Max Thrusters

[Josephina]

Thisis probably a really stupid noob question, but I'm stuck in the advanced construction tutorial whilst placing the RCS thrusters!! I've followed the steps - added 4 symmetrical thrusters on the FL-T100 tank, moved them with the offset tool, reset them using space, and then turned off the COM indicator but it won;t let me click "Next". Not sure what I'm doing wrong! I'll try and insert a picture below:

In simulation I built some command laws taking into account the fact that we can control each thrusters independently. The goal is to stabilize the ROV in a certain pitch and roll using Python and ROS.

However, override function only allows to command directly the orientations and the forward/backward, upward/downward movements. That is why I want to know if there is a way to control each thrusters independently.

Note that control in any particular direction or rotation axis is only possible with appropriately placed thrusters. The BlueROV2 Heavy can control both pitch and roll, whereas the standard frame configuration has no pitch control capabilities.

Thanks for that answer. That is very interesting and I think it should work using pymavlink. However, I am using mavros and as far as I know, it is impossible to use pymavlink and mavros at the same time right ?

I cannot find a description of the RCS thrusters on the current incarnation of Starship. I assume that "using ullage gas" means venting boil-off as cold gas. Or is it heated before discharge? Or even ignited?

However, this was always just a stop-gap solution. One of the goals of the Starship system is to simplify the architecture, and having another propellant on the vehicle runs counter to that. The plan was to develop methalox hot-gas thrusters.

However, in a 2021 Starbase tour with Tim Dodd, the Everyday Astronaut, Elon Musk mentioned that they were using ullage gas thrusters on the Super Heavy booster and only using the methalox thrusters on Starship. When Tim Dodd asked why not also use the ullage gas thrusters on Starship, Elon Musk started to answer why it doesn't make sense but realized halfway through the answer that it actually does make sense. Since then, the methalox hot-gas thrusters were never seen again and never mentioned anywhere.

Shortly after this interview, it could be observed that the location and shape of the ullage vents on both Super Heavy and Starship changed significantly. If you look at the ullage vents now, they have a distinctly shaped nozzle and many of them are arranged in the tell-tale three-axis pattern. Some of them were also moved away from the center of gravity where there is a larger lever arm.

So, the best guess we have now is that both the Super Heavy booster and Starship use ullage gas thrusters. Note that this is not unheard of. In fact, Elon Musk mentioned that SpaceX actually uses the passivization vent of the Falcon 9 upper stage to help reorient it for disposal, by orienting the rocket in the right way before opening the valve.

You might have seen a couple of joke posts on ? during the last couple of hours, saying it is Tim Dodd's fault that Ship 28 tumbled out of control during re-entry: that interview is what they are referring to.

It is working like a cold-gas thruster in that there is no combustion and no chemical reaction. It's just opening a valve on a pressure vessel. However, the gas is not actually "cold". In fact, you want your ullage gas to be as hot as possible without compromising the structure of the tank wall.

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When I tested my first few vessels dependent on a small number of azimuth thrusters, I found a worse dynamic fault response than I was used too. But the offshore industry likes azimuth thrusters and I recently read industry guidance recommending them over fixed direction thrusters. I wondered if I was wrong, and I thought this mismatch might be worth looking at.

Azimuth thrusters have a number of advantages over conventional thrusters. They take up less room than a shaft line and are easy to place in large, relatively flat, hull sections. Their ability to direct the thrust where it is most needed sometimes allows replacement of multiple fixed thrusters and they are more efficient than tunnel thrusters. They are ideal for large semi-submersibles, where multiple thrusters can be placed far apart from each other, to prevent losses from thruster interaction (thruster wash across another thruster reduces thrust). They can be stuck out of the bottom of the pontoons and the wash angled slightly down to avoid hull interaction (thruster wash across the hull reduces thrust). There may be mission critical areas or transducers that their wash must not affect, but with many thrusters, large vessels, and more than two redundant groups, this is not a significant problem.

