Raptor failure modes and gimbaling

[Slarty1080]

I believe that Raptor has been designed to gimbal and Starship has a multiple engine out capacity on landing. If they use three engines for landing and one cuts out for whatever reason how does Starship respond?

It would be possible to light some of the other engines to compensate, but I suspect this would not happen quickly enough in all circumstances so presumably they would gimbal the 2 remaining good engine(s) to balance and then throttle up to balance instead?

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

[Lars-J]

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

I'm not sure you understand where the gimbal axis of rotation is - it is just above the engine. It will not have an nozzle edge angled up into the ship.

There is going to be plenty of ground clearance, you don't need to worry about a gimbaled engine touching the ground. The vehicle will not be not sitting on its nozzles. And before liftoff they will run gimbal tests of all engines anyway as part of the startup.

[Keldor]

It might be single engine out capability.  Then if one of the side engines fails, cut off the opposite one and land just on the center engine.  Otherwise, if the center engine fails, continue on the two outer engines.  These both keep the thrust centered through any single engine failure.

[Lars-J]

It might be single engine out capability.  Then if one of the side engines fails, cut off the opposite one and land just on the center engine.  Otherwise, if the center engine fails, continue on the two outer engines.  These both keep the thrust centered through any single engine failure.

If losing one engine means that you need to shut off the opposite engine, that is NOT single engine out capability.

All Starship Raptor engines will likely gimbal (and certainly the landing ones), so while shutting down another engine is a possibility it means that you are starting the landing burn with 3x the engines you need! (If you have 3 landing engines) Which seems excessive and unlikely for performance/margin reasons. So more likely significant gimbal action will be required by the remaining running engines.

[Slarty1080]

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

I'm not sure you understand where the gimbal axis of rotation is - it is just above the engine. It will not have an nozzle edge angled up into the ship.

There is going to be plenty of ground clearance, you don't need to worry about a gimbaled engine touching the ground. The vehicle will not be not sitting on its nozzles. And before liftoff they will run gimbal tests of all engines anyway as part of the startup.

You are quite right to assume that my understanding of gimbaling is rudimentary.

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

[ZachF]

Raptor gimballs, I've circled the gimbal actuators right here (These might be stand-ins however):

[envy887]

I believe that Raptor has been designed to gimbal and Starship has a multiple engine out capacity on landing. If they use three engines for landing and one cuts out for whatever reason how does Starship respond?

It would be possible to light some of the other engines to compensate, but I suspect this would not happen quickly enough in all circumstances so presumably they would gimbal the 2 remaining good engine(s) to balance and then throttle up to balance instead?

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

Starship will retain full control with 2 engines, and they probably won't need to shut down the opposite engine if a side engine fails. It's still possible to null all rates and land the vehicle upright with asymmetric thrust - it is just a slightly more complex control problem. They would avoid landing on a single engine if at all possible because A) that reduces roll control to RCS only and B) that eliminates all propulsion redundancy.

For a Mars landing I would expect all engines to be burning. It's going to be coming in about 3 or 4 times faster than an Earth landing, and will be heavier with the extra landing fuel. The faster you can scrub off that velocity the less fuel you need for landing.

[Lars-J]

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

No, SH would still not land on its nozzles. The original idea was to have the SH land in the launch mount, so thread the needle and land in the middle of a circle. The SH never rests on its nozzles, it would be supported by launch mounts/receivers arranged in a circle around it.

But now that legs are back on the table (it seems), SH would land on legs.

Rocket nozzles are fragile and never support a rockets weight. (at least for liquid engines)

[geza]

Raptor gimballs, I've circled the gimbal actuators right here (These might be stand-ins however):

Earlier we debated in another thread, how the Raptor was supposed to gimbal: hydraulically (with what kind of fluid?), or electromechanically? (Merlin is gimbaled with kerosene hydraulics, not available here.) Now we have this picture. Can an expert determine the gimbal mechanism from this?

[Johnnyhinbos]

I believe that Raptor has been designed to gimbal and Starship has a multiple engine out capacity on landing. If they use three engines for landing and one cuts out for whatever reason how does Starship respond?

It would be possible to light some of the other engines to compensate, but I suspect this would not happen quickly enough in all circumstances so presumably they would gimbal the 2 remaining good engine(s) to balance and then throttle up to balance instead?

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

Not sure you understand just how rapidly engines can gimbal. Here's a video of a SpaceX TVC test that gives a pretty good example of the responsiveness these systems have.

