Firefly Space Systems Aerospike

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Firefly Space Systems Aerospike

Postby rick m » Mon Dec 01, 2014 10:18 am

This sounds like a new team for going into orbit using an aerospike.

http://arstechnica.com/science/2014/11/ ... rth-orbit/

I wish them success.

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Re: Firefly Space Systems Aerospike

Postby Jay » Sat Dec 06, 2014 10:31 pm

I wish them success too, but they seem to be schooled in the same “minimize weight/maximize performance” mentality of the last fifty years. That’s not surprising since I notice a couple of their technical gurus came from Lockheed Martin and Rocketdyne. Carbon fiber composites are very expensive not only in terms of raw materials, but also in terms of the time, equipment and the fabrication expertise required to produce precise and reliable finished components.

I’m inclined to favor the Minimum Cost Design (MCD) protocols first validated in the 1960’s. Although the MCD argument that costs can be spectacularly reduced by sacrificing performance was specifically directed toward the development of large numbers of heavy-lifters, I’m convinced the same principles can be applied to high-volume spaceflight companies carrying smaller payloads as well. The MCD concept compensates for the higher weight of low-tech aluminum and steel simply by making the rocket bigger. The increased fuel cost is trivial compared to the dramatically reduced fabrication costs. A spaceflight company planning to make hundreds of launches will benefit from the lessons of commercial mass production, which epitomizes minimum cost design.

I like Firefly’s liquid methane/liquid oxygen combination, which is completely consistent with MCD. Across the board the savings are great.

Central to the MCD concept is the use of simple low-cost pressure-fed engines instead of complex and pricey pump-fed designs. Ars Technica clearly describes the Firefly fuel tanks as self-pressurizing and the alpha engine appears to be pressure-fed rather than pump-fed, but the article did not make that quite as clear. A pressure-fed aerospike configuration is certainly compatible with the MCD protocols.

Unfortunately, whenever I look at “plug cluster aerospike” engines, I see bell-nozzles -- lots of them -- which would seem to be counter productive. Everyone knows the arguments favoring aerospike engines in theory, but reducing the theory to practice has proven to be elusive. Don’t get me wrong; Firefly’s engineers are no slouches, but I won’t believe the brochure until I see the data.

MCD was ultimately rejected in favor of the pork barrel space shuttle program based on the conclusions of a couple of clearly biased studies by aerospace industry and government insiders with vested interests in high-priced spaceflight. That bias has continued ever since. Today, the same vested interests feed the media with soundbites describing MCD as old news and no longer relevant with new technology. Even the Ars Technica article parrots the party line, “Metal rockets are for suckers.” The fact that we have abandoned the space shuttle in favor of fifty-year-old concepts such as expendable rockets and crew capsules splashing down in the ocean seems to have escaped their notice. Newer is not always better.

Nevertheless, the economic and physical constraints underlying MCD are inescapable and with the ongoing commercialization of spaceflight services the first company to embrace the protocols will leap ahead of the competition. Other companies will then be forced to adopt MCD as well and spaceflight costs will plummet as a result.

There, that should stir some debate!

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Re: Firefly Space Systems Aerospike

Postby rick m » Mon Dec 08, 2014 6:45 am

Jay wrote: ...Carbon fiber composites are very expensive not only in terms of raw materials, but also in terms of the time, equipment and the fabrication expertise required to produce precise and reliable finished components.

Because we are using a very low Isp propellant in Sugar Shot to Space, we 'had been using' lighter weight composite motor cases that were more expensive in cost and time.

Jay wrote:I’m inclined to favor the Minimum Cost Design (MCD) protocols first validated in the 1960’s...The MCD concept compensates for the higher weight of low-tech aluminum and steel simply by making the rocket bigger. The increased fuel cost is trivial compared to the dramatically reduced fabrication costs.

