From owner-skunk-works-digest@netwrx1.com Fri Apr 2 14:30:45 2004 Date: Fri, 02 Apr 2004 13:27:02 -0600 From: skunk-works-digest Reply-To: skunk-works@netwrx1.com To: skunk-works-digest@netwrx1.com Subject: skunk-works-digest V13 #8 skunk-works-digest Friday, April 2 2004 Volume 13 : Number 008 Index of this digest by subject: *************************************************** Re: Scram Jet Re: Scram Jet RE: NASA's X-43A Proves Hypersonic Scramjet Flight RE: Scram Jet RE: Scram Jet RE: Scram Jet Re: Scram Jet Re: Scram Jet Francis Gary Powers' "Operation Overflight" again in print RE: Scram Jet RE: Scram Jet TEAL DAWN factoids TEAL RAIN, AMBER, Global Hawk, Predator Re: TEAL DAWN factoids Re: TEAL DAWN factoids Re: TEAL DAWN factoids *************************************************** ---------------------------------------------------------------------- Date: Sun, 28 Mar 2004 14:29:13 +0100 From: "David" Subject: Re: Scram Jet First of all: sincere congratulations to the X-43 team for it's milestone. I hope it will encourage others working on scramjets to keep the faith. We'll never know what benefits scramjets might offer if we don't fly 'em. A brief answer to your question Frank: Maintaining even combustion in a scramjet engine was explained to me by a hypersonics expert as "..like trying to light a match in a hurricane." You're right of course, a scramjet (in principle) is a simple duct with no moving parts. However, in reality, it's a deceptively-some would (justifiably) call it a fiendishly simple engine. The devil however, is in the detail. Designing a duct geometry/injector layout that will enable efficient, even fuel injection into the M+ airflow is unimaginably difficult without causing a shockwave that will 'choke' the engine. The airflow through a scramjet will shock down to Is the scramjet really that complicated? It seems like a rather simple > device that ought to work provided that you can get it up to its necessary > speed and keep it pointed into the airstream. In the first test, those > issues caused a failure; in the second, it worked as predicted. No doubt I > am missing something (probably quite a lot.) > > > > From what I understand, in essence, a scram jet is a heat-resistant funnel > that captures air and compresses it until, at the narrowest point, hydrogen > is injected and the ambient heat causes the mixture to ignite and expand. > After the ignition point, the tube enlarges again to accommodate the > expanding gas and the heated gas escapes out of the rear of the 'engine.' > In very simple terms, a scramjet is an hourglass shaped tube into which > hydrogen is injected at the narrowest point t when in a fast-enough > airstream for the hydrogen to be ignited by the highly compressed air. The > expanded gas drives the device forward which captures and compresses more > air continuing the process until the hydrogen is exhausted (or turned off.) > There are, as I understand it, no moving parts except, perhaps, a valve to > turn on and off the hydrogen flow. > > > > Other than the obvious problems of attaining the necessary speed and > controlling the vehicle at that speed (neither of which are unique to > scramjets), are there any moving parts or complications? Of course, there > are also materials problems caused by the heat but, again, those are hardly > unique to this new technology. So what is the significance of yesterday's > test other than as a limited proof-of-concept? ------------------------------ Date: Sun, 28 Mar 2004 12:53:07 -0700 From: Brad Hitch Subject: Re: Scram Jet The scramjet is essentially a desperate response to the incredible pressures and temperatures encountered in a subsonic-diffused ramjet operating at high Mach numbers. It is very difficult to get the heat release neccesary to obtain efficient propulsion or even a positive net thrust with combustor velocities in excess of Mach 2. There is a huge amount of ram drag and total pressure loss due to boundary layer friction throughout the engine as well as losses induced by the shocks and shear layers associated with fuel/air mixing, making it very difficult to fully expand the flow in the exhaust nozzle again without introducing alot of external wave drag. Obtaining efficient mixing and combustion heat release with a residence time of a millisecond or so is very hard to do. The heat content of the fuel combustion process is dwarfed by the kinetic energy of the incoming air stream. Shock angles are low and inlets become very long at high Mach = lots of surface area to cool and boundary layer shear surface area to degrade the total pressure of the flow. And since it's not a rocket, you only have the fuel flow (no help from cryogenic