Aerospike Engines: Why aren't we using them now? [video]

https://www.youtube.com/watch?v=K4zFefh5T-8

by hinkley • 8 years ago

111 36 8 years ago

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exabrial• 8 years ago

Why did Firefly aerospace switch after their takeover

hliyan• 8 years ago

Summary:

1. An aerospike engine tries to overcome a problem in the traditional bell-shaped nozzles: traditional nozzle shape and size can only be optimized for one specific pressure (e.g. either sea level or a higher altitude). At all other altitudes, it is inefficient.

2. An areospike is kind of an inverted bell -- there is a spike in the middle and the rocket exhaust flows around it. This forms a sort of virtual bell nozzle (the inner wall is the spike's surface, the outer wall is the air itself). As the atmospheric pressure varies, the virtual nozzle shape adjusts too. This makes the areospike efficient at all altitudes (but not as efficient as a bell nozzle at it's most optimal).

3. Ideal for single-stage-to-orbit

4. Work stopped (author claims) because in the space industry, tried and tested is favored. And apparently, fuel is not such a large component of the cost of a rocket launch.

wolfram742• 8 years ago

When sentiments like this [1] are floating around, it's an easy argument to make that companies aren't interested in fundamental research, and at most tweaking off the shelf parts.

-- [1] https://www.ft.com/content/869b514c-727d-11db-a5f5-0000779e2...

carapace• 8 years ago

Choose the subscription that is right for you

Seems unrelated.

hinkley2• 8 years ago

Got another article without a paywall?

valuearb5• 8 years ago

The fundamental research on aero-spikes has been done, and they flunk the rocket equation.

dTal• 8 years ago

fuel is not such a large component of the cost of a rocket launch

Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload.

ThenAsNow6• 8 years ago

Surely it's a massively important cost in terms of weight? Engine efficiency isn't just about cutting costs, it's about maximising payload - and the majority of a rocket's mass is fuel. Even a 5 percent efficiency gain is a huge win for payload.

It's not that easy an answer. This point about maximizing efficiency to maximize payload only really matters if you are trying to increase the payload delivered for an existing system of fixed size. If you have the latitude to design a new /derivative vehicle, then you have the latitude to size the tanks larger to account for increased propellant. Propellant is cheap compared to the other costs of manufacturing and operating a rocket-propelled vehicle, though as re-use becomes more and more common, the sensitivity to propellant costs might be higher.

Another thing to keep in mind is that efficiency in the form of increased Isp can be traded for engine thrust-to-weight. The SpaceX Merlin engine for example, uses an "inefficient" (i.e., low Isp) gas generator kerosene cycle, but has incredible thrust-to-weight (so is very mass-efficient).

Aerospike engines will tend to be heavier due to distributing the thrust chamber exhaust circumferentially as opposed to through a small central throat.

In practice, with a multi-stage system it is very hard to realize a system-level performance benefit from an aerospike configuration, which is the principal reason why they have not been used. Worth noting, the original Space Shuttle Main Engine configuration Rocketdyne developed was an aerospike, which makes sense given the engine fires from ground level to orbit. NASA Marshall apparently felt this was excessively high risk, so mandated a bell nozzle. The high area-ratio SSME nozzle is operating at the hairy edge of internal separation, and visibly "twangs" as the engine starts up and the internal shock blows out of the nozzle. Very low-throttle operation at sea level involves running with the shock and separated flow inside the nozzle and results in debits against the nozzle life.

Interest in the aerospike configuration waxes and wanes within the U.S. DoD & NASA, but currently the Air Force Research Lab is considering pursuing a modular aerospike engine program. The benefits are not in the realm of performance, but rather the aerospike Isp benefit is used to offset the lower thrust-to-weight. The real motivation is the desire to gang smaller thrust chambers and turbomachinery sets to provide more of a "Lego-like" low-cost way to design, test, and scale up these engines.

valuearb5• 8 years ago

Aero spikes only make sense for single stage to orbit, and because of the rocket equation, SSTO doesn’t make much sense itself.

First, Aerospikes only have better efficiency over staged rockets over part of the launch to orbit. At sea level it has vurtually no advantage, it’s not until Tye rocket ge s to thin air that the Aerospike efficiency is higher. But guess what happens shortly thereafter? The first stage is dropped and a vacuum optimized second stage takes over, again reducing the aerospike efficiency advantage to near zero. And that slightly better efficiency over part of the flight isn’t free, it costs extra weight over bell shaped nozzles,

So the big win is for Aerospikes is SSTO, that’s where it’s far more efficient than fixed nozzles. But because of the rocket equation, SSTO doesn’t make sense. Dropping off a big chunk of mass half way up (the first stage) provides substantial efficiency gains.

