Wiki Super Heavy/Starship - General Development Discussion

[mongo]

This isn't really true. Gimballing engines can provide translation without a lean. It's more complex to do both hover and translation only using the engines because it means precise control over both angle and thrust level, but the requirements may be well within the vehicle's capabilities. Still, horizontal thrusters would make control simpler, with the engines providing lift (and roll) and the thrusters providing translation.

Translation without rotation requires a net thrust vector though the center of mass with a horizontal component. Gimballing engines at the bottom can't do that unless they also apply differential thrust to shift the thrust vector off center at the source thus allowing it to point through the center of mass and avoid torque. Pure gimballing can only tilt, translate, anti-tilt like balancing a broom on one's finger.

This is demonstrated by the Shuttle powerslide or the Astra launch that was missing an engine.

I will be interested to see how SpaceX deals with booster versus crosswinds and the amount of tilt they can handle at catch.

 

[Max Spaghetti]

Translation without rotation requires a net thrust vector though the center of mass with a horizontal component. Gimballing engines at the bottom can't do that unless they also apply differential thrust to shift the thrust vector off center at the source thus allowing it to point through the center of mass and avoid torque.

Correct.

Physics dictates that a gimballing thrust vector below the CofG can only translate laterally by also tilting the rocket off vertical.

There may be a certain amount of off-vertical tolerance that the chopsticks can cope with, but it presumably won’t be much.

There is also wind to account for.

A vertical descent that is completely on-target will still need a sideways component of thrust to account for wind. That would require a vertical tilt if the gimbaling engines are the only source of thrust. Top mounted thrusters would surely be required IMHO.

 

[Electroman]

Translation without rotation requires a net thrust vector though the center of mass with a horizontal component. Gimballing engines at the bottom can't do that unless they also apply differential thrust to shift the thrust vector off center at the source thus allowing it to point through the center of mass and avoid torque. Pure gimballing can only tilt, translate, anti-tilt like balancing a broom on one's finger.

Are you using the terms, 'tilt' and 'rotation' here in the sense that they mean the same ?

 

[JB47394]

This is a still taken from a video on the AnthroFuturism YouTube channel. This image is something I've always wanted to see, in order to be reminded of scale.

The video is, as you might expect, highly optimistic.

 

[JB47394]

Physics dictates that a gimballing thrust vector below the CofG can only translate laterally by also tilting the rocket off vertical.

My instincts tell me that gimbaling off-axis thrust vectors with a sufficiently low CG should permit translation without inducing a rotation. I have no idea of the magnitude of that force, but I think that, academically, it's an accurate statement. If they used the second ring of gimbaling engines, they should be able to get a larger effect because they're farther off-axis.

It would be complicated by cross winds trying to tip over the booster. Low CG, but high CP (center of pressure).

Perhaps I'm assuming too low a CG, and that the methane load (and, possibly, a return cargo load) would move it too high to allow this sort of maneuver.

 

[Max Spaghetti]

My instincts tell me that gimbaling off-axis thrust vectors with a sufficiently low CG should permit translation without inducing a rotation. I have no idea of the magnitude of that force, but I think that, academically, it's an accurate statement. If they used the second ring of gimbaling engines, they should be able to get a larger effect because they're farther off-axis.

I hadn't considered the potential of off-axis thrust vectors. Plausible.

 

[EVCollies]

Given the mass of the booster, any guess to speed of wind gusts required to move Super Heavy?

 

[JB47394]

Given the mass of the booster, any guess to speed of wind gusts required to move Super Heavy?

Short answer: Typical sustained winds at Boca Chica won't be a problem. Gusts of 30 mph would be a concern.

Long answer:

I found a simple wind load calculator. It takes a wind velocity and surface area to give a load in Newtons. I'm sure the idea is that you've got a flat plate of a certain surface area, with the wind hitting it.

