Passenger market for suborbital flights.

I saw that this weeks Space Access '09 conference, http://www.space-access.org /, will have several presentations by companies working on suborbital
flights for tourism. According to this article, Virgin Atlantic is planning on marketing just suborbital flights at $200,000 and it reports a survey said orbital flights might be commercially viable at $500,000:
Space tourism survey targets cost factor. Online results hint at future price points for suborbital and orbital flights. By Leonard David Senior space writer updated 4:53 p.m. ET, Tues., Oct. 3, 2006 "Pricey seats. "So far, orbital space tourism has been the propelled province of well-heeled millionaires. Even for projected suborbital jaunts — up to the edge of space and return to Earth — the price tag for a Virgin Galactic spaceliner seat slaps your purse or wallet for roughly $200,000. Several key results of the space tourism survey point out: The prices of current space treks into suborbital and orbital are generally too high at present, with only 7 percent registering for a suborbital flight and 4 percent for an orbital adventure at current price levels. Suborbital flights would really take off at $25,000, and orbital flights at $500,000, if such price levels were compatible with an operator’s business plan. If price were not an issue, nearly two-thirds of the respondents would want to go on a round-the-moon adventure." http://www.msnbc.msn.com/id/15120091 /
I want to argue here that it would be feasible to provide service also for a much larger market: suborbital, hypersonic passenger flights for transcontinental and intercontinental transportation. A round trip cross-Atlantic ticket on the Mach 2 Concorde cost around $10,000. I don't think it's out of the question that a substantial number of business executives and wealthy vacationers would be willing to pay $100,000 to make a cross-Atlantic or cross-U.S. trip that took less than an hour, especially when it included making a short stint to space in the process. Likewise I think there would be a substantial market at $100,000 per ticket for a trip to Asia that only took 2 or 3 hours, compared to a full day as it does now.
You can make a calculation for how much fuel you would need for a rocket flying horizontally to reach a certain distance by using the rocket equation for velocity:
Vf -Vi = Ve*ln(Mi/Mf), where Vf, Mf are the final velocity and final mass, and Vi, Mi are the initial velocity and initial mass, and Ve is the exhaust velocity. The formula still works for intermediate points in the trip where you burned only a portion of the fuel, where Vf and Mf are the values at these intermediate times. Let's say you're burning propellant at a rate r kgs/sec. Then the mass of the vehicle at time t will be Mf = Mi-rt. I'll say the initial velocity Vi is zero, and let the velocity at time t be V(t). Then the formula becomes:
V(t) = Ve*ln[Mi/(Mi-rt)]. Then we can integrate this formula for velocity to get the distance traveled, S(t):
S(t) = Ve*t - (Ve/r)*(Mi-rt)*ln[Mi/(Mi-rt)]
This formula is for the case of constant thrust, where the acceleration will gradually increase since the mass is decreasing as the fuel is used up. It might be more comfortable for the passengers if instead we used a constant acceleration flight. This would be accomplished by making the fuel flow rate, and therefore thrust, decrease as the weight decreases. The formulas for this case can be constructed in an analogous fashion to those of the classic rocket equation. I haven't calculated it but my guess is the total fuel usage would be the same as for using the fuel at a constant rate. In any case, I will assume that just as for SpaceShipOne it will have aerodynamic shape to allow lift so that most of this propulsion can go towards providing horizontal thrust. I didn't include the drag in this first order calculation of the constant fuel rate case, but it can be added in a more detail examination. You can reduce the drag by having the craft undergo the hypersonic flight at high altitude. You can save fuel to reach this altitude by using a carrier craft such as the White Knight for SpaceShipOne. Note that you don't have to use the fuel on the carrier craft or suborbital vehicle to get to a height of say 100 km, but only to get to high enough altitude to reduce the drag and heating on the vehicle at the