Musk has shown that the key to making space flight affordable isn’t breakthroughs in engine performance, and its not reusability, at least not yet. Before SpaceX ever landed a booster they were 30x cheaper than the Shuttle, and 20x cheaper than the SLS. How did they do it when the Falcon 9 engine has very average performance, and when they didn’t reuse any of them?

The reason is same as Henry Fords, assembly line manufacturing. They mastered making Merlins cheaply by building them in high volumes and constantly improving at it. Nine engines per rocket made that possible, just as it’s a key to their making reuse work.

phkahler• 8 years ago

It looks like SpaceX are perfectly capable of doing simulation and testing of an engine like this. If they have it would be great to see a presentation on their results and conclusions.

valuearb5• 8 years ago

They’d tell you that staging reduces the efficiency if Aerospikes enough that they are not worth the weight penalty.

qdonnellan2• 8 years ago

We used an aerospike nozzle for our senior design project at Texas A&M. I thoroughly enjoyed the research and theory behind the concept, but when it came down to actually building the thing we ran into a lot of - obvious in hindsight - complexity.

The nozzle must be made out of graphite, the manufacturing of which is much more complicated than aluminum, so throw out the door any pre-conceived notions you may have that making any shape is possible. Our nozzle was a three-part component: the inner plug (think of one of those "nose suckers" you have to get boogers out of a baby's nose) and the outer nest in which the plug sat. Between the nest and the plug you have "struts" supporting the plug to allow for a gap (through which your fuel will be exhausted). The plug is secured to the nest via some high-heat epoxy.

So yeah, way more complicated than a single bell of graphite, which is probably the #1 reason why these haven't seen industry use (the cost of complexity must be weighed against the cost of inefficiency, both are important factors in any rocket application).

I am very interested in the "linear aerospike"[1] engine though, which would obviously have design implications on the rest of the spacecraft, but manufacturing a linear aerospike would be much simpler than what we attempted.

[1] https://en.wikipedia.org/wiki/Aerospike_engine [The main image]

newnewpdro• 8 years ago

Does 3D printing make a difference here?

qdonnellan2• 8 years ago

Potentially! Actually Relativity Space [1] is attempting to do just that (I don't believe they manufacture aerospikes, but I don't see why they couldn't).

[1] https://www.relativityspace.com/

[EDIT] If you're reading this and you work at Relativity, I'd love to know if you actually 3D print your nozzles...

burfog2• 8 years ago

I can see that Texas A&M made you use graphite, but otherwise I don't see why the nozzle must be made out of graphite.

For a turbine blade, you'd use a single-crystal superalloy. The machining is just 3D printing of wax; you then use a lost-wax process to cast it. Cooling channels are built into it.

That ought to work. You don't even need a particularly corrosion-resistant superalloy if you cool it with liquid methane and then let that leak out to form a moving film on the surface.

avmich2• 8 years ago

Cooling channels are built into it.

How would you build cooling channels into something like this?

ThenAsNow6• 8 years ago

Regeneratively cooling nozzles (i.e., circulating liquid or gaseous propellant in embedded cooling channels) is standard fare for liquid rocket engines. GP used graphite because it was a student project. Probably not what would be done for a real launch vehicle.

One thing to keep in mind about spike nozzle cooling is that you get compression shocks from the aerospike flowfield that strike the centerbody. These shock impingements drive up local heat transfer rates. As the flowfield changes with increasing altitude (decreasing ambient pressure), the impingement locations move. So when you size your cooling capacity, you have to account for this which tends to make the spike "overcooled" as compared to what you'd have to design for with the bell. This tends to correlate with increased pressure losses from the associated high coolant flow rates, causing a system-level mass hit and detracting from the nozzle Isp efficiency benefit. Take a look at XRS-2200 test video to watch the spike ramps ice over hard after engine shutdown. While not unique to that engine, the magnitude of that effect has a lot to do with how heavily cooled the ramps were.

caio1982• 8 years ago

Because two stage rockets are now cheaper and easier to develop given track records of bazillions of launches until today. Reusable rockets have substantially reduced interest in aerospike design because it simply does not solve an actual problem anymore.

jl2718• 8 years ago

One thing I never understood about SpaceX is whether it’s really worth it to build a multi-use rocket. It’s a really simple design, and the materials are not that expensive. When I was studying this stuff, it seemed that we were moving in the direction of things like cheap self-consuming solid fuel engines. It’s just a controlled explosion; seems like trying to create a refillable stick of dynamite. Not only that, but reuse increases weight, which increases size, which increases weight.... We all knew why launches were overpriced, and it had nothing to do with the cost of the fuselage.