oxidizer) to cool everything. Look at Heiser and Pratt's "Hypersonic Airbreathing Propulsion" book for a more in-depth discussion that still doesn't give you enough info to design a working system: The acceleration from separation to Mach 7 was achieved by diving, not by engine thrust. Watch for a specific impulse number to come out. They know exactly what flight profile they were expecting given the predicted engine performance as well as how the measured performance should correspond to a full-size engine. Predicted full-size engine Isp is one of the first things they will calculate from the test data because it is such an important figure of merit. Even though it was over a weekend, the silence about performance so far makes me suspicious that it didn't do as well as they had hoped. It's not clear to me that this is really a problem worth solving since rockets such as ICBM's and IRBM's with improvements in guidance technology could probably perform the mission of a hypersonic cruise missile as well or better at less overall development cost. Don't get me wrong, though - I'm not opposed to research. This is a sexy and difficult problem - most things you try at this level won't work, but you *will* learn interesting and useful things in the process. It was a well-run and not very expensive program. Chuck McClinton rocks. It is a research effort that is more akin to the nuclear fusion program, however. Don't hold your breath waiting to buy a ticket on that Orient Express or a Mr. Fusion for your home. Brad Hitch David wrote: > First of all: sincere congratulations to the X-43 team for it's > milestone. I hope it will encourage others working on scramjets to > keep the faith. We'll never know what benefits scramjets might offer > if we don't fly 'em. > > A brief answer to your question Frank: > > Maintaining even combustion in a scramjet engine was explained to me > by a hypersonics expert as "..like trying to light a match in a > hurricane." You're right of course, a scramjet (in principle) is a > simple duct with no moving parts. However, in reality, it's a > deceptively-some would (justifiably) call it a fiendishly simple > engine. The devil however, is in the detail. Designing a duct > geometry/injector layout that will enable efficient, even fuel > injection into the M+ airflow is unimaginably difficult without > causing a shockwave that will 'choke' the engine. The airflow through > a scramjet will shock down to > The University of Queensland's vice-chancellor professor John Hay > summed up the problem perfectly when asked about the importance of the > 'HyShot' scramjet flight test programme it was conducting with QinetiQ > of the UK and various international industrial partners: > > "The objectives of the HyShot program can be equated to breaking the > sound barrier in flight, but for HyShot, it will be the combustion > barrier that is broken." > > A key benefit of the X-43 flight is the length of time the engine > could be studied in free flight - I believe around ten seconds, > compared with the duration shock tunnels tests where functionality is > measured in milliseconds. > > Hope this helps a little - oh - and it's nice to see the list working > again. > > Hit it Larry...! > > David > > > > >>Is the scramjet really that complicated? It seems like a rather > > simple > >>device that ought to work provided that you can get it up to its > > necessary > >>speed and keep it pointed into the airstream. In the first test, > > those > >>issues caused a failure; in the second, it worked as predicted. No > > doubt I > >>am missing something (probably quite a lot.) >> >> >> >>From what I understand, in essence, a scram jet is a heat-resistant > > funnel > >>that captures air and compresses it until, at the narrowest point, > > hydrogen > >>is injected and the ambient heat causes the mixture to ignite and > > expand. > >>After the ignition point, the tube enlarges again to accommodate the >>expanding gas and the heated gas escapes out of the rear of the > > 'engine.' > >>In very simple terms, a scramjet is an hourglass shaped tube into > > which > >>hydrogen is injected at the narrowest point t when in a fast-enough >>airstream for the hydrogen to be ignited by the highly compressed > > air. The > >>expanded gas drives the device forward which captures and compresses > > more > >>air continuing the process until the hydrogen is exhausted (or > > turned off.) > >>There are, as I understand it, no moving parts except, perhaps, a > > valve to > >>turn on and off the hydrogen flow. >> >> >> >>Other than the obvious problems of attaining the necessary speed and >>controlling the vehicle at that speed (neither of which are unique > > to > >>scramjets), are there any moving parts or complications? Of course, > > there > >>are also materials problems caused by the heat but, again, those are > > hardly > >>unique to this new technology. So what is the significance of > > yesterday's > >>test other than as a limited proof-of-concept? ------------------------------ Date: Sun, 28 Mar 2004 18:23:52 -0800 From: "Smith, Larry O" Subject: RE: NASA's X-43A Proves Hypersonic Scramjet Flight Yes he is !! He's doing cartwheels !! My aching back !! :) I'd love to find out more details, but the phrase "positive acceleration" is the keyword here. If you can get some acceleration, you can obviously do hypersonic cruising (where thrust matches drag). For them to get acceleration at M7 (where thrust overcame drag), and do some climbing as well, they did their jobs well. I'd love to know more !! So based on the above, we have at least begun a new era here, because at least, vehicles that boost themselves with a rocket and then cruise with a scramjet are now proven feasible. And there are programs to integrate a rocket with a scramjet, as well as a turbine engine with a scramjet, to get the scramjet up to cruise or lightoff speed. The next flight is supposed to be to Mach 10, to get some acceleration there. And I might add, the X-43A shape is a shape more amenable to an eventual manned vehicle than the other shapes that have flown previously, which were missile configurations. Not for a minute to minimize those successes, mind you. The DAWN of a new AGE !! Finally, the dreams of the late 50's are becoming a reality !! I'm very pleased !! I'll be watching for more technical info. I'll comment more later. Regards, Larry - -----Original Message----- From: owner-skunk-works@netwrx1.com [mailto:owner-skunk-works@netwrx1.com] On Behalf Of Andreas Gehrs-Pahl Sent: Saturday, March 27, 2004 11:33 PM To: Skunk Works List Subject: NASA's X-43A Proves Hypersonic Scramjet Flight I bet Larry was very happy to read this NASA Dryden Flight Research Center Press Release about the successful X-43A flight: NASA News ... March 27, 2004 ... RELEASE: 04-16 NASA's X-43A Proves Hypersonic Scramjet Flight ... - -- Andreas - --- - --- Andreas Gehrs-Pahl E-Mail: GPahl@CharterMI.net 415 Gute Street or: Andreas@DDPSoftware.com Owosso, MI 48867-4410 or: Andreas@Aerospace-History.net Tel: (989) 723-9927 Web Site: http://www.Aerospace-History.net - --- - --- ------------------------------ Date: Mon, 29 Mar 2004 00:17:20 -0800 From: "Smith, Larry O" Subject: RE: Scram Jet Hello Brad, long time since we've talked. >The acceleration from separation to Mach 7 was achieved by diving, not by engine thrust. This doesn't seem to agree with what Huebner said in the press release: "We achieved positive acceleration of the vehicle while we were climbing, ...". I'm anxious to hear more detailed information though, as well. Larry ------------------------------ Date: Mon, 29 Mar 2004 01:56:14 -0800 From: "Smith, Larry O" Subject: RE: Scram Jet >The scramjet is essentially a desperate response to the incredible pressures and temperatures >encountered in a subsonic-diffused ramjet operating at high Mach numbers. I don't think it's desperate. I think it's rather elegant. Methods to cool subsonic diffused high supersonic speed inlets sound more desperate to me, as neat as those ideas are. >The heat content of the fuel combustion process is dwarfed by the kinetic energy of the incoming >air stream. ... >Look at Heiser and Pratt's "Hypersonic Airbreathing Propulsion" book for a more in-depth >discussion that still doesn't give you enough info to design a working system: For guidance here, in Heiser and Pratt, look at the M10 scramjet modelled by the H-K (Enthalpy-Kinetic Energy) diagram on page 84 (Fig. 2.19). Yes, this is a perfect case modelling of a scramjet flying at M10. But it illustrates what's trying to be accomplished here given some of the parameters your mentioning, namely fuel heating and kinetic energy. Summarizing greatly, the M10 scramjet process modelled by that diagram has the heat release from the fuel causing a total temperature change in the air of 1.4X. After expanding that total temperature in the nozzle, the kinetic energy change from inlet freestream to exhaust has been around 1.2X (including total pressure losses - unclear exactly which). Thus the thrust. This assumes an inlet with high kinetic energy efficiency, which scramjets can do. You're point about minimizing wave drag in the inlet and nozzle is well taken, as well as the goal of minimizing skin friction drag. These are optimizations that will improve with time. >It's not clear to me that this is really a problem worth solving since rockets such as ICBM's and >IRBM's with improvements in guidance technology could probably perform the mission of a hypersonic >cruise missile as well or better at less overall development cost. Don't get me wrong, >though - I'm not opposed to research. We've done enough simulations of scramjets. It's time to fly and develop the technology for real. We need to give it to the hypersonics nuts (people like me) who will work on it like the Wrights did their flyer. Larry ------------------------------ Date: Mon, 29 Mar 2004 08:49:02 -0800 From: Erik Hoel Subject: RE: Scram Jet Hello all. Is there a decent web site that discusses scramjets in terms that a Walter Middy-like aerospace fan like myself can understand? A few pictures would also be nice. Thanks, Erik ------------------------------ Date: Mon, 29 Mar 2004 12:41:18 -0700 From: Brad Hitch Subject: Re: Scram Jet NASA-Glenn has a nice overview of airbreathing propulsion, though not on the scramjet in particular. A scramjet is a ramjet in which the inlet air flow is not diffused to subsonic velocities but is instead maintained supersonic (for the most part) throughout the engine. This avoids turning the engine into a heavy pressure vessel but introduces problems with getting good fuel injection, mixing and combustion heat release in the very short time that the air is in the engine. Engine structural cooling is a real problem as well. Brad Hitch Erik Hoel wrote: > Hello all. > > Is there a decent web site that discusses scramjets in terms that a Walter > Middy-like aerospace fan like myself can understand? A few pictures would > also be nice. > > Thanks, > Erik ------------------------------ Date: Mon, 29 Mar 2004 09:52:29 -0700 From: Brad Hitch Subject: Re: Scram Jet Smith, Larry O wrote: > Hello Brad, long time since we've talked. Hi Larry - the mail list still works! > > >>The acceleration from separation to Mach 7 was achieved by diving, not > > by engine thrust. > > This doesn't seem to agree with what Huebner said in the press release: > > "We achieved positive acceleration of the vehicle while we were > climbing, ...". > > I'm anxious to hear more detailed information though, as well. > > Larry > > > IIRC from briefings I have heard at conferences I have attended and conversations I have had with Chuck McClinton (NASA Langley), they expected to have to dive to maintain Mach 7 since the flow path in the vehicle is very small and has lots of skin friction - essentially measuring thrust by the change in acceleration/deceleration of the vehicle. I admit I don't know the details of what maneuvers were actually performed to trim the velocity after separation to hit their Mach 7 condition, though. I can't seem to find much on it on the web. Proving a positive net thrust in free flight with unvitiated air would certainly be a great accomplishment. The hydrocarbon-fueled X-43C was recently canceled, by the way. Brad Hitch ------------------------------ Date: Mon, 29 Mar 2004 14:24:14 -0600 From: "Dan Lasley" Subject: Francis Gary Powers' "Operation Overflight" again in print All, About once a month Francis Gary Powers, Jr., distributes an email in which he talks about the Cold War museum he is attempting to establish at an old Nike missile base in Virginia. In his most recent email (copied below), he announces: "... my father's autobiography, Operation Overflight, has been republished for the first time in over 30 years." I sent him a check for the book the next day (see his email for ordering information). The story of FGP's flights over the former USSR, his being shot down, and his trial, imprisonment, and release is fascinating--it's one of the few books I've picked up recently and just could not put down. Highly, highly recommended. Regards, Dan P.S. Other than supporting it with my hard earned cash, I have no affiliation with the Cold War museum. I have no affiliation with the publisher of "Operation Overflight". - ------------FGP Jr.'s email attached-------------- > Dear Friends and Supporters of The Cold War Museum, > > Volume 4, Issue 1 of our e-newsletter, The Cold War Times is available for > viewing at http://www.coldwar.org/education/coldwar_magazine.html. Our next > Spies of Washington Tour is scheduled for Saturday March 27. Visit > www.spytour.com for more information. > > The Cold War Museum continues to make great strides in their efforts to > preserve Cold War history and honor Cold War veterans. Please help spread > the word about the Museum. Together we can make this vision a reality. If > you should have any questions or want additional information, please do not > hesitate to email. Tax-deductible contributions and artifact donations to > the Museum will ensure that future generations will remember