throughout the launch environment, in all axes. The point is that, again assuming cognizant selection of attach location (so, not in the middle of a bare panel between two stringers) there's really minimal constraints on where attach points might be located.

Rest of post snipped since you agree with me.

an interesting idea, tail to tail, 240m long, crew at the tips, slow spin for ~.2 g, Mars g is 0.38 earth g so not enfeebled

Yeah, no question a rigid attachment is the right solution for this situation. Obviously there's no need to go way up to 1g during a mars transit; certainly simulating some gravity (I'd surmise close to mars gravity, if possible) would be useful from both a mental and physiological perspective. And of course, the transient and steady state management of the contemplated tether solution would be unnecessarily complicated.

Its also worth noting a few things with respect to this contemplated rigid solution:
--There could be an intermediate frame between the two vehicles, further spacing them apart. This frame could easily be assembled in space, and its volume could even be used for transporting additional supplies, emergency habitat, etc. The additional distance between vehicles provided by this frame would improve the intended gravity environment.
--The vehicles don't necessarily have to be tail-to-tail. In fact, nose-to-tail has a few nice benefits. 1) It enables the stack to have primary propulsion, which would likely de-complicate logistics as the stack could be assembled in earth orbit before an escape burn and, if one is planed, disassembled after a mars capture burn. 2) It would almost certainly move the CG of the stack away from the centroid of the system, which of course would increase the gravity effect on the far starship tip. (Its likely only one starship would be used for primary habitation during transit)
--One could even imagine a complex stack configuration, where the vehicles aren't necessarily coaxial during the gravity spin. The goal here would be to leverage more favorable inertial properties for a more stable system.

Ultimately, these kinds of solution based thought experiments are useful, but are most useful from a "what problem are we trying to solve" perspective. First principals and all. In other words, hyper focusing on how to make bolas out of two starships might generate some good solution based conversation, but it pretty well side steps the problem....

 

[winfield100]

Nice thoughts! Starship is 50m long, so it may be too short for meaningful artificial gravity in tail-to-tail configuration. (Starship + Super Heavy is 120m.) At 2rpm spin, gravity in the crew compartment (~10m from the nose) would be about 0.15g. Having the spacecraft rigidly connected would help with some of the stability issues, and this sort of docking will probably be required for propellant transfer (unless an external depot is used for all transfers), but I still think a nose-to-nose tether-based approach for artificial gravity is probably ideal, mostly because the coriolis forces are less problematic; a tethered system can scale up closer to 1G for long-duration trips. My guess is that the health effects of reduced gravity will turn out to be roughly proportional to G forces; e.g. 0.5G would incur half the bone loss, etc. Much research still to do on this, but of course it could only be done in an environment where intermediate G-forces (such as 0.7G) can be sustained! So tether-based artificial gravity would almost certainly be needed for such research, even if it turns out to not be optimal for deep-space travel.

you are correct. I'm incorrect
Also, there are a lot of very bright folks whom have given a lot of thought to this.
Mars Society has one thought, take bottom booster along for the ride, run teathers between the 2, spin.

(aerobraking landing on earth burns off 99.9% of v, on Mars, 99%, on Moon, 0% and delta V to Mars is less than Moon)

I bow to the literally 100's if not 1,000's whom have given thought to this, far more than I, including the EssEff stories.
wasn't sure I would see a Mars colony in my lifespan.

there is, by the way a _free_ 4 day, online virtual Mars Society Convention in mid October that may discuss this problem, and others

Space Exploration Economics to be Discussed by Voyager Space’s Dylan Taylor at Mars Society Convention - The Mars Society

The Mars Society is pleased to announce that Dylan Taylor, Chairman & CEO of Voyager Space, a multi-national space holding firm that acquires and integrates leading space exploration enterprises globally, will address the 24th Annual International Mars Society Convention in October on...

www.marssociety.org

there is probably a lot of info at
The 24th Annual International Mars Society Convention - The Mars Society (as a starting point)
and
SpaceX Starship - Wikipedia (there are a lot of links)

 

[Ben W]

Yeah, no question a rigid attachment is the right solution for this situation. Obviously there's no need to go way up to 1g during a mars transit; certainly simulating some gravity (I'd surmise close to mars gravity, if possible) would be useful from both a mental and physiological perspective. And of course, the transient and steady state management of the contemplated tether solution would be unnecessarily complicated.