hypersonic velocities. XCOR is planning on using kerosene and LOX for their engines so I'll use this type of engine for getting the Ve number. Kerosene/LOX engines can have Isp of 360 s at high altitude, which I am assuming will be the only time the rocket will be used. So Ve will be in the range of 3600 m/s at high altitude. First let's say you want to go across the continental U.S., 4500 km. For a first generation transport vehicle let's say it's comparable in size to SpaceShipOne about 1,000 kg empty and 3,000 kg fully loaded with fuel to carry one pilot and two passengers. Let's put in some numbers in order to calculate the distance, S(t): say t = 2500 s, about 42 minutes, r = 1 kg/s, and Mi consists of a 1000 kg vehicle with passengers and 2500 kg fuel, for a total of 3500 kg. Then we calculate: S(t) = 3600*2500 - (3600/1)*(1000)*ln (3500/1000) = 4,490,000 meters, or 4,490 km. The time of 42 minutes compares to about 6 hours for a normal passenger jet to travel this distance. The maximum speed would be Vf = 3600*ln(3500/1000) = 4500 m/s, or Mach 15, quite a high speed. The X-15 was able to reach Mach 6.7 and was planned on being able to reach Mach 8. It had an Inconel skin with a titanium frame to resist the heat loads at these high Mach numbers. Still for Mach 15 you might need materials even more heat resistant. In this article Burt Rutan says SpaceShipOne's carbon composite structure would not be sufficient for even the Mach 6.7 speeds of the X-15:
X-15 and today’s spaceplanes. by Sam Dinkin Monday, August 9, 2004 http://www.thespacereview.com/article/204/2
Still carbon-carbon composites are used for the leading edges of the wings for the Space Shuttle which have to withstand the highest temperatures of re-entry even at Mach 25, so presumably would also work at Mach 15. These carbon-carbon composites became infamous though for how they fractured under impact by foam in the Columbia accident. It turned out they are even more brittle than fiberglass. This is a bit puzzling because the type of carbon composites used extensively for example in modern race cars is actually more fracture resistant than steel. This makes them an ideal material for race cars since they have greater strength than steel while being more fracture resistant and at a fraction of the weight. I can only assume that at the time the shuttle was being designed, these highly fracture resistant carbon composites were not available. Then the recommendation for the thermal protection is the carbon-composites of this highly fracture resistant type. For the vehicle to be useful as a transport craft it will have to be able to take-off and land at least at international airports. Airport safety managers might not be too enthusiastic about rocket takeoff at their airports, and certainly not enthusiastic towards deadstick landings. At least for the takeoffs this uncertainly be could ameliorated by the jet engine carrier craft. For the landings I suggest these rocket craft also have their own small jet engines so that they can do powered landings. There are some lightweight jet engines that could work for our 1000 kg first generation craft. For instance there is the TRS-18-1 engine that can produce 326 pounds of thrust and only weighs 85 pounds:
Microturbo TRS-18-1 Engine Specifications. http://www.bd-micro.com/FLS5J.HTM#ENGINE
Two of these would probably be sufficient for landing our 1000 kg rocket plane assuming at subsonic speeds the craft had a lift/drag ratio typical for jets, which can be at 10 and above. A more high performance and more extensively tested jet engine to use might be the PW610F. This weighs 260 pounds and can produce 900 pounds of thrust:
Pratt & Whitney Canada PW600. http://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PW600
One of these would probably sufficient for our purposes. For this more high performance engine we might even be able to use it for takeoff to reach high altitude for the rocket plane, dispensing with the need for the carrier craft. At this early stage, we would have separate jet engines and rocket engines. The jet intakes would be closed off when the rocket is operating and opened to be used only during low speed, subsonic flight. However, we can imagine with further development we would get a type of hybrid engine, as for example envisioned for the Skylon craft, where the jet and rocket engine are combined into one.
