Astra’s Frugal Design Leads To Latest Unusual Failure

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We have all heard it being said, and it bears repetition: it’s hard to go to space.But it will actually be even Harder The smaller the booster. It is used for rocket structures, engines, avionics, and useful payloads that make up a small portion of the lift-off mass, while the rest are propellants that must be consumed to reach orbital speed. Because it is. So if the Falcon 9 tilts the scale at 549,054 kilograms (1,207,920 pounds), it can only lift a payload of 22,800 kg (50,265 pounds). This is about 4% of the takeoff weight.

As you can imagine, there is a lower bound that the hardware equations needed to build a fully functional rocket do not have enough mass. But where are the limits? That’s what Astra, a newcomer to aerospace, is trying to find. Their Rocket 3 is one of the smallest orbital boosters in flight to date, closer in size and mass to the German V2 of World War II than the towering vehicles manufactured by SpaceX and Blue Origin. Thing. Even Rocket Lab Electron is a very smooth rocket in itself and is quite large.

The reason they are trying to make such a small rocket is, of course, very simple: small means cheaper. Assuming you have a payload that’s lightweight and compact enough to fit in your launcher, Astra says you can get it on track for about $ 2.5 million. Compete with SpaceX’s “Ride Share” program for less than half the cost of a dedicated flight on Rocket Lab’s Electron. Such low ticket prices are immeasurable 10 years ago and promise to shake the already competitive commercial launch market. But of course, before celebrating this new spaceflight milestone, Astra needs to make sure things fly.

Their latest mission ended with a complete loss of vehicles and payloads when the upper tier went out of control about three minutes after a complete takeoff from Cape Canaveral Air Force Base in Florida. Such problems are not uncommon in new orbit boosters, and historically few rockets go into normal operation without the loss of one or two payloads. However, this failure, which was broadcast live on the Internet, was extremely rare. Due to the unconventional design of Astra’s small rocket, the upper tier appeared to be stuck. inside Booster after the payload fairing could not be fully opened.

A little second stage

To understand this failure, we need to take a step back and consider the design of a traditional multi-stage rocket. It’s oversimplified, but in general it’s no exaggeration to say that the top row is a smaller version of the first row. It’s short in length, probably narrow in diameter, and has fewer engines. But basically its design is similar to the design of a larger counterpart. The payload is mounted on top of the top row, which is surrounded by an aerodynamic fairing or nose cone.

The first and second stages of Falcon 9 are separated by a black intermediate stage and have the same diameter.

However, Astra’s Rocket 3 is unique in that its second stage is not quite similar to the first. The design is reminiscent of a satellite, not a smooth rocket. It has an exposed tank and a skeletal structure that does not survive flying in the dense lower atmosphere. This stage passes through a faint upper atmosphere where drag is not an issue, so Astra decided to remove it to the essentials to reduce its mass.

The unusual appearance of the upper stage, like the satellite of Rocket 3, partially hidden inside the conical middle stage.

The drawback of this design is that it requires covering the fragile upper tier until the rocket reaches a considerable altitude. Therefore, it is necessary to enclose the entire second stage, rather than just placing the payload in the protective aerodynamic fairing. The lower part of the second stage is pushed into a hollow mid-stage, and an elongated fairing prevents the payload and its movement into space from being exposed to supersonic airflow in the early stages of flight.

Costly compromise

The beauty of this design is that the material used to build the payload fairing (usually carbon fiber or fiberglass) is very lightweight. Even considering the additional fairing length required, the overall weight will be lighter than if the second stage had a traditional rigid fuselage. However, if you are trying to make a rocket that is as cheap as possible, there are obvious problems with this approach. Manufacturing these lightweight payload fairings is very expensive.

Assembling the Rocket 3 aluminum payload fairing.

In fact, in the 2020 interview Ars TechnicaChris Kemp, co-founder of Astra, admitted that the astronomical cost of carbon fiber fairings forced them to abandon them in order to use aluminum for Rocket 3. $ 2,500. This is a significant cost savings for such low cost vehicles, but the trade-off is that the new metal fairing weighs 20% more than the original fairing.

The switch to heavier aluminum payload fairings means that the massive savings in Astra’s unique second-stage design are not as substantial as originally intended. In practice, this could hurt the total load capacity of the vehicle, but it was still considered a good call from a cost standpoint. But given this recent failure, perhaps the changes have had a greater impact than are now becoming apparent.

Clogged

Watching the live video from the launch on February 11th, you can clearly see a series of events where the upper tier lost control and eventually lost track. In almost exactly 3 minutes of flight, the payload fairing trembles visibly but cannot be opened. After 4 seconds, the second stage, propelled forward by a spring-loaded mechanism, hits the fairing but cannot be knocked on. Finally, 3 minutes and 11 seconds after the mission, the second stage engine ignites while still in the middle stage. This buildup of pressure blows off the fairing, but unfortunately it also destabilizes the second stage and sends tumbling.

I can see clearly what It happened, but Astra’s job is to understand now why It happened. The company, based in Alameda, California, is still investigating with the Federal Aviation Administration (FAA) and is currently making an official statement as to why the payload fairing did not open properly when commanded. Not announced.

But that doesn’t mean we can’t guess as an armchair engineer. Is it necessary to strengthen the mechanism used to separate the payload fairings as the mass of aluminum increases? We also suspect that a lightweight carbon fiber payload fairing could give way and save the mission after the second stage crash. In any case, there is one thing for sure. It seems that Astra’s low-cost rockets have the trick to fail in an unusual way.

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