The cross section of the booster is 639 m2, and so I used that as the area of the flat plate. For the purpose of this very rough calculation, the angle won't matter (it's a cylinder), so the wind is always hitting that plate at 90 degrees.

The average wind speed in Boca Chica in April is 13.7 mph (6.124 m/s). The average over the past five years is 11.7 mph.

The April number gives us 14,095 Newtons of force.

At landing, figure the booster will be at its dry mass (200 tons) plus hover propellant and reserve. The Raptor's propellant mass flow is apparently 650 kg/s of LOX plus 140 kg/s of CH4. A single engine can hover a booster's dry mass, and if we want 30 seconds of hover time plus a 10 second reserve, that's 31.6 tons. I'm ignoring the number of engines running and their throttle levels.

14,683 Newtons applied to a 231 ton vehicle will accelerate it at 0.07 m/s2.

Assume a gust of 30 mph (13.411 m/s). That results in 70,417 Newtons of force. (The force scales by the square of the wind speed)

70,417 Newtons applied to a 231 ton vehicle will accelerate it at 0.34 m/s2.

I doubt that sustained winds would be much of a problem, but a strong gust for a few seconds could get it moving at a couple mph. That could be bad, particularly for the final alignment of a catch.

If we want to get crazy, how about landing in a thunderstorm and getting hit by a microburst? Those are 100 mph, and can be considerably higher. On top of that, they're sustained for several minutes.

A 100 mph wind just pushed it with 782,431 newtons of force. It will accelerate the vehicle at 2.39 m/s. After 10 seconds, it's going to be moving at 53 mph (86 km/h). Well, assuming that the engines didn't counter the acceleration. I'm not going to do the math for the tilt needed to oppose that while trying to hover. This has gone on long enough.

 

[ecarfan]

Great analysis. Given the level of expertise that SpaceX has acquired in successfully landing over 200 boosters with fairly high precision, I’m confident that they have a pretty good idea of what will be safe weather conditions for catching a Starship booster and ship. Of course the catching process will require greater precision than an ASDS landing. It’s going to be fascinating to watch SpaceX learn how to do it! Excitement guaranteed…

 

[ecarfan]

It’s unclear exactly what SpaceX is doing to the chopsticks, but the interesting part of this video is at about the 23:00 point showing closeups of a guy working on the tiles at the nose of S29.

 

[scaesare]

Good article at Ringwatchers with anticipated changes on "Version 2" of Starship & Booster: Something New: What's the Hardware Status of Starship Version 2?

 

[JB47394]

NASASpaceFlight's video on Elon's presentation, and their take on the significance of it all. One point that was interesting was that they didn't run the Raptors anywhere near their maximum performance on IFT-3.

 

[ecarfan]

And lots of interesting speculation about the hardware changes between the various versions. Well worth watching.

 

[scaesare]

Not sure what S/N this is, but cool shot:

https://twitter.com/x/status/1779588366792556715

 

[JB47394]

Not sure what S/N this is, but cool shot:

I think it's Ship 29. They've been giving the tiles a real going over this week. Perhaps 10% have been removed from all over the vehicle - which is a lot of tiles.

 

[Buckminster]

https://twitter.com/x/status/1781188584084582905

 

[JB47394]

tweet

For the curious, 700,000 pounds is 317 metric tons.

 

[Buckminster]

For the curious, 700,000 pounds is 317 metric tons.

So possibly referring to a Raptor 3.5?

 

[JB47394]

So possibly referring to a Raptor 3.5?

I think Elon is just being his usual speculative self.

 

[scaesare]

I think Elon is just being his usual speculative self.

He generally doesn't say stuff like this if he doesn't see a physics-based path towards it.

Timelines, and business aspirations he has a shakier record on, but engineering things he's more solid on.

As a reference point, the Original Merlin 1A, produced 76K lbs thrust, the 1D Full Thrust version 190K lbs. That's 2.5 X. So, for raptor to go from 185K lbs to 317, or 1.7X might not be out of the realm of possability.