[Keldor]

It might be single engine out capability.  Then if one of the side engines fails, cut off the opposite one and land just on the center engine.  Otherwise, if the center engine fails, continue on the two outer engines.  These both keep the thrust centered through any single engine failure.

If losing one engine means that you need to shut off the opposite engine, that is NOT single engine out capability.

All Starship Raptor engines will likely gimbal (and certainly the landing ones), so while shutting down another engine is a possibility it means that you are starting the landing burn with 3x the engines you need! (If you have 3 landing engines) Which seems excessive and unlikely for performance/margin reasons. So more likely significant gimbal action will be required by the remaining running engines.

Single engine out capability just means that if any single engine fails, it can still land.  There's nothing about whether you'd have to turn off a second engine to do so successfully.

The problem I see with asymmetric thrust is that even though it will still have control in the air, the rocket will have to tilt a bit to keep the center of thrust beneath the center of mass.  This means that when it touches down, it will be on a single landing leg because the other two will still be in the air.  Is the leg strong enough to take the full weight of the rocket while it tips back fully upright for the other two legs to touch down?  It might buckle.  Though admittedly the engines are close enough to each other than the rocket wouldn't have to tip very far.  Also, maybe the shock absorbers have enough range of motion for the other two legs to touch down without the first leg bottoming out?

If there is enough range of motion in the shock absorbers, it might even open the door to two engine out landings.

Concerning raptor performance, the rocket weighs 85 tons, but Raptor can produce northward of 200 tons of thrust.  This gives SS a TWR greater than 1, even if it's laden with returning cargo or passengers.  Though doing so would mean the entire landing approach would have to be at low decelleration, and thus take more fuel.  Maybe a single engine can simply buy enough time for another pair of engines to start up?  Or even all four other engines.  Then they could initially decellerate rapidly while higher up, knowing there's time to fire up the other engines and decellerate very rapidly for a moment if needed, and only using the slow decelleration the the final few seconds before touchdown, allowing a single engine landing with a much smaller fuel penalty.  Raptor was also designed to max out at 250 tons thrust.  Even though the early prototypes may not be able to sustain this for long without damage, maybe they can do it for just long enough.

A final thing to note is that SS is perfectly capable of flight with unbalanced thrust even if it can't touch down.  It could descend most of the way down, only cutting the second engine to balance at the last moment to straighten out for touchdown.  It could even do something similar with only a single side engine running, gimbling into an unstable vertical attitude just before touchdown, much like an airplane landing in a strong crosswind flies in with the nose pointing off to the side to compensate, then straightens out right before touchdown so that the wheels line up with the ground.

[Hog]

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

Rocket nozzles are fragile and never support a rockets weight. (at least for liquid engines)

During a launch, as soon as the vehicle is off the launch mount, don't said nozzles support not only the vehicles weight, but also the force of thrusting?

Of course that is much different scenario than having a vehicle sitting on a launch/land mount with the nozzles as the point of contact.

[Slarty1080]

I believe that Raptor has been designed to gimbal and Starship has a multiple engine out capacity on landing. If they use three engines for landing and one cuts out for whatever reason how does Starship respond?

It would be possible to light some of the other engines to compensate, but I suspect this would not happen quickly enough in all circumstances so presumably they would gimbal the 2 remaining good engine(s) to balance and then throttle up to balance instead?

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

Not sure you understand just how rapidly engines can gimbal. Here's a video of a SpaceX TVC test that gives a pretty good example of the responsiveness these systems have.

Looks like that's the way to do it. If it can gimbal that quickly then I think it should be able to land vertically on a single side engine as it can thrust to the left then thrust to the right very fast and repeatedly and spend slightly more time on one side than the other to account for the fact that the engine is off centre. The Starship itself would never be able to react as quickly and would only feel the average thrust vector.

If this is possible with Raptor then I assume they could lose 4 of the 6 engines and still survive, possibly even lose 5 assuming 185 tons total. Although I understand that loss of the engines would be altitude dependant. Loss of 3-5 engines at very high altitude would probably mean disaster as the remaining engines would require too much fuel. Perhaps abort to orbit?

[Lars-J]

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

Rocket nozzles are fragile and never support a rockets weight. (at least for liquid engines)

During a launch, as soon as the vehicle is off the launch mount, don't said nozzles support not only the vehicles weight, but also the force of thrusting?