Cold War events > and personalities that forever altered our understanding of national > security, international relations, and personal sacrifice for one's country. > > FYI, I am please to announce that my father's autobiography, Operation > Overflight, has been republished for the first time in over 30 years. Copies > are available for purchase for $25 + $3.85 S&H, by replying to this email. A > portion of the proceeds will benefit The Cold War Museum. > > Please feel free to pass this on to anyone whom you think might also be > interested in our efforts. If you would prefer not to receive informational > e-mails of this kind, please type REMOVE in the subject line and you will be > removed from our database. > > Very truly yours, > > > Francis Gary Powers, Jr. > Founder > The Cold War Museum > P.O. Box 178 > Fairfax, VA 22030 > (703) 273-2381-p > (703) 273-4903-f > gpowersjr@coldwar.org > www.coldwar.org ------------------------------ Date: Tue, 30 Mar 2004 13:54:24 -0800 From: "Smith, Larry O" Subject: RE: Scram Jet There are some great sites that NASA has had on the different X-43 versions (X-43A/B/C). These have had very nice CG artwork of the different X-43 versions in different settings. There has also been some nice CG concept video of some of these vehicles operating. Let me see if I can run some of these URLs down. I recall visiting these sites about a year ago I think. If anyone else has the current URL of these don't be shy. Larry - -----Original Message----- From: owner-skunk-works@netwrx1.com [mailto:owner-skunk-works@netwrx1.com] On Behalf Of Erik Hoel Sent: Monday, March 29, 2004 8:49 AM To: skunk-works@netwrx1.com Subject: RE: Scram Jet Hello all. Is there a decent web site that discusses scramjets in terms that a Walter Middy-like aerospace fan like myself can understand? A few pictures would also be nice. Thanks, Erik ------------------------------ Date: Tue, 30 Mar 2004 19:18:56 -0800 From: "Smith, Larry O" Subject: RE: Scram Jet I found the site I visited about a year ago. Check out: http://www.fusiononline.com/space.htm Look at the RBCC and ISTAR sections for hypersonic airbreathing concepts. You may also want to download a copy of the following paper which is a good intro with some nice drawings etc, of the X-43 program, including the later variants of X-43. http://hypersonic2002.aaaf.asso.fr/papers/17_5175.pdf Larry - -----Original Message----- From: owner-skunk-works@netwrx1.com [mailto:owner-skunk-works@netwrx1.com] On Behalf Of Smith, Larry O Sent: Tuesday, March 30, 2004 1:54 PM To: skunk-works@netwrx1.com Subject: RE: Scram Jet There are some great sites that NASA has had on the different X-43 versions (X-43A/B/C). These have had very nice CG artwork of the different X-43 versions in different settings. There has also been some nice CG concept video of some of these vehicles operating. Let me see if I can run some of these URLs down. I recall visiting these sites about a year ago I think. If anyone else has the current URL of these don't be shy. Larry - -----Original Message----- From: owner-skunk-works@netwrx1.com [mailto:owner-skunk-works@netwrx1.com] On Behalf Of Erik Hoel Sent: Monday, March 29, 2004 8:49 AM To: skunk-works@netwrx1.com Subject: RE: Scram Jet Hello all. Is there a decent web site that discusses scramjets in terms that a Walter Middy-like aerospace fan like myself can understand? A few pictures would also be nice. Thanks, Erik ------------------------------ Date: Fri, 2 Apr 2004 07:06:32 -0600 From: "Allen Thomson" Subject: TEAL DAWN factoids This came up in connection with a discussion on Usenet, and I thought it might be of interest here: >From Jim Karam's page, http://www.karam.com/Photo_Gallery_Core.htm [Picture of] Convair's AGM-129A Advanced Cruise Missile, a direct descendant of one of my DARPA initiatives that was nick-named Teal Dawn[*]. Gee, I wonder where he came up with that name? And, no, this picture is not upside-down and backward. Also, don't let anyone tell you how easy it is to design forward swept wings because of today's powerful analytic software. [*] points to picture of his daughter, Dawn Which his resume http://www.karam.com/Jim_Karam's_Resume.htm dates: Program Manager, Strategic Technology Office, Defense Advanced Research Projects Agency, Rosslyn, VA (1975-1978) Conceived & executed three major advanced cruise missile technology thrusts (approx. $15M annually). "Zero-CEP" guidance incorporated active (laser & mmw) & passive sensors with sophisticated image processing. Demonstrated compound rotary and reciprocating engine concepts for reduced fuel consumption, small propulsion. More survivable airframes used radical shaping and new advanced materials, i.e., the