It seemed like Elon didn’t understand a lot of fundamentals, and I observed that he had a lot of trouble recruiting Astro engineering experts because of this, so he ended up with mostly young MechE students out of Cornell’s SAE. Retro-propulsive landing is an interesting show piece, but how does it change the economics? Note that SpaceX is still being propped up by NASA COTS, so it’s far too early to call it a commercial success.

Can anybody better-informed help me understand what we’ve learned since about the advantages of reuse?

exelius• 8 years ago

It’s not the cost of the fuselage itself, but the infrastructure to build it and the throughput with which they can build them.

If you want to increase the number of rocket launches, you need to increase the number of rockets. We will always launch them as fast as we can build them, and it’s probably cheaper to develop reusable rocket tech than it is to build a single additional rocket production facility. It makes every production facility afterwards that much more efficient.

It was a great economic move if your goal is to increase the number of launches as quickly as possible and make spaceflighy routine.

nkoren• 8 years ago

Note that SpaceX is still being propped up by NASA COTS, so it’s far too early to call it a commercial success.

I'm sick of anti-Elon hysteria, and I while I respect differences of opinion, I downvote FUD that's based on misinformation. SpaceX commercially bid for, and won, a launch services contract, which they are delivering at a considerably lower cost than the competition. Being paid for a commercial service is not the same as being subsidized -- And in any case, NASA is not their major source of revenue.

But if your question is sincere: the cost of the fuselage (and engines, and communications and control systems, and the considerable amount of tech that goes into rockets) are considerable. Scattered around various tweets, Elon has indicated that the breakdown of costs for a (pre-Block 5) Falcon 9 are as follows:

- Payload fairing: $6M

- 2nd stage: $12M

- 1st stage: $24M

- Everything else -- propellant, operations, permits, etc.: $6M

So recovery of the 1st stage would save about 50% of the launch costs, if such a recovery were itself cost-free. Obviously it does require some extra hardware, propellant, and operational complexity -- but as long adds less than $24M per launch, it's a net positive. I've seen no suggestion that it adds more than a few million per launch.

pjc50• 8 years ago

Reusable rockets make sense for the same reason that reusable aircraft make sense.

bewo001• 8 years ago

Reusable airplanes would make less sense if they had to fly back empty all the time..

perl4ever• 8 years ago

Does anybody believe SpaceX is getting a subsidy relative to any of its competitors? If not, I don't see this common criticism as making any sense.

valuearb5• 8 years ago

SpaceX isn’t getting subsidized, and it’s costs were a fraction of other commercial providers even before reuse. It’s because Elon understands more about the economics of space launches than anyone, and he focused them on manufacturing efficiencies.

Liquid engines are far more flexible that solid, and due to the Falcon 9 design using nine engine, SpaceX has built over 500 Merlin engines. That gave them an assembly line approach instead of a hand building approach, which led to both massive cost reductions, and continuous improvement processes.

That nine engine design was also key to mastering retro-propulsion, because it allows for the deep throttling required.

And if you don’t understand the quantum leap reusability is, you don’t have any understanding of launch economics. Solid rockets will never be economic because they aren’t reusable. Every launch, SLS is going destroy $500M of hand built custom rocket parts (ignoring another couple billion in development and operating costs).

If the BFR cost $500M each, and can fly 100 tines with only minor refurbishment, that’s only $5M per launch. With refurb, operating and fuel costs, total launch cost would be near $10M, for a cargo capacity 59% bigger than the SLS.

Reusability brings flight costs close to fuel costs, which are trivial. The Saturn V cargo cost was about $10;000 per pound, the Shuttle $40,000 per pound, commercial rockets were around $5,000-$10,000 per pound. The Falcon 9 is under $1,500 per pound, the Falcon Heavy under $1,000 per pound, and if the BFR concept works it will cost between $20-$50 per pound.

jpk• 8 years ago

One of the premises of this video is wrong. The author claims that rockets are staged primarily because the engine bells need to be a different shape at different altitudes to maximize their efficiency.

While it is true that engines designed for atmospheric and vacuum flight employ different bell shapes for the reasons the author describes, this is not why rockets are staged. The primary driver behind staging is to shed a portion of structural mass after it's no longer needed.

For instance, there's no reason most rockets couldn't be designed to lift their payloads directly to orbit in one stage, but by the time the vehicle reaches the end of its flight profile, it would be accelerating a great deal of structural mass to orbital velocity before deploying the payload. The near empty fuel tanks, large engines needed to lift all that fuel you've since consumed, etc are all dead weight at that point. If instead the vehicle drops that mass mid flight and continues on to orbit in a much lighter configuration, the total possible deltaV of the system is greatly increased. Using second stage engine with a bell designed for vacuum only makes sense here, but that's not why rockets are staged.

Aerospike Engines: Why aren't we using them now? [video]

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