You might have heard about a secretive space flight effort in the wilds of Texas that sustained an unfortunate crash recently. That was the “Blue Origin” project. (http://www.blueorigin.com/)

The flight was aborted by “a flight instability” as the team called it. That, in my opinion, is a Texas sized understatement.

I’m not going to call the crash an accident, because such a crash was completely predictable just from taking a quick glance at its design. And of course they didn’t intend for it to crash, its probably just that they didn’t quite understand some fundamental rocket flight dynamics.

It is also my opinion, that until they change the fundamental design of their rocket, it will continue to crash – no matter what they do to their flight control systems.

What’s Wrong?

The reason can be understood from a simple experiment you can do at home playing with an arrow or a badminton shuttlecock.

Take an arrow or a shuttlecock and drop it tail first from say a third story window. What happens? (You already know the answer to this don’t you?)

Within a few arrow lengths the arrow / shuttlecock swaps ends and proceeds tip first, tail behind.

Why is that?

Arrows and shuttlecocks work because for a body moving in air their center of drag will “flag” behind their center of motive force – otherwise known as center of mass.

More accurately, its center of acceleration (gravitational force / mass) is ahead (in the direction it is being forced) of its center of drag at that moment.

The Flag Principle

More generally any dropped object will tumble whenever its center of dynamic drag is ahead of its center of mass. When its center of drag is behind its center of mass – it stops tumbling.

You see an analogy of this every day when you see a flag streaming back from its flagpole. The center of motive force is the flagpole – even though it is not moving relative to the ground, it is moving relative to the wind.

Spacecraft are not necessarily unstable in flight through air, but powered spacecraft are(1).

Powered spacecraft are unstable because when you turn the rocket propulsion on — the center of force moves backwards, typically getting behind the center of drag (though not always if the fins extend far enough behind the rocket).

This analysis boils down to whether the motive force (gravity or a rocket motor) is pushing or pulling the object. Pulling is dynamically stable, pushing is not.

Rockets Need Dual Stability Systems

Rockets use both passive and active stability systems. Passive stability systems include trailing edge fins fixed in place. Active stability systems include steerable rocket exhaust nozzles or adjustable flying surfaces (and differential throttles as Roga gently reminded me).

The Blue Origin’s main problem is that it has a whole set of things that add up to instability.

First on the list are the tiny little passive-stability fins. Adding to that is the short body and bulbous nose that creates serious drag pushing the nose sideways whenever the rocket veers slight off going exactly straight ahead and blocks air flow to the fins. (And is it my imagination or does the nose have a larger diameter than the rocket body?)

The Blue Origin is very much like a shuttlecock or an arrow being pushed backwards through the air; it is a tumble waiting to happen.

Imagine the rocket encountering the first slight wind shear at supersonic speeds – and here we go a-tumbling again.

How to Fix Blue Origin

A proper rocket design needs an adequate passive-stability system first; before adding propulsion and steering. This is because an active stability system can only provide limited control; it won’t have enough power to correct for a grossly aerodynamically unstable rocket.

When you start with a aerodynamically passively-stable rocket – then your steering system only has to guide the vehicle where you want it to go. It doesn’t have to do any work to just keep it flying in a straight line. You can get rid of an entire software program. (That’s not completely true, but its close enough.)

Because of Blue Origin’s short body, its power-off center of drag and gravity (motive force) are close together, possibly only inches apart. Those centers can move dramatically and even swap ends with different fuel loads, payloads, speeds and angles of attack. Its easy to get them backwards. With rocket power on — the center of motive force moves strongly backwards making the craft even less stable.

However, once you launch a rocket into the air – when the centers of drag and motive force are backwards (negative stability) by a little bit – the error is often immediately obvious.

Shooting an Arrow Backwards

The “Backwards Arrow” I see is that the Blue Origin rocket as a “whole aerodynamic design” does not have the minimum shapes needed to be stable when it is flying without power. (That includes a large margin of error for us not knowing with certainty where the fuel and payload are, but giving it every possible benefit towards stability.)

The analogy is how when you drop an arrow facing backwards (feathers down) it will tumble until it straightens out and goes tip first. Similarly I suspect that if dropped nose down Blue Origin will tumble; that it will not continue in a stable fashion with its nose down.

The direct version is — If you shot an arrow backwards with the feathered end facing the target – it will tumble. It is my strong suspicion that if you shot a model of Blue Origin towards a target – it would tumble just like the backwards facing arrow.

So, to get back on track the Blue Origin team needs to stop and do some fundamental flight tests so they can see and viscerally understand what’s wrong. They seem to be relying too much on new (and untested) ideas rather than on empirical (actual physical) tests.

In spite of its daunting reputation, most rocket science is not difficult to understand or explain. If your rocket designer or engineer cannot show you how your rocket stability works in a simple fashion – then maybe you should find one who can.

They need to start with unpowered “flights” so they can see a dramatic demonstration of how poor their existing design is; and how a proper rocket works.

They can do this very quickly and inexpensively with drop tests of a model of their spacecraft. My guess is that their existing design will tumble and will not provide anything resembling a stable “flight.”

They can then compare that to an arrow, a shuttlecock or a model of any NASA rocket – and see how those aircraft fly / fall with beautiful passive-stability.

There’s no shame in using tried and true designs to achieve a new goal.

Wishing them good luck.

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Notes:

1. Its not entirely true that all rockets become unstable as they turn on their motors. Robert Goddard’s early models had designs that “pulled” — instead of pushed as most modern rockets are designed. This design was stable while power was on because the center of drag always remained behind the center of motive force.

For further reading:

OpenRocket simulation software