Most of what we're doing now is aluminum and steel and discarding the 'dual-pulse' motor design that has proven problematic and are going with a conventional 2-stage rocket to reach space. We shall see if that choice will be successful for us.

Jay wrote:Central to the MCD concept is the use of simple low-cost pressure-fed engines instead of complex and pricey pump-fed designs. Ars Technica clearly describes the Firefly fuel tanks as self-pressurizing and the alpha engine appears to be pressure-fed rather than pump-fed, but the article did not make that quite as clear. A pressure-fed aerospike configuration is certainly compatible with the MCD protocols.

Sounds like instead of a pump they will have to pressurize the entire rocket (the tanks). That seems like it will add weight unless eliminating pumps makes up for that; certainly this sounds like a 'simpler' system.

Jay wrote:Unfortunately, whenever I look at “plug cluster aerospike” engines, I see bell-nozzles -- lots of them -- which would seem to be counter productive. Everyone knows the arguments favoring aerospike engines in theory, but reducing the theory to practice has proven to be elusive.

I saw all those 'bell-nozzles' and though simpler and less weight than larger ones, I wonder if the aerospike function will actually work or is this simply a cluster like design (Falcon 9) without the central nozzles. We are developing a very simple aerospike for our Sugar Shot motor that if successful will truly economize the nozzle in terms of mass and fabrication. Though our motors burn for short periods of time, I think this nozzle design could also be used for longer burning liquid propellant motors since the fuel and oxidizer could be plumbed in for heat transfer.

Jay wrote:Even the Ars Technica article parrots the party line, “Metal rockets are for suckers.” The fact that we have abandoned the space shuttle in favor of fifty-year-old concepts such as expendable rockets and crew capsules splashing down in the ocean seems to have escaped their notice. Newer is not always better.
There, that should stir some debate!

I see all these 'new propulsion' ideas to get around chemical rockets to space but don't see any actually working (at least for getting of the surface of our planet). As for metal vs composites, each has its own ideal application. I saw something about 3-D printing rockets from plastics without the need for metals...we shall see. Currently we have been using steel bell-nozzles because they are easy to machine and the lower combustion temperatures of sugar allows us to. We did embark on experimenting with composites and ablatives for reducing the weight but I'm ready to pull the plug on that testing because of time and expense.

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Re: Firefly Space Systems Aerospike

Postby Jay » Tue Dec 09, 2014 7:37 pm

I’ve said it before, but it bears repeating, the dedication and hard work of the whole SugarShot team is truly impressive. You guys are leaving no stone unturned in your quest for ‘sweet’ success! Anything I might say here you probably already know, but I’ll babble on for the sake of others who might be quietly following the conversation.

rick m wrote:…we 'had been using' lighter weight composite motor cases that were more expensive in cost and time. ...Most of what we're doing now is aluminum and steel…


Many people are surprised to find that composites often prove to be undesirable. Compared to the low cost and ease of traditional work with aluminum or steel, carbon fiber and other composites can be really expensive and finicky materials.

The argument put forward for carbon fiber by Firefly and everyone else is basically “low weight and high strength equal better performance.” Even though your sugar propellant does have a very low specific impulse, the acceleration at launch is still tremendous. Many people seem to think carbon fiber or some other composite is “obviously the only way” to achieve the necessary balance between strength and weight, but SugarShot appears to have unexpectedly confirmed the Minimum Cost Design (MCD) analysis that the advantages of carbon fiber are rarely worth the trouble and cost. Newer does not always equal better.

MCD works by compensating for increased weight with increased size; so the concept scales up, but not down. In other words, although larger stronger tanks and more fuel cause the weight to increase, the thrust increases faster than the weight. This is a simplified explanation of course, but the result is that the increase in fuel cost is trivial compared to the tremendous reduction in fabrication cost, as well as other expenses related to the more complex launch requirements of pump-fed systems.