Bob Clark
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Just saw this article on Rocketplane XP, which plans to offer suborbital, tourism rocket flights, while using jet engines for take- offs and landings:
Rocketplane reset by Jeff Foust Monday, November 5, 2007
The revised Rocketplane XP design (above) is intended ultimately to be more competitive in the emerging suborbital space tourism conference. http://www.thespacereview.com/article/994/1
The Second Space Age. March 6, 2008 Patrick Mahoney "Ready for a space cruise? The technology is taxiing to the runway." http://machinedesign.com/article/the-second-space-age-0306
Rocketplane XP's current design was modified from the original Lear Jet base airframe but still has the look of a passenger business jet, with a rocket in the tail. It has some titanium and steel portions to withstand the heat of reentry in addition to an aluminum frame. This makes it heavier than a Lear Jet and it has to use a long military base runway for take-offs and landings. However, quite likely if it used all composite materials, as does SpaceShipOne, to replace the heavy steel, titanium, and aluminum it could take off and land from a standard sized airport runway.
Bob Clark
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The Europeans have also proposed a business jet model for a suborbital tourism rocket:
DATE:14/06/07 SOURCE:Flightglobal.com PICTURES: Astrium aims for 2012 suborbital tourism flights. By Rob Coppinger "The space jet will take off from a conventional runway, powered by two jet engines, and fly to 39,300ft (12,000m), where it will ignite its liquid oxygen, methane rocket engine providing an ascent acceleration of 3g. After 80s the jet will reach 196,000ft and coast to its apogee." http://www.flightglobal.com/articles/2007/06/14/214632/pictures-astrium-aims-for-2012-suborbital-tourism-flights.html
Interestingly, they consider this as a precursor to a point-to-point transport.
Another article on the proposed Astrium rocketplane:
Space planes 'to meet big demand'. By Jonathan Amos, Science reporter, BBC News Monday, 17 March 2008, 13:38 GMT "Aerospace giant EADS says it will need a production line of rocket planes to satisfy the space tourism market." http://news.bbc.co.uk/2/hi/science/nature/7298511.stm
There is a link to a nice video of a simulated flight on this page. In the video the passengers are wearing helmets with closed visors. But it doesn't look like they are wearing actual spacesuits with independent air supplies because the helmets are not connected to the rest of the suits. The helmets have more the look of motorcycle helmets. I don't know if this is really supposed
Bob Clark
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Another suborbital, tourism rocket plane based on a business jet model:
Bristol Spaceplanes - Ascender. http://www.bristolspaceplanes.com/projects/ascender.shtml
There have been several studies showing just for tourism there would be a sufficient market for such suborbital flights to be profitable. I have to think there would be a bigger market for cases where the traveler would actually want to go somewhere and this method could get him there in 1/10th the time. As a point of comparison I did a search on the Japan Airlines site for round trip business class tickets from my town of Philadelphia to Tokyo. It ranged from $6,600 to $21,000:
================================================== Select Your Flights
Philadelphia to Tokyo Thursday, April 9, 2009 Tokyo to Philadelphia Tuesday, April 14, 2009 Travelers: 1 Travel class: Business and First
Select your fare: Price differences within a fare type may be due to flight connections or availability. Prices are per adult passenger and include Taxes and Surcharges.
Fare type Fare description Lowest price Business Saver Special Restricted. Bed-style seating on most long- haul routes - Executive Class. more details $6,672.48 Business Saver Restricted. Bed-style seating on most long-haul routes - Executive Class. more details $7,611.48 Business Normal Flexible. Bed-style seating on most long-haul routes - Executive Class. more details $12,330.48 First Normal Flexible. World-renowned service and comfort - First Class. more details $21,589.48 ==================================================
Note also, that the $200,000 ticket price mentioned for suborbital flights on SpaceShipOne is only for the first few flights. After, a few years the price is expected to come down to $20,000.
