Wiki Super Heavy/Starship - General Development Discussion

[JB47394]

Anything more than connected is the same thing though and it's safer to begin a linear approach from a distance to allow more time to react to problems than create a near miss situation to start with.

I don't understand. Here's a top notch graphic showing the approach. The black circles are the ships, and the red boxes are their docking mechanisms. What can go wrong? If the approaching ship has a failure, it's going to slide right past. If everything goes as expected, the approaching ship brakes without blasting the other ship with its thrusters, and stops within whatever distance can be tolerated.

Also keep in mind both objects are orbiting and can't maintain a fixed gap without continual thruster intervention unless they are pro/retrograde with identical speeds, in which case they aren't closing the gap.

How much of a drift are we talking about here with two ships moving with a 0.1 m/s differential? I have no idea how to calculate it.

 

[mongo]

I don't understand. Here's a top notch graphic showing the approach. The black circles are the ships, and the red boxes are their docking mechanisms. What can go wrong? If the approaching ship has a failure, it's going to slide right past. If everything goes as expected, the approaching ship brakes without blasting the other ship with its thrusters, and stops within whatever distance can be tolerated.

View attachment 1042933


How much of a drift are we talking about here with two ships moving with a 0.1 m/s differential? I have no idea how to calculate it.

Red is the cone of badness if there is excess velocity or error on the trajectory, blue is the effective path, cyan is equivalent straight in. What does the tangential approach buy you?

400 km vs 401km is about 2km/hr difference or half a meter per second for a circular orbits.

 

[JB47394]

What does the tangential approach buy you?

The elimination of essentially all of the cyan component. On failure of cyan, you smack the target. On thrusting with cyan, you have to thrust at the target. I don't see any justification for the cyan approach.

 

[mongo]

The elimination of essentially all of the cyan component. On failure of cyan, you smack the target. On thrusting with cyan, you have to thrust at the target. I don't see any justification for the cyan approach.

How do you end up docking if you don't thrust at the target? Cyan is a long coast toward the target, tangental approach is a coast toward a much smaller target.

 

[JB47394]

How do you end up docking if you don't thrust at the target? Cyan is a long coast toward the target, tangental approach is a coast toward a much smaller target.

This may be a terminology thing. I'm talking about "firing my engines at the target". I don't want to do that. I want my exhaust to go either past the target in this tangential approach, or I want my exhaust to go away from the target so that my ship can go straight at it.

When I fire a thruster on my left side to go right, am I thrusting left or right? Words are a pain.

 

[mongo]

This may be a terminology thing. I'm talking about "firing my engines at the target". I don't want to do that. I want my exhaust to go either past the target in this tangential approach, or I want my exhaust to go away from the target so that my ship can go straight at it.

When I fire a thruster on my left side to go right, am I thrusting left or right? Words are a pain.

Ah, gotcha I had picked up the splash connotation, but also thought maybe velocity vector since we're discussing collisions.
In a straight on senario it shouldn't need to exhaust at the station either, wheras in the tangential approach it might if there was a late adjustment.

 

[JB47394]

In a straight on senario it shouldn't need to exhaust at the station either, wheras in the tangential approach it might if there was a late adjustment.

That's the opposite of my intuition, but unless we sit down at a pub and rely on the old standbys of a salt shaker and a pepper shaker to illustrate what we're talking about, I don't think we'll sort this out.

 

[ecarfan]

Glad you guys got that sorted.

In this new NSF video at the 16:05 point, Jack starts talking about ship propellant transfer and what NASA showed at a recent presentation. Here’s the NASA slide.

 

[JB47394]

Glad you guys got that sorted.

In this new NSF video at the 16:05 point, Jack starts talking about ship propellant transfer and what NASA showed at a recent presentation. Here’s the NASA slide.

And here's Ars Technica's article on it all

NASA lays out how SpaceX will refuel Starships in low-Earth orbit

"The fundamental flow mechanism is the pressure delta across the umbilical."

arstechnica.com

 

[ecarfan]

Road closure tomorrow from 7AM to 7PM CDT, possibly to move S30 to the suborbital pad for a static fire.

 

[ecarfan]

What I imagined from the description is that the thrust is continuous, and thus the "pressure differential" doesn't need to come from pressurizing the tank. It would just be the thrust providing an artificial gravity: connect the tanks and they'll tend to equalize their levels.

Okay but that won’t lead to the ship having a full tank. The tanker ship will never arrive in orbit with a full tank. So how does the ship needing prop ever get full if no pumps are used?