Of course that is much different scenario than having a vehicle sitting on a launch/land mount with the nozzles as the point of contact.

You answered your own question. The thrust chamber and throat is where the lifting load is carried. Not the tip of the nozzle.

[rakaydos]

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

Rocket nozzles are fragile and never support a rockets weight. (at least for liquid engines)

During a launch, as soon as the vehicle is off the launch mount, don't said nozzles support not only the vehicles weight, but also the force of thrusting?

Of course that is much different scenario than having a vehicle sitting on a launch/land mount with the nozzles as the point of contact.

You answered your own question. The thrust chamber and throat is where the lifting load is carried. Not the tip of the nozzle.

I've seen plenty of "factory floor" pics of engines sitting on their nozzeles, though...

[Twark_Main]

The problem I see with asymmetric thrust is that even though it will still have control in the air, the rocket will have to tilt a bit to keep the center of thrust beneath the center of mass.  This means that when it touches down, it will be on a single landing leg because the other two will still be in the air.

...

A final thing to note is that SS is perfectly capable of flight with unbalanced thrust even if it can't touch down.  It could descend most of the way down, only cutting the second engine to balance at the last moment to straighten out for touchdown.  It could even do something similar with only a single side engine running, gimbling into an unstable vertical attitude just before touchdown, much like an airplane landing in a strong crosswind flies in with the nose pointing off to the side to compensate, then straightens out right before touchdown so that the wheels line up with the ground.

Yes, a clever landing algorithm (which SpaceX has) will have no problem landing on all legs simultaneously using one or two engines. It will build up sideways speed in the opposite direction, then tip upright just before touchdown such that the horizontal velocity, vertical velocity, altitude, tip angle, and angular rates all hit zero at the same time.

Sounds hard (and it is), but this is the same problem the guidance loop solves during every hoverslam. There's no special case here, just a more constrained actuator model with the failed engines deactivated. It wouldn't surprise me if the math actually gets easier because of the reduced degrees of freedom, though it will burn through some propellant margin.

"Sometimes the best hardware option is software."

Looks like that's the way to do it. If it can gimbal that quickly then I think it should be able to land vertically on a single side engine as it can thrust to the left then thrust to the right very fast and repeatedly and spend slightly more time on one side than the other to account for the fact that the engine is off centre.

Why incur the cosine loss instead of simply pointing the engine continuously in the average direction? Is it to avoid striking a failed nozzle at that position?

I've seen plenty of "factory floor" pics of engines sitting on their nozzeles, though...

That's only supporting the weight of the engine, not the entire vehicle.

[Slarty1080]

The problem I see with asymmetric thrust is that even though it will still have control in the air, the rocket will have to tilt a bit to keep the center of thrust beneath the center of mass.  This means that when it touches down, it will be on a single landing leg because the other two will still be in the air.

...

A final thing to note is that SS is perfectly capable of flight with unbalanced thrust even if it can't touch down.  It could descend most of the way down, only cutting the second engine to balance at the last moment to straighten out for touchdown.  It could even do something similar with only a single side engine running, gimbling into an unstable vertical attitude just before touchdown, much like an airplane landing in a strong crosswind flies in with the nose pointing off to the side to compensate, then straightens out right before touchdown so that the wheels line up with the ground.

Yes, a clever landing algorithm (which SpaceX has) will have no problem landing on all legs simultaneously using one or two engines. It will build up sideways speed in the opposite direction, then tip upright just before touchdown such that the horizontal velocity, vertical velocity, altitude, tip angle, and angular rates all hit zero at the same time.

Sounds hard (and it is), but this is the same problem the guidance loop solves during every hoverslam. There's no special case here, just a more constrained actuator model with the failed engines deactivated. It wouldn't surprise me if the math actually gets easier because of the reduced degrees of freedom, though it will burn through some propellant margin.

"Sometimes the best hardware option is software."

Looks like that's the way to do it. If it can gimbal that quickly then I think it should be able to land vertically on a single side engine as it can thrust to the left then thrust to the right very fast and repeatedly and spend slightly more time on one side than the other to account for the fact that the engine is off centre.

Why incur the cosine loss instead of simply pointing the engine continuously in the average direction? Is it to avoid striking a failed nozzle at that position?

I've seen plenty of "factory floor" pics of engines sitting on their nozzeles, though...

That's only supporting the weight of the engine, not the entire vehicle.