Bob Clark
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In their usual good reporting, the BBC discusses directly the safety issues for these suborbital tourism flights:
Space ships: the next generation - Space Tourist- BBC Science & Nature.
http://www.youtube.com/watch?v=g_ROkapCj14&feature=related

Bob Clark
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In this animation of the SpaceShipTwo suborbital flight, the passengers do not appear to be wearing actual spacesuits, though they do have helmets:
Virgin Galactic SpaceShipTwo Animation.
http://www.youtube.com/watch?v=Cw1WaW8JsFs

Bob Clark
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Another suborbital tourism project was posted on the SpaceFellowship.com forum:
Project Enterprise. http://www.european-spacetourism.eu/index2.html
I noted in their simulated video of the trip, it has the passengers wearing oxygen masks.
Bob Clark
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This site is an informative compendium of articles related to space flight, and particularly space tourism:
SPACE FUTURE. http://www.spacefuture.com/home.shtml
Here's an article re-printed on the site on commercial orbital and suborbital flights:
New Commercial Opportunities in Space. D M Ashford The Aeronautical Journal, February 2007. "Of aeroplanes that have actually flown, and with the possible exception of SpaceShipOne, the one most suitable for providing the basis of a space tourism industry is perhaps the Saunders Roe SR.53 rocket fighter that first flew in 1957. This is probably the most practical rocket-powered aeroplane yet built. If it had entered service, the RAF would soon have had a mature rocketplane with long life and rapid turnaround. With straight-forward development, the SR.53 could have had sub-orbital performance. Indeed, when it was cancelled as a fighter in 1958, Saunders Roe did propose a space research variant(17)." http://www.spacefuture.com/archive/new_opportunities_in_commercial_space.shtml
Note that that the example of the Saunders Roe SR.53 shows that an aircraft can successfully operate with both rocket and jet engines.
Bob Clark
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More on the Saunders-Roe SR.53:
Saunders-Roe SR.53. http://en.wikipedia.org/wiki/Saunders-Roe_SR.53
Bob Clark
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wrote:

variant(17)."http://www.spacefuture.com/archive/new_opportunities_in_commercial_sp ...
More on the Saunders-Roe SR.53:
Saunders-Roe SR.53. http://en.wikipedia.org/wiki/Saunders-Roe_SR.53
Bob Clark ===================================Note that the example of the 1940s German V1 shows that an aircraft can successfully operate with both rocket and jet engines.
http://www.battlefield-site.co.uk/V1cutaway.gif
Big hairy deal.
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On the Space Future site, another interesting article:
Flight Mechanics of Manned Sub-Orbital Reusable Launch Vehicles with Recommendations for Launch and Recovery. Marti Sarigul-Klijn Ph.D. and Nesrin Sarigul-Klijn*, Ph.D AIAA 2003-0909 January 2003 (Revised April 2003)
http://www.spacefuture.com/archive/images/flight_mechanics_of_manned_suborbital_reusable_launch_vehicles_with_recommendations_for_launch_and_recovery.7.png
Lockheed NF-104 (USAF image) http://www.spacefuture.com/archive/flight_mechanics_of_manned_suborbital_reusable_launch_vehicles_with_recommendations_for_launch_and_recovery.shtml
It discusses four separate combined jet/rocket aircraft.