Its also worth noting a few things with respect to this contemplated rigid solution:
--There could be an intermediate frame between the two vehicles, further spacing them apart. This frame could easily be assembled in space, and its volume could even be used for transporting additional supplies, emergency habitat, etc. The additional distance between vehicles provided by this frame would improve the intended gravity environment.
--The vehicles don't necessarily have to be tail-to-tail. In fact, nose-to-tail has a few nice benefits. 1) It enables the stack to have primary propulsion, which would likely de-complicate logistics as the stack could be assembled in earth orbit before an escape burn and, if one is planed, disassembled after a mars capture burn. 2) It would almost certainly move the CG of the stack away from the centroid of the system, which of course would increase the gravity effect on the far starship tip. (Its likely only one starship would be used for primary habitation during transit)
--One could even imagine a complex stack configuration, where the vehicles aren't necessarily coaxial during the gravity spin. The goal here would be to leverage more favorable inertial properties for a more stable system.

Ultimately, these kinds of solution based thought experiments are useful, but are most useful from a "what problem are we trying to solve" perspective. First principals and all. In other words, hyper focusing on how to make bolas out of two starships might generate some good solution based conversation, but it pretty well side steps the problem....

Agreed that "what problem are we trying to solve" is the right question. My assumption was that there are two main goals: first (and most importantly) to keep the crew in optimal physical and mental health during the long trip, and second, to gradually acclimate the crew to the gravity at the destination. For one-way trips to Mars (i.e. colonization), Mars gravity is probably sufficient, but for a round-trip, I would think being able to spin up closer to Earth gravity would have real benefits.

A rigid connection solution is probably not feasible for much above Mars gravity; either the Coriolis forces would become too strong (if the ships were directly attached at the tail), or the mass of the rigid connecting frame would become prohibitive. Even the lightest-weight flexible tether in my scenario would mass at least a ton or two, so moving from there to a rigid structure, especially one capable of withstanding any sort of compressive G-forces (e.g. from an escape burn) would severely eat into payload capacity. A tether would obviously only be deployed during the coast phase, after escape burn. (No capture burn needed for Earth/Mars.)

The other issue with tail-to-tail is that the crew-compartment spin configuration would have its gravity direction reversed from the launch configuration. I would think it's probably a lot simpler and more efficient to design the crew compartment to have a consistent "down" direction. (Anticipating that the launch chairs would likely double as sleeping beds. If you spin nose-outward, you'd have to move all the beds from floor to ceiling after launch, then back to floor for landing.)

Finally I really don't think managing the stability of the spin configuration would be all that difficult (for a computer); spacecraft already do it all the time. E.g. Dragon approaching and docking at ISS has to precisely control all 6 degrees of freedom. And while spinning, only 3 degrees of freedom are relevant, all controllable by flywheel/gyro, so in that sense it would seem to get easier, not harder. But then again, I am not a rocket scientist.

 

[Ben W]

Mars Society has one thought, take bottom booster along for the ride, run teathers between the 2, spin.

Starship doesn't have an orbital booster to do this with, so you'd need two Starships. (It could technically be one crew Starship and one cargo Starship, but artificial gravity is pointless for cargo; you'd maximize payload capacity by pairing up crew Starships with tethers, and leaving the cargo Starships floating in zero-G.)

there is, by the way a _free_ 4 day, online virtual Mars Society Convention in mid October that may discuss this problem, and others

Looking forward to this! I attended one Mars Society conference back in the 90's in Boulder; very inspirational. Thanks for the links!

 

[Buckminster]

SN 20 passed pressure test yesterday. Road closure on Monday. Assuming not for a launch? Still need to be stacked and test fired I think.