Re the Cosine loss issue I was thinking of take-off from an unprepared or minimally prepared surface on Mars or the Moon. In that way more debris might be directed laterally out and away from the ship. As soon as the ship was 100m or so up the engines could be realigned into the vertical plane.

[edzieba]

Yes I see it now - of course doh! – it’s sitting on its legs when it lands so there would be no issue with gimbaling. In fact it would be possible and might even desirable in some cases to gimbal all engines outward for take-off.

But presumably Superheavy *would* have this issue if it turns out that it doesn’t have legs?

Rocket nozzles are fragile and never support a rockets weight. (at least for liquid engines)

During a launch, as soon as the vehicle is off the launch mount, don't said nozzles support not only the vehicles weight, but also the force of thrusting?

Of course that is much different scenario than having a vehicle sitting on a launch/land mount with the nozzles as the point of contact.

You answered your own question. The thrust chamber and throat is where the lifting load is carried. Not the tip of the nozzle.

Not really: if the nozzle bell bore no vertical load, it would be redundant and could be omitted as dead weight.
If you look at a rear-on view of an engine with bell, then the portion of the vertical load carried by that engine is distributed according to apparent surface area (handily, the portion carried by the bell vs. past the throat is also the expansion ration). There is also the additional non-vertical load from containing the expanding gasses - which will be much higher in the combustion chamber - but in terms of 'supporting the weight of the rocket' the bell is doing the majority of the work. Due to the gas pressure acting outwards on the bell it could not support the weight under static conditions (i.e. you couldn't just stick a bell-shaped plug in there and sit the rocket onto it), but the actual vertical component of thrust during operation is absolutely not borne just by the combustion chamber & throat.

[magnemoe]

I believe that Raptor has been designed to gimbal and Starship has a multiple engine out capacity on landing. If they use three engines for landing and one cuts out for whatever reason how does Starship respond?

It would be possible to light some of the other engines to compensate, but I suspect this would not happen quickly enough in all circumstances so presumably they would gimbal the 2 remaining good engine(s) to balance and then throttle up to balance instead?

If so what happens to the gimbaled engine when it gets to ground level? Wouldn’t the gimbaled engine present one nozzle edge angled up into the ship and one nozzle edge angled down slightly below all the other engines? If so wouldn’t landing on it in that orientation destroy the engine nozzle? Or doesn’t gimbaling work that way?

Starship will retain full control with 2 engines, and they probably won't need to shut down the opposite engine if a side engine fails. It's still possible to null all rates and land the vehicle upright with asymmetric thrust - it is just a slightly more complex control problem. They would avoid landing on a single engine if at all possible because A) that reduces roll control to RCS only and B) that eliminates all propulsion redundancy.

For a Mars landing I would expect all engines to be burning. It's going to be coming in about 3 or 4 times faster than an Earth landing, and will be heavier with the extra landing fuel. The faster you can scrub off that velocity the less fuel you need for landing.

During burn to orbit they will use all engines but probably shut down some at end of burn as you will get to high trust and g load. They have engine out capability might even loose two but this might leave to little fuel to land.
no need to shut down engine on other side.

During landing they will use 1 or 3, my guess is they start with 3 but only use center for touchdown.
If center engine is out they will probably need to land on two engines as having just one engine on the side will probably be to much for gimbal.

[Lars-J]

You answered your own question. The thrust chamber and throat is where the lifting load is carried. Not the tip of the nozzle.

Not really: if the nozzle bell bore no vertical load, it would be redundant and could be omitted as dead weight.
If you look at a rear-on view of an engine with bell, then the portion of the vertical load carried by that engine is distributed according to apparent surface area (handily, the portion carried by the bell vs. past the throat is also the expansion ration). There is also the additional non-vertical load from containing the expanding gasses - which will be much higher in the combustion chamber - but in terms of 'supporting the weight of the rocket' the bell is doing the majority of the work. Due to the gas pressure acting outwards on the bell it could not support the weight under static conditions (i.e. you couldn't just stick a bell-shaped plug in there and sit the rocket onto it), but the actual vertical component of thrust during operation is absolutely not borne just by the combustion chamber & throat.

Yes, the apparent surface area (from below) does carry the load. But that is the entire engine - Not the nozzle edge alone! (which has a tiny area - the area is a thin ring)  If you think about it, it will makes sense.

The edge of the nozzle is not flimsy - it supports the engine itself - but it will NOT support 100s of tons of mass pushing down on it.