Bob Clark
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The launch of a military satellite at the NASA's Wallops Island facility in Virginia reminded me that commercial launches are also being made from the site:
Mid-Atlantic commercial spaceport makes 1st launch. Posted 12/16/2006 7:20 AM ET http://www.usatoday.com/tech/science/space/2006-12-16-commercialrocket_x.htm?POE=TECISVA
There was a push by some to offer a prize for a suborbital commercial transatlantic transport to launch from the site but I don't know if that is still being considered:
Hypersonic Cruise for the V Prize. Wednesday 2008.01.23 by gravityloss http://gravityloss.wordpress.com/2008/01/23/hypersonic-cruise-for-the-v-prize /
This article did not optimize the cruise velocity to get the best fuel fraction and gets a too high cruise speed of 6,000 m/s. A later article on this web site discusses how this could be optimized, giving a cruise velocity of only 3,000 m/s if you can get a high lift/drag ratio of 7 at hypersonic speeds:
Optimum Rocket Cruise. Friday 2009.03.20 by gravityloss http://gravityloss.wordpress.com/2009/03/20/optimum-rocket-cruise /
Some hypersonic waverider shapes can get a lift drag ratio in the range of 6 to 8 when optimized for a set hypersonic cruise speed:
Waverider Design. http://www.aerospaceweb.org/design/waverider/waverider.shtml
If the delta-v required is only 3,000 m/s this could be obtained with a rather low mass ratio: Mi/Mf= exp(delta-V/Ve) = exp(3,000/3600) = 2.3, assuming an exhaust velocity Ve of 3,600 m/s, which can be reached by kerosene/LOX rockets. This is less than the mass ratio of SpaceShipOne.
Bob Clark
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Saw these articles linked on the Wikipedia X-33 page:
Lockheed Test Flies Space Plane Prototype. By Leonard David Special Correspondent, SPACE.com posted: 24 April 2008 11:41 am ET "Lockheed Martin has tested a prototype reusable launch system by flying a sub-scale flight demonstrator from the site of New Mexico's proposed Spaceport America. "The successful test flight of the proprietary vehicle took place in December and was only recently disclosed. A company official said Lockheed Martin is planning more tests using ever-larger vehicles. "Lockheed Martin Space Systems teamed with launch provider UP Aerospace of Highlands Ranch, Colo., Dec. 19 to conduct a small demonstration launch at Spaceport America in southern New Mexico to evaluate proprietary technology the company currently has under development." http://www.space.com/businesstechnology/080424-lockheed-spaceplane-test.html
Lockheed loses prototype of Space reusable launch vehicle. 18 August 2008 "Golden, Colorado: Lockheed Martin's second test flight of a prototype reusable launch system failed with the craft going out of control and becoming seriously damaged, rendering it unusable. The test, conducted 12 August, saw the winged craft take off from a launch rail under its own power and fly for some 12.5 seconds of a planned flight of less than a minute before it crashed." ... "The 200-pound (91 kg) vehicle reached its planned altitude of roughly 1,500 feet (457 meters). It is 8 feet (2.4 meters) long with a wingspan of about 6 feet (1.8 meters) and is roughly one-fifth in scale. It is being flown to develop techniques and procedures for quick launch, ease of operations and low cost access to space." http://www.domain-b.com/aero/space/launch_veh/20080818_lockheed_martin.html
Anyone know what the scoop on this is? Since the Lockheed X-33 was canceled because of the relatively trivial problem (compared to the complexity of the rest of the system) of the composite hydrogen tank debonding, I'm inclined to think Lockheed solved that problem and it's a version of the X-33. The dimensions given though are small for it actually being a one-fifth scale of the X-33:
X-33 Advanced Technology Demonstrator. X-33 Specifications. Length: 69 ft Width: 77 ft Takeoff weight: 285,000 lbs Fuel: LH2/LO2 Fuel weight: 210,000 lbs Main Propulsion: 2 J-2S Linear Aerospikes Take-off thrust: 410,000 lbs Maximum speed: Mach 13+ Payload to Low Earth Orbit: N/A http://en.wikisource.org/wiki/X-33_Advanced_Technology_Demonstrator
If it is a version of the X-33, I wonder why it would now be classified. Perhaps Lockheed found it would be able to provide the suborbital troop delivery system wanted by the defense department. If so, it could also work as a suborbital commercial transport.
