again, i think the main argument gets lost as neither side here can act as adults.
“can be used as” and “are designed for” are VERY different things.
The grid fins on Falcon 9 are not designed as airbrakes, they are designed as stabilizers and guidance surfaces. I can find NO reliable sources that these specific ones are designed to used as airbrakes.
Do they add to drag? Absolutely. Can they be used to brake the decent? Absolutely. Are they designed for that purpose? no, not what i can find anywhere.
i actually found (cant seem to find it again though…) some places state the opposite. They have designed them with the front facing side having “peaks” at the intersections making them have lower zero-lift wave drag. This means that they have the lowest drag when not turning and thus needs to angle themselves to create meaningful drag.
Had they been designed with “valleys” instead i would have leaned more towards agreeing with you. Then they would have had better lift to drag ratios but would have created more drag at an alfa of zero making them inherently air-brake.
Yes, i know those curves. they do however tell us nothing on whether the Falcon 9 fins were designed to achieve more (or less) drag than other designs of grid fins.
The choice of a design with “peaks” however does tell us something. It tells us that they made choices in the design to have the fins crate LESS drag at zero-lift. if you wanted them to act as airbrakes why would you design them to have less drag?
The fins having more or less drag at specific speeds is just the same thing as anything else would. that doesn’t mean that they were designed with the extra drag as an intent.
again “can be used as” is not the same as “are designed for”.
They are quite literally visibly tailored for that specific purpose. The basic design of the fin is best suited as both a surface to maximize drag (minimize terminal velocity) and also to help aid the precision landing at lower atmosphere when the RCS boosters are ineffective. The TVC isn’t useful when the motors aren’t firing as well.
These were required to reduce wave drag preventing control at all in the regions between subsonic and supersonic. They weren’t necessary on the thinner aluminum fins. They increased thickness and moved to titanium, at the same time they stopped using the landing gear as drag devices.
None of that is coincidence. If the grid fins weren’t necessary to slow the descent, they would have never made them larger, draggier, and out of titanium to handle the higher heat from the increased drag.
The fins themselves need a high level of precision over a wide range of speeds. That’s why they have peaks, it would be bad to lose all control to wave drag in transonic regions as this is a critical point in the boosters flight path.
Any grid fin not designed to slow something down would have ultra thin tips on the grids and the opposite is seen here.
well yes, the basic design of ANY grid fin. not necessarily those specific ones. Edit: you know what, i re-read that and no, that isn’t a basic design characteristic. otherwise it wouldn’t be used on missiles.
Do you have a source for that? i have found nothing saying anything close to that.
Oh they absolutely did. The original aluminium fins burned up at reentry and couldn’t be re used. so they did titanium for RE-usability and then size/peaks for control authority, not increased drag. as specifically stated by musk: one , two and three.
Edit: i even found one where he explicitly states that the increase in size does allow for a longer glide but that it is a “minor factor”
this doesn’t make sense. Its a zero-lift drag reduction. meaning they reduce drag when NOT angled and when angled they have higher drag compared to that of valley types which does the opposite; more drag at no angle (working as air-brakes) but better drag to lift ratio when turning. so the opposite as to what you claim here.
this depends on speed, you cant have to thin leading edges at too high speeds. The forces would bend the metal and/or the heat-soak would be too little making them heat up MUCH faster and thus risk burning and braking due to the friction induced heat.
The specific fins on Falcon 9 were designed with RE-usability and control authority in mind, NOT air-braking. They absolutely do decelerate the decent, perhaps even more than i think, but that doesn’t matter to the discussion as my claim is that they were not DESIGNED with that as their purpose.
You misunderstand. The basic design of the falcon 9’s grid fins optimized drag in comparison to grid fins used on missiles. The grids are extremely thick and flat. This is not beneficial to the control of the rocket, instead only improved the terminal velocity by increasing drag.
All launches are filmed.
The control authority was already there. The titanium fins didn’t need to be so much thicker, but they are. The peaks allow better flow in spite of the thicker fins. Optimization in all areas.
Is inches thick necessary for re-entry? I don’t think so. It’s considerably thicker than the aluminum fins. This wouldn’t be necessary.
I wonder how they stabilize the pod if they don’t move? Perhaps drag?
The general lack of understanding of grid fins is why people fail to understand the R-77 around here.
You still don’t understand, the fences themselves are thicker and flat. This is not ideal for reducing drag. More spacing was required in part of the lattice to even allow any flow at higher mach numbers.
Edited to include test of thin walls, note how the size of holes matter more.
they also didn’t change the thickness of the walls between the versions.
the thickness could be to make them reusable and withstand the forces and heat, not for more drag.
its very relevant, it shows the change in drag when changing only wall thickness and changin only hole size.
highly relevant as the change for Falcon nine was larger holes, but no change in wall thickness. meaning less drag.
You are not observing the taper, or considering the additional area of the lattice from the extension or the peaks. There is a reason they have foregone using the landing gear as a method of reducing the terminal velocity.
the increased area from the peaks results in higher drag and better control at angles, not at zero alfa.
if you want them as air brakes you want the higher drag when not angled.
The peaks are added to make the zero alfa drag less.
the additional area would not increase the drag more than the peaks decreases it at zero alfa.
that is the function of valleys.
i have yet to se a source on this. until you provide one i have only seen statements of the opposite and will believe as such.
The rocket maintains ~20° AoA in descent to slow down, increase drag, and avoid wasting fuel. Once it is subsonic, slowing down is not necessary and control is transferred mostly to the engines TVC systems.
Elon Musk stated it himself and I already provided the source.
The peaks were added to ensure proper flow in spite of the thicker grids, improving control in several regions and also providing additional drag.
well yes. but that would be counter productive as you would have less area for steering and more area for drag.
so its a question of reducing area by making the walls thinner or holes bigger to achieve less drag.
