Will the New Glenn have grid fins? Why or why not?

Question

The January 2019 Blue Origin video New Glenn: The Road to Space shows the rocket with static fins at the bottom and four articulated fins near the top of the first stage.

Is this just notional, or has Blue Origin pretty much decided not to use grid fins for the reusable first stage's reentry and landing?

SpaceX's Falcon 9 uses grid fins, and I'd thought they, or something else with substantial aerodynamic lift and high thermal resistance were necessary, and those grid fins looked to be a pretty good solution.

Can normal fins serve the similar purpose, or is Blue Origin likely to replace those fins with grid fins in the future?

Answer 1

The Falcon 9's grid fins exist to solve a particular problem: Control during engines-first descent. Aerodynamic stability during that phase comes from having the center of mass well below/ahead of the center of pressure due to heavy engines at the bottom. But a stable descent to the wrong place isn't the desired outcome. Further, Falcon 9 flies a specific profile that includes some attitude changes, so active control is needed. SpaceX chose grid fins over adding more-capable thrusters or other aerodynamic solutions, but we don't know exactly why: Cost? Weight (which is basically cost)? Reliability? Reusability?

What about a larger rocket, like new Glenn?

If you just scale-up an aerodynamic shape, keeping everything else in the design constant, it becomes more stable: deviations due to aerodynamic disruptions (wind) and gravity torque (i.e. not straight above engines) will cause slower acceleration.

Why? The moment of inertia $mL^2$ gets significantly larger when scaling up to a larger object of the same shape. $m$ is bigger (by scaling, as the cube of $L$), $L^2$ is bigger. The scaling is generally as $L^5$. But also note that torques are increased: they act over a longer $L$, and come from a larger area (as $L^2$). So, again just as scaling, an angular acceleration of an aerodynamic error goes as $1/L^2$, decreasing as objects get bigger. For gravity, the forces scale like $m$, the torque like $mL$, so angular acceleration goes like $1/L$; decreasing with size, but not as much as the zero case. Bottom line: Controls (fins or otherwise) can get comparatively smaller (still bigger, but not as much bigger as the rocket) as rockets grow in size.

But for fins to work, you have to have airflow. Note that Blue Origin seems to prefer landing via a hover phase. Fins aren't useful then, grid or otherwise. Thrusters are needed.

From the look of the rocket, they're using non-grid fins to move the center of pressure closer to the center of mass (i.e., toward the bottom) during flight, which reduces the control authority they need during the descent. Thrusters might then be enough. Combined with a desire for a hover phase, they might be taking a Giant Thruster Capacity approach. A bigger rocket already has more capacity for thrusters; Blue Origin might have been able to find a way to add some more capacity and get away from a complicated movable-fin system.

Answer 2

Fins are usually used for aerodynamically controlling the vehicle. Grid fins are a simpler and more storable form (as mentioned folding flat, especially during ascent) of fins as compared to the conventional ones. The New Glenn seems to follow on the legacy of fins developed, tested, and validated during the New Shepard flights, that have used conventional fins to achieve control in the atmospheric parts of the flight.

Grid fins are typically tail controlled, so they're providing larger control moments in comparison to fins closer to the the center of mass, which is typically the case of ailerons as a control surface, versus elevators or rudders. They work to serve as control actuators as well as drag generators (they're designed to sustain the heat, and SpaceX has improved this over time). Larger rockets perhaps already have sufficient area for drag, and thus require less of the grid fins help, and/or the Blue Origin engineers have planned to achieve it differently from what SpaceX is planning. I'm not sure if this answers your question completely, but I feel like I'm adding on some information to other answers.

Answer 3

Grid fins stabilise the vehicle, as you mentioned. Now imagine this: When you balance a broomstick by the bottom of the handle, its much easier to balance than if you tried the same thing with a pencil. This is because the pencil's moment of inertia is much higher, meaning a larger object, the broomstick, is easier to balance. This is why the New Glenn, a larger vehicle than the Falcon 9, doesn't require grid fins. If you see renders of SpaceX's Starship, there are no grid fins, i'm guessing for a similar reason. Hope this helps.