This article suggests the composite hydrogen tank debonding has been solved:
New Composite Hydrogen Fuel Tank For RLVs Successfully Tested. Huntsville - Dec 22, 2003 "A team of engineers from Northrop Grumman and NASA's Marshall Space Flight Center, Huntsville, Ala. Have demonstrated that a new, specially designed fuel tank made from composite materials can safely hold and contain liquid hydrogen under simulated launch conditions." ... "Fuel tank problems on the X-33 Venture Star project were critical to ending what was the last major new space transportation R&D program at NASA." http://www.spacedaily.com/news/rlv-03p.html
This article says XCOR, the suborbital tourism company, is developing a composite cryogenic tank:
An update on composite tanks for cryogens. More automation, improved materials bring composite fuel tanks for space applications closer to reality. Contributed by: Sara Black, Technical Editor Article Date: 11/1/2005 http://www.compositesworld.com/articles/an-update-on-composite-tanks-for-cryogens.aspx
Bob Clark
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I was looking up other references on the X-33/VentureStar composite tanks when I found this:
Space Access Update #91 2/7/00. The Last Five Years: NASA Gets Handed The Ball, And Drops It. "The advantage of a lifting-body is that it doesn't (shouldn't) need wings to maneuver during orbital reentry then land on a runway; it thus saves the weight of wings - minimum vehicle weight of course being critical to a successful Single Stage To Orbit vehicle. The disadvantage is that the vehicle shape is such that propellant tanks can't be the optimum circular cross-section shape that minimizes tank weight for a given mass of propellant - any tank shape that doesn't have a round cross-section will tend to try to balloon out to being round anyway as soon as it's pressurized; preventing non- round tanks from doing this requires heavy reinforcement. What lifting bodies gain from omitting wings they more than lose again on heavier tanks, at least with traditional tank construction methods. "Lockheed claimed to have solved the tank-weight problem, via "multi- lobed" tanks that combined partial circular sections with clever internal bracing. Further, these tanks were to be made out of graphite-epoxy composite rather than aluminum. The result was supposed to be complex-shaped tanks that would conform to the lifting-body shape while being just as light as conventional circular-section tanks. We'll come back to this." ... "And of course, part of L-M X-33's weight growth was the "multi- lobed" propellant tanks growing considerably heavier than promised. Neither Rockwell nor McDonnell-Douglas bid these; both used proven circular-section tanks. X-33's graphite-epoxy "multi-lobed" liquid hydrogen tanks have ended up over twice as heavy relative to the weight of propellant carried as the Shuttle's 70's vintage aluminum circular-section tanks - yet an X-33 tank still split open in test last fall. Going over to aluminum will make the problem worse; X- 33's aluminum multi-lobed liquid oxygen tank is nearly four times as heavy relative to the weight of propellant carried as Shuttle's aluminum circular-section equivalent." http://www.space-access.org/updates/sau91.html
This states that the X-33 tank mass to propellant mass carried is actually worse than for usual rockets, such as the space shuttle. This is surprising since I thought the composite tanks were to reduce the weight. However, as described here, the higher mass is because of the unusual shape of the tanks. The author of that article does not like the VentureStar or the decision to fund it. But it is important to keep in mind that the primary subsystems for the X-33 were shown to be viable including the aerospike engines and the metallic thermal protection system, and it was only the comparatively trivial problem of getting the lightweight tanks to work that caused the program cancellation:
X-33/VentureStar - What really happened. January 4th, 2006 by Chris Bergin http://www.nasaspaceflight.com/2006/01/x-33venturestar-what-really-happened /
The greater tank mass to propellant mass ratio for the X-33 tanks can be confirmed by using the data for the X-33 tank sizes here:
X-33 Program in the Midst of Final Testing and Validation of Key Components. Marshall Space Flight Center Lockheed Martin Skunk Works Sept. 28, 1999 http://www.xs4all.nl/~carlkop/x33.html
compared to the space shuttle external tank data given here:
EXTERNAL TANK. http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/et.html