 

[HVM]

https://twitter.com/x/status/1451353320627507203

https://twitter.com/x/status/1451353565029707778

https://twitter.com/x/status/1451354108531728384

 

[EVCollies]

Nice article from Eric Berger about Vacuum Raptor's test fire.

How SpaceX ignited a new Raptor engine on Starship without an explosion

So why don't all rocket engines have giant nozzles?

arstechnica.com

 

[favo]

https://twitter.com/x/status/1452062147479883776

 

[scaesare]

Nice article from Eric Berger about Vacuum Raptor's test fire.

How SpaceX ignited a new Raptor engine on Starship without an explosion

So why don't all rocket engines have giant nozzles?

arstechnica.com

So my understanding is that theoretically an infinitely large nozzle would be desirable, but the practical considerations of flow separation limit the nozzle size within the atmosphere, and assumedly packaging/size limitations limit the size in vacuum.

So, it would seem that by designing an engine with such a massive chamber pressure that you can operate a vac Raptor, you are going to have an non-vac engine that needs ginomous nozzles even a sea-level, or risk an exhaust that's likely going to be over-expanded. Are the Raptor non-vac engines equipped with nozzle larger than typical for a throat that size given the record-breaking chamber pressures they are reaching?

 

[mongo]

So my understanding is that theoretically an infinitely large nozzle would be desirable, but the practical considerations of flow separation limit the nozzle size within the atmosphere, and assumedly packaging/size limitations limit the size in vacuum.

So, it would seem that by designing an engine with such a massive chamber pressure that you can operate a vac Raptor, you are going to have an non-vac engine that needs ginomous nozzles even a sea-level, or risk an exhaust that's likely going to be over-expanded. Are the Raptor non-vac engines equipped with nozzle larger than typical for a throat that size given the record-breaking chamber pressures they are reaching?

A smaller nozzle would result in underexpanded exhaust with a big flame cone trailing behind.

Higher chamber pressure would typically drive a higher exhaust to throat ratio. They could reduce the throat of the combustion chamber to decrease exhaust pressure and increase efficiency, but that would reduce thrust which is the critical parameter for first stage liftoff. Starship is a little different due to liftoff occuting on Mars or the Moon, but Earth landing is at higher atmospheric pressures.

 

[JRP3]

First orbital SpaceX Starship to splash down, sink off Kauai

Nov. 7—At the end of the first orbital test flight for its 164-foot Starship, SpaceX envisions a reentry into the atmosphere at speeds approaching Mach 25, or 19, 000 miles per hour, followed by 15 minutes of hypersonic flight. During this time, the spacecraft will hurtle sideways, generating...

www.yahoo.com

 

[Stook02ss]

an interesting idea, tail to tail, 240m long, crew at the tips, slow spin for ~.2 g, Mars g is 0.38 earth g so not enfeebled
View attachment 711285

How SpaceX Will Create Artificial Gravity on the Journey to Mars

Watching Scott Kelly struggle to walk after a year in space should be a sobering reminder to those making plans to travel to Mars. In…

medium.com

there has been a bunch of ideas for artificial g over the years
(weekends and retirement (7 saturdays in a week) are great for thinking)

Elon already said they nixed this, but it was a good concept and one they had planned on using for a while. It seems they are now going to lineup side by side and transfer fuel that way.

 

[Grendal]

Starship update from Elon:

 

[Cosmacelf]

That was an awesome Q&A session. Great questions and great answers. Well worth your listening time (dive right into the Q&A part).

 

[Nikxice]

Elon seemed relaxed and animated with the Zoom format, especially during the Q&A. He mentioned there'd be maybe 12 to 15 Starship test flight next year. That sounds like plenty of iterating will be happening during 2022. Quite optimistic about future success, but he's less certain that the first Orbital test flight will succeed.
Entertaining...starting with his talkative lap kid....then learning that Elon likes stainless steel so much they should probably get a room...Also, was I the only one expecting Elon would respond to that tall tailed black cat stealing his scene?

 

[Grendal]

All around good stuff. He even handled the "preserve planets" person very well. Preservation is wonderful but becoming multi-planetary is a higher priority.