beginnings of "stealth". Several eventually entered Full Scale Development and/or production by the Air Force and Navy. There's also a short write-up on pdf p.73 of http://www.darpa.mil/body/pdf/transition.pdf ADVANCED CRUISE MISSILE A DARPA program, TEAL DAWN, developed key technologies and a design later incorporated into the Air Force Advanced Cruise Missile (ACM). In the early 1980s, the Air Force assumed responsibility for the ACM Program and successfully managed the system through concept demonstration; engineering and manufacturing development; production; and development. The TEAL DAWN Program involved a series of studies and developments related to the development of a long-range stealthy strategic cruise missile. DARPA experience in low observables was incorporated into the design of the low-signature engine inlet and nozzle. Other technologies included the unique aerosurface sweep angles that provided a benefit to the aerodynamic performance. Clearly recognized performance goals (signature, range, flight profile)were successfully demonstrated during the DARPA phase of the program. Wind tunnel and radar ranges testing also were accomplished by the Air Force under DARPA sponsorship. The follow-on Air Force program could then focus on operational test and evaluations (OT&E) and manufacturing objectives with a high degree ofconfidence that program objectives would be realized. ------------------------------ Date: Fri, 2 Apr 2004 09:49:53 -0600 From: "Allen Thomson" Subject: TEAL RAIN, AMBER, Global Hawk, Predator Probably y'all knew this, but I didn't: http://www.darpa.mil/body/strategic_plan/strategic_text.htm The Global Hawk and Predator Unmanned Aerial Vehicles have been prominent in Operation Enduring Freedom in Afghanistan and other parts of the world. DARPA began working on Global Hawk in the 1970s as the TEAL RAIN program; the Global Hawk high altitude endurance unmanned aerial vehicle (UAV) transitioned from DARPA to the Air Force in 1998. Development of Predator began in 1984 as DARPA^Rs AMBER program. The Tier 2 Predator medium-altitude endurance UAV evolved directly from DARPA^Rs AMBER and Gnat 750-45 designs, and was operationally deployed in the mid-90s. ------------------------------ Date: Fri, 2 Apr 2004 20:29:21 +0200 From: Andreas Parsch Subject: Re: TEAL DAWN factoids On Friday 02 April 2004 15:06, Allen Thomson wrote: > > From Jim Karam's page, http://www.karam.com/Photo_Gallery_Core.htm > > [Picture of] Convair's AGM-129A Advanced Cruise Missile, a > direct descendant of one of my DARPA initiatives that was > nick-named Teal Dawn. [...] As an addendum, here are drawings of supposed TEAL DAWN designs: http://www.designation-systems.net/temp/teal-dawn.jpg The images are from J.Jones "Stealth Technology, The Art of Black Magic", 1989. Whether they have any base in reality is anyone's guess ;-). Regards Andreas ------------------------------ Date: Fri, 2 Apr 2004 13:14:57 -0600 From: "Allen Thomson" Subject: Re: TEAL DAWN factoids > As an addendum, here are drawings of supposed TEAL DAWN designs: > > http://www.designation-systems.net/temp/teal-dawn.jpg > > The images are from J.Jones "Stealth Technology, The Art of Black Magic", 1989. Whether they have any base in reality is anyone's guess ;-). B) seems to be not dissimilar to what eventually got built and was mentioned on Dr. Karam's page. ------------------------------ Date: Fri, 2 Apr 2004 13:25:04 -0600 From: "Allen Thomson" Subject: Re: TEAL DAWN factoids > The images are from J.Jones "Stealth Technology, The Art of Black Magic", 1989. BTW, not having the book, I just went to Amazon and found that their associated dealers have a bunch Real Cheap. I ordered one, Like New!, for $5.72 (Including Shipping Price). ------------------------------ End of skunk-works-digest V13 #8 ******************************** To subscribe to skunk-works-digest, send the command: subscribe skunk-works-digest in the body of a message to "majordomo@netwrx1.com". If you want to subscribe something other than the account the mail is coming from, such as a local redistribution list, then append that address to the "subscribe" command; for example, to subscribe "local-skunk-works": subscribe skunk-works-digest local-skunk-works@your.domain.net To unsubscribe, send mail to the same address, with the command: unsubscribe skunk-works-digest in the body. Administrative requests, problems, and other non-list mail can be sent to georgek@netwrx1.com. A non-digest (direct mail) version of this list is also available; to subscribe to that instead, replace all instances of "skunk-works-digest" in the commands above with "skunk-works". 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