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There have been a few instances of landing failures related to the grid fins and their hydraulic control - one where the hydraulic fluid ran out, and the latest one where a hydraulic pump stalled.

Given that these grid fins are located near the top of the rocket, rather far from the RP-1 tank, why can't they be controlled via electric motors and the onboard battery system? Would the weight of these motors be more than the associated pump and hydraulic lines (or soon to be two pumps if a redundant one is added)?

Perhaps the force that is neeeded is rather high - but it would seem that even relatively small electric motors could be geared down appropriately to provide the force. Is there a reason they are better controlled via hydraulics than through electric motors?

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  • $\begingroup$ I don't have enough aerodynamics to calculate the torque, but in a reentry at some point I expect it to be of the order of 1E+04 Nm (very roughly). That's the same ballpark as a Tesla car at the wheel, after all gearing. The motor torque is much lower. $\endgroup$ – uhoh Dec 8 '18 at 4:37
  • $\begingroup$ 1000 Nm? That might clarify it a bit, in that if they geared a motor low enough to produce high torque, they would lose the speed they require or the motor size would have to increase to more weight than the pump + lines. Thanks! $\endgroup$ – Ehryk Dec 8 '18 at 5:22
  • $\begingroup$ It's 10,000 Nm but it is just a guess. $\endgroup$ – uhoh Dec 8 '18 at 5:57
  • $\begingroup$ Oh. Why did you write it as 1E+04? $\endgroup$ – Ehryk Dec 8 '18 at 5:58
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    $\begingroup$ @uhoh Sure, but 10,000 Nm is more clear to me. $\endgroup$ – Ehryk Dec 8 '18 at 6:22
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As a general rule electric motors excel at high speed low loads while hydraulics work best at the low speed high torque end. Especially where the range of motion is small, the forces large and precise control is required there is a lot of knowledge and existing design around hydraulics. In particular with the grid fins there is the need to make rapid precise changes to the position.

For hydraulics the moving assembly can be compact,simple,light weight and low inertia. It will also consume very little power when not moving, and hold position well while also allowing precise small movements limited largely by the point at which surface tension prevents making the flow through the valve more constricted.

For electric drive there will need to be a gearbox which adds space and weight and a lot of backlash. So while high speeds are achievable rapid small reversing moves requires driving the motor quickly to take up the backlash and then a bit more to get the required motion. The backlash also means that if the force on the load changes direction there can be motion of the load (grid fin) as the backlash 'swaps ends' even with the motor locked. These two factors make the control process much more complicated. Driving in and out of a stalled condition at low speed can also be complex for an electric drive due to the large number of friction points in the motor and gear train to be unstuck without producing excessive motion.

So yes it can/has done, but would bring new problems. If using electric motors for flight control it might actually be better to redesign the control scheme to use a larger number of smaller control surfaces that better match the nature of electric drive.

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  • $\begingroup$ Thanks for the addition of precision as a factor. This could of course be mitigated through a brake/clamping system, but again increases the weight penalty. That was helpful! $\endgroup$ – Ehryk Dec 8 '18 at 5:23

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