Keep in mind these observations are in response to the reference to the Firefly alpha engine, which uses the LNG/LOx propellant combination. The solid-fuel costs for the SugarShot program don’t exactly fit this ‘mold.’ The MCD protocols are based only on the analysis of liquid-fuel/liquid-oxidizer systems.


rick m wrote:…we're…discarding the 'dual-pulse' motor design that has proven problematic and are going with a conventional 2-stage rocket to reach space. We shall see if that choice will be successful for us.


Three cheers to the SugarShot “Rocket Boys” for doing the hard work of investigating a tantalizing concept! Even if you didn’t get the results you wanted the experiments with the dual-pulse engine were well worth the effort because you discovered and documented previously unknown information about a very challenging rocket motor configuration. Two-stage is a tried and true technique which I’m sure will serve you well.


rick m wrote:I wonder if the aerospike function will actually work or is this simply a cluster like design (Falcon 9) without the central nozzles.


I wonder too. The argument for the aerospike concept is that eliminating the bell nozzle and “inverting” the exhaust jet to form an aerospike significantly increases efficiency during the atmospheric stages of the flight. An engine that relies on a cluster of bell nozzles “hanging out in the wind” to generate the aerospike would appear to be self-contradicting. It seems like the worst of both worlds. I’ll be happy to be proven wrong though and I’m sure the designers at Firefly will be happy to do it!


rick m wrote:We are developing a very simple aerospike for our Sugar Shot motor that if successful will truly economize the nozzle in terms of mass and fabrication. Though our motors burn for short periods of time, I think this nozzle design could also be used for longer burning liquid propellant motors since the fuel and oxidizer could be plumbed in for heat transfer.


A low-cost solid-fuel aerospike motor would be amazing. No matter whether the rocket is a liquid-fuel tortoise or a solid-fuel hare, as long as the jet of exhaust maintains the aerospike form at all altitudes during the rocket’s rise through the atmosphere the theoretical benefits of aerospikes remain the same. Would both SugarShot stages use the aerospike design, or just the first stage?


rick m wrote:Sounds like instead of a pump they will have to pressurize the entire rocket (the tanks). That seems like it will add weight unless eliminating pumps makes up for that; certainly this sounds like a 'simpler' system.


The Firefly alpha motor does appear to be pressure-fed. If so, their “plug cluster aerospike” should be simple, fairly inexpensive and reliable. Not as simple or as cheap as a single nozzle, but we’ll see.

Yes, pressure-fed engines do rely on heavier pressurized tanks instead of pumps, but eliminating the pumps is not intended to make up for the added weight of the tanks. Instead, you allow the whole rocket to be larger and heavier to make up for the reduced performance. Extensive testing confirms that larger and heavier but simpler rockets can carry the same payloads as minimum-weight/maximum-performance boosters at a fraction of the cost. Under MCD, cost is the enemy, not weight.

The mantra of the very expensive status quo for the last fifty years has been, “Save weight. Save weight. Save weight. At all cost!” The mantra of MCD is, “We can deal with the extra weight. Save money! Save money! Save money!” Since reducing cost is the only way to democratize access to space, cost must be the governing criterion for design, not performance.


rick m wrote:I see all these 'new propulsion' ideas to get around chemical rockets to space but don't see any actually working (at least for getting of the surface of our planet).


Exactly right. For any conventional payload, the cheapest proven way to get off the surface and into orbit is a multi-stage pressure-fed liquid-fuel expendable rocket built according to the principles of Minimum Cost Design. Advances since the 1960’s in materials engineering and computerized production such as CNC machining and additive manufacturing (3-D printing) only make MCD even more valid than ever before.

Government agencies (and their contractors) have always benefited from maximizing cost, but unsubsidized commercial competitors (in any business) live or die by minimizing cost. As space transportation becomes more and more privatized, companies like Firefly Space Systems can not afford to ignore the wisdom of MCD as readily as NASA, the U.S. Air Force and the aerospace industry’s “members-only club” have done for the last fifty years.

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