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

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?

• slightly related: Why didn't Saturn V have grid fins? Long March and Soyuz do! – uhoh May 10 '19 at 1:01
• Grid fins aren't magic. They are easier to fold flat though. – JCRM May 10 '19 at 5:25
• This question is almost impossible to answer, there are such a myriad of factors and BO aren't saying anything more than you already know. – ANone May 10 '19 at 14:48
• @ANone there are quite a large number of active and knowledgable users here with a diverse set of backgrounds and knowledge bases. I'm not sure how one can be so certain ahead of time that absolutely none of them are able to think of a helpful answer. If you can suggest a way I can modify the question to improve it, let me know. Until then, let's see how it goes, someone may surprise you. After all, it's not rocket science. Oh, wait... – uhoh May 10 '19 at 15:19
• Maybe. Why but as stands this is about blue origins intents and reasoning. And SpaceX's. To answer you'd have to intimate knowledge of their reasoning. If you did, you probably shouldn't be telling on the internet. Maybe you could phrase it as what are the relative advantages disadvantages, and constraints of the 2 systems. – ANone May 13 '19 at 10:09

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.

• Wonderful answer, thank you! – uhoh May 11 '19 at 15:23
• Yes, those are all angular accelerations. Thanks for catching the missed word, will edit. – Bob Jacobsen May 11 '19 at 15:24
• but the fins are moveable? – JCRM May 14 '19 at 13:44

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.

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.

• This is interesting, but can you also add the explanation without the "imagine this" part? What is it exactly about the larger mass, or moment of inertia, or both that makes grid fins better in some cases and normal fins in other cases? Does it have to do with tradeoffs between fins and cold gas thrusters perhaps? – uhoh May 10 '19 at 9:06
• Can you explain why a pencil has a higher moment of inertia than a broomstick? Also a golf ball is smaller than a broomstick but is easier to balance. This answer needs work. – Organic Marble May 10 '19 at 12:15
• The moment of inertia of a pencil is not larger than the moment of inertia of a broomstick. The moment of inertia of a thin rod about its short axis is 1/12 mL^2. This should be roughly accurate for pencils and broomsticks. The broomstick has both a greater length and a larger mass. – WaterMolecule May 10 '19 at 16:08
• The moment for the larger & heavier bar is significantly larger. $m$ 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, angular acceleration of aerodynamic error tends to go like$1/L^2$, decreasing as objects get bigger. For gravity, the force scales like$m$, torque like$mL$, so acceleration goes as$1/L$; decreasing with size, but not as much as the aero case. – Bob Jacobsen May 10 '19 at 23:53 • @BobJacobsen you might want to repost your comment, it looks like a missing or extra $ is parsing your text into MathJax. – uhoh May 11 '19 at 22:45