 

[scaesare]

What I imagined from the description is that the thrust is continuous, and thus the "pressure differential" doesn't need to come from pressurizing the tank. It would just be the thrust providing an artificial gravity: connect the tanks and they'll tend to equalize their levels.

I didn't get that impression for the afore-mentioned Ars article:

Once the two Starships come together, they will connect using the same ports SpaceX uses to load propellants on the launch pad. Then, SpaceX will fine-tune tank pressures and fire propellant settling thrusters. "At that point, they'll open up and let the propellant flow."

The propellants will flow from one vehicle to the other using a pressure differential, or "delta," between the donor tank and the recipient tank. This is a simpler solution than relying on pumps.

"The fundamental flow mechanism is because of the pressure delta across the umbilical," Kshatriya said.

They seem to make the distinction that the thrusters are for settling the prop, and then use "fine tuned tank pressures" for the prop flow...

 

[scaesare]

I don't understand. Here's a top notch graphic showing the approach. The black circles are the ships, and the red boxes are their docking mechanisms. What can go wrong? If the approaching ship has a failure, it's going to slide right past. If everything goes as expected, the approaching ship brakes without blasting the other ship with its thrusters, and stops within whatever distance can be tolerated.

View attachment 1042933


How much of a drift are we talking about here with two ships moving with a 0.1 m/s differential? I have no idea how to calculate it.

This scenario requires the ships to null out their "parallel" velocity, but then still move (avoiding "thrust" as a confusing term) directly towards each other right?

The idea being this maximizes the time where an error won't produce a collision, and makes the "head on" scenario of a shorter duration?

 

[scaesare]

How do you end up docking if you don't thrust at the target? Cyan is a long coast toward the target, tangental approach is a coast toward a much smaller target.

Tangential is a coast towards a smaller target, or a smaller coast towards the same sized target?

I would think the latter...?

 

[JB47394]

So how does the ship needing prop ever get full if no pumps are used?

Sustained pressure differential. The receiving tank will continue to vent boiloff in order to keep its tanks at low pressure. Meanwhile, the sending tank will continue to boil propellant to keep its tanks at high pressure.

They could pressurize the sending tank via a number of different techniques, including any combination of these:

1. Passively, with the sending ship in the sun and the receiving ship in the shade
2. Actively, by tapping a methalox settling thruster for hot gas
3. Actively, by using electric heaters.

I have no idea what the practical limit for the pressure differential might be. The difficult part on the receiving side is making sure that your vents only vent gas. Keeping the receiving tanks cold, and limiting boil, would be a huge win.

Tapping a methalox settling thruster seems the most natural choice because it allows for nice synergy; it has to be firing for the duration of the transfer, and the pressure on the sending tanks has to be kept high for the duration of the transfer. We know that SpaceX has worked on methalox thrusters, but they may be very simple affairs, without the ability to provide that hot gas. Would they build a full flow staged combustion thruster that kicks out on the order of 10 tons of thrust? It would be a mini-Raptor.

The idea being this maximizes the time where an error won't produce a collision, and makes the "head on" scenario of a shorter duration?

That's the thinking. It also means that the exhaust from the approaching vehicle's thrusters aren't directly impinging on the target vehicle. There would still be some splash as the exhaust expands widely in vacuum.

Tangential is a coast towards a smaller target, or a smaller coast towards the same sized target?

Tangential doesn't have to do with the size of who is moving. It has to do with whether the approaching vehicle would strike the target vehicle if it didn't alter its path. If I set up an approach at some speed and then never adjust my line, but miss the target, then that's a tangential approach. If I hit the target, then it's a collision approach. Everyone seems to use a collision approach.

It may be considered more practical because it doesn't require both vehicles to be designed to allow this "slide past" thing. Each has to have the docking side clear of any projections that would hit the other vehicle when approaching or, in the case of a failure, sliding past. The technique only has to allow for a corridor, but it's obviously less demanding from a geometric design standpoint to allow a ship to nose straight in with its docking port.

 

[ecarfan]

Sustained pressure differential. The receiving tank will continue to vent boiloff in order to keep its tanks at low pressure. Meanwhile, the sending tank will continue to boil propellant to keep its tanks at high pressure.

Thanks. I wonder what the efficiency loss will be, i.e. what percent of the prop needs to be lost to boil off to maintain the needed differential? We shall see…

 

[scaesare]

That's the thinking. It also means that the exhaust from the approaching vehicle's thrusters aren't directly impinging on the target vehicle. There would still be some splash as the exhaust expands widely in vacuum.