The above drawing shows an additional complication that reduces the healthy and fault response of azimuth thrusters. Smaller ships and semi-submersibles need to worry more about thruster interaction and work & transducer zones that cannot have thruster wash. These forbidden zones limit the directions that the remaining healthy azimuth can thrust, which reduces healthy DP capability and dynamic failure response. For example, if forward wash is forbidden then capability will be limited on the stern, and after a fault, the remaining thruster may be slowed by having to go the long way round to correct position. Even healthy stern sway and yaw thrust will be limited. The fixed thrusters have defined wash areas that the work areas (but not the transducer in the picture) are placed out of, so its work and capability can remain unaffected. Large forbidden zones can make azimuth thrusters a poor choice.

Finally, hidden thruster azimuth feedback calibration faults are more of a concern when you have a smaller number of main azimuths, and unlike with a rudder swing, there may be no wash to diagnose the problem with. An azimuth thruster that fails to hidden azimuth offset or rotation is often minor when a vessel has twelve main azimuth thrusters, but can be critical when it has two. Hidden azimuth angle control faults have generally been the most insidious azimuth faults that I have found during testing. Hidden faults in thrust speed or pitch control can be detected by comparison with thruster load, but azimuth angle usually lacks an independent feedback and has common failure modes. This makes regularly checking the angle of orientation of the azimuth thrusters especially important. Azimuths that provide independent local indication (not through the common feedback gearbox) can be regularly compared with feedback or cameras installed to allow the DPO direct access (preferred solution). Without independent feedback or direct physical indication of angle, azimuths need regularly tested (e.g. during setup or before critical operations).

The industry loves azimuth thrusters for their obvious and immediate benefits, but there are limitations that need to be considered and managed. Small numbers of azimuths are less suited to tight position keeping, more vulnerable to faults, and more impacted by forbidden zones, but the drawbacks of azimuth delay were smaller than I expected. All thrusters have trade-offs and there are some solutions, but azimuths are not always the right tool for the job.

Thrusters are what propels a ship when flying in normal space. Upgraded thrusters can accommodate heavier ships, increase ship speed and improve manoeuvrability. If the Optimal Mass of a ship is exceeded, it will be unable to travel at its top speed until its mass is lowered below the Optimal Mass threshold.

A ship's top speed is based on the ratio of optimal mass of the equipped thrusters to the total mass of the ship. This ratio is only linear in the case of C-rated thrusters. A- & B-rated thrusters are positively concave, meaning that they benefit more from a total mass lower than optimal mass and have less of a disadvantage from a total mass higher than optimal mass. D- & E-rated thrusters have the opposite effect.[1]

At a total mass identical to the optimal mass of the equipped thrusters, a ship's top speed will always be identical to the advertised speed, no matter the rating. This does not mean that a higher rated thrusters won't have a benefit over lower ratings, as they have a higher optimal mass.[1]

Power Distributor rating and class appears to have no effect on top speed (outside of its mass), but the number of pips allocated to engines do. Top speed per pip allocated is a linear function, meaning that 2 pips will have a speed exactly between 4 pips and 0 pips, but the speed modifier of 0 allocated pips depends on a hidden value belonging to the ship, as can be seen in the table below.[1][2]

The main thruster(s) obviously push the ship forward, but generally also have flaps to redirect thrust or are even capable of being swivelled and re-pointed, which allows them also to control the direction of the ship. However all ships have multiple smaller thrusters situated in relevant locations over their hull: these are the manoeuvring thrusters, and allow for more refined directional changes.

Manoeuvring thrusters fire automatically in response of the pilot's stick movements. Pilots will see them as small jets emitting from points along their ship as they move the stick. Turning off Flight Assist will give the manoeuvring thrusters more control over the ship's orientation at the cost of less control over its forward motion.

The force applied by the manoeuvring thrusters relative to the ship mass is used to specify a ship's manoevrability rating, but there are exceptions. It is assumed that upgrading thrusters also upgrades manoeuvring capability, but the effect has not yet been quantified.

Although they are clearly based on a real physical model, the individual manoeuvring thrusters or arrays were never named by any official publication from Frontier Developments, but in a simplified way they can be labelled by their real-life equivalents.

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