Ah... I hadn't thought about deceleration thrust affecting the opposite vehicle. I suppose there would be some of that at times, although does Dragon do that with ISS, or does it just approach slowly enough that that the docking adapter absorbs the kinetic energy?

Tangential doesn't have to do with the size of who is moving. It has to do with whether the approaching vehicle would strike the target vehicle if it didn't alter its path. If I set up an approach at some speed and then never adjust my line, but miss the target, then that's a tangential approach. If I hit the target, then it's a collision approach. Everyone seems to use a collision approach.

It may be considered more practical because it doesn't require both vehicles to be designed to allow this "slide past" thing. Each has to have the docking side clear of any projections that would hit the other vehicle when approaching or, in the case of a failure, sliding past. The technique only has to allow for a corridor, but it's obviously less demanding from a geometric design standpoint to allow a ship to nose straight in with its docking port.

Right... that was in response to @mongo 's post where he referred to a "smaller target"... was asking for clarification on what he meant.

But given the docking adapters still require a "head on" approach, it would seem that the riskiest portion of the maneuver (when you are closest) remains. Tha farther away, the more time you have to correct.

So, I guess the issue becomes: are the orbital mechanics challenges @mongo outlines less risky than eliminating the low(er) risk parts of a direct approach?

 

[Matias]

The first ship will carry whatever payload it wants to transport to Moon or Mars. The 2nd one will have propellant as its payload, and that propellant will refill the first ship whose tanks should be empty when it reached orbit

Because propellant transfer is being driven only by pressure difference, it will end when both vehicles are equally full?

edit: https://teslamotorsclub.com/tmc/members/jb47394.218854/ mentioned other options.

 

[JB47394]

Ah... I hadn't thought about deceleration thrust affecting the opposite vehicle. I suppose there would be some of that at times, although does Dragon do that with ISS, or does it just approach slowly enough that that the docking adapter absorbs the kinetic energy?

With such a small vehicle doing the maneuvering at such slow speeds, the thrust levels are tiny. If it was a battleship approaching a space station and using packs of Raptor engines for fine maneuvers, I think impingement would be more of a concern. Raptors ablate steel at an appreciable rate.

But given the docking adapters still require a "head on" approach, it would seem that the riskiest portion of the maneuver (when you are closest) remains. Tha farther away, the more time you have to correct.

When approaching tangentially, you have the exact same amount of time. You approach from the side, and stop a millimeter from the docking port. Then the docking ports can mechanically reach out to soft capture. The goal is to eliminate as much of the "head on" movement as possible. Theoretically, it can all be eliminated.

Alternately, you approach from the side and aim to stop right at the capture mechanism - which sticks out a couple meters from the target vehicle. You slide into the mechanism from the side and it draws the two vehicles together. Design the mechanism so that if the speed of the approaching vehicle is too high, it automatically releases and lets the approaching vehicle slide past.

Because propellant transfer is being driven only by pressure difference, it will end when both vehicles are equally full.

Except that the two vehicles should actively manage their pressures. The target vehicle should vent its pressure while the delivering vehicle should build its pressure.

You may be right about the first test, however. They may not do anything to attempt to keep the process going, but I'd be surprised by that. I would expect SpaceX to make the attempt at managing the pressures. They do this with their autogenous pressurization system to keep tank pressures high while they drain the tanks through the engines.

 

[scaesare]

With such a small vehicle doing the maneuvering at such slow speeds, the thrust levels are tiny. If it was a battleship approaching a space station and using packs of Raptor engines for fine maneuvers, I think impingement would be more of a concern. Raptors ablate steel at an appreciable rate.

Hmm... aren't we talking small RCS thrusters here? How would the Raptor thrust impinge on the other ship? Orientation reference:

(NOT a picture of how baby Starships are made)

When approaching tangentially, you have the exact same amount of time. You approach from the side, and stop a millimeter from the docking port. Then the docking ports can mechanically reach out to soft capture. The goal is to eliminate as much of the "head on" movement as possible. Theoretically, it can all be eliminated.

Alternately, you approach from the side and aim to stop right at the capture mechanism - which sticks out a couple meters from the target vehicle. You slide into the mechanism from the side and it draws the two vehicles together. Design the mechanism so that if the speed of the approaching vehicle is too high, it automatically releases and lets the approaching vehicle slide past.

OK... I do recall you mention a grappling mechanism earlier. That eliminates the need to use thrust to move towards each other... gotcha.