I had earlier proposed solving the problem of getting lightweight tanks by using high strength materials that, at their highest strengths, are only available in microscale fibers or particles, by using them in hollowed out form:
Newsgroups: sci.astro, sci.space.policy, sci.physics, sci.energy Date: Tue, 29 Jul 2008 09:59:14 -0700 (PDT) Local: Tues, Jul 29 2008 12:59 pm Subject: High strength fibers for hydrogen storage on the VentureStar. http://groups.google.com/group/sci.astro/browse_frm/thread/c3ed912875d2d96c
A problem though is that these fibers or particles might be only 10 microns wide. The wall thickness of the aluminum alloy space shuttle external tank is in the range of 5 to 10 millimeters with the tank diameter on the order of 10 meters. This is a diameter to wall thickness ratio of around a 1000 to 1. If the microscale hollow fibers were to match or exceed this they would have a wall thickness of only 10 nanometers or less. This would cause extreme problems in manufacturing them and handling them so as not to damage them. However, this was based on the idea that you had to get a material that was stronger for weight than aluminum alloy. But the real problem is that the X-33 tanks are not of the usual cylindrical or spherical shape. Then we can get the minimal weight by using numerous small diameter cylindrical tubes to make up the shape of the conformal tanks without requiring the ultrahigh strength of the microscale fibers. Because of the large size of X-33 tanks we might even be able to have these cylindrical tubes be as large as say 10 centimeters across, and have them be of varying lengths so when bundled together they make up the conformal shape of the X-33 tanks. If they are 10 cm across and using the aluminum alloy, to be of the 1000 to 1 diameter to thickness ratio of the shuttle ET, they would have to have a 100 micron wall thickness. This is easy to achieve since for example common household aluminum foil may be only 16 microns thick:
Reynolds Wrap* Aluminum Foil. http://vwrlabshop.com/reynolds-wrap-aluminum-foil/p/0014244 /
Smaller diameter tubes if necessary would have a smaller wall thickness. Even tubes 1 cm wide requiring wall thickness of 10 microns is well within the range of commonly used aluminum sheeting. The effect of bundling the many tubes together would also give the complete tank strength as it would be be in the form of a honeycomb, a structure of inherently high strength to weight ratio. The viability of this idea would be easy test by using tubes made from common aluminum foil and testing how well they hold up to the pressures and temperatures seen in the cryogenic tanks. The tubes could be formed by epoxying the edges together and then epoxying the tubes together to form the shape of the conformal tanks. To form stronger tubes and bonds between the tubes we could also use a light brazing technique. The aluminum foil doesn't have the same strength to weight ratio of the aluminum alloys but it would serve to give a first level indication of how well the idea would work. At this first level you would probably also want to use liquid nitrogen rather than liquid hydrogen as well. Assuming the multitube method works to provide similar tank mass to propellant mass ratio as the shuttle ET, the bare mass of the X-33 liquid hydrogen tanks could be reduced from 12,000 lbs. to 6,000 lbs, and the liquid oxygen tank from 6,000 lbs. to 1,500 lbs, quite a large mass saving for a vehicle of bare mass of 65,000 lbs. For so many small cylindrical tanks, probably you would not want to have separate valves for each cylinder that all had to operate in unison. A couple of ways to release the fuel in a throttleable fashion might be to have each small cylinder be completely used up once it is opened, with a group of cylinders being opened sequentially, or to have one end of the cylinders be closed off and a single cap cover the other open end of all the cylinders which would be used to connect to a single valve for the tank. NASA does not seem to have much interest in funding further X-33 development. However, the U.S. Air Force is interested in developing a suborbital troop transport (which of course could also work as a commercial suborbital passenger transport):
Pentagon seeks military role for space tourism technology. By Stephen Trimble DATE:23/02/09 SOURCE:Flight International http://www.flightglobal.com/articles/2009/02/23/322951/pentagon-seeks-military-role-for-space-tourism-technology.html
Then considering the low cost nature of the idea, the Air Force might fund this since if viable it would lead to a suborbital transport in the X-33, and likely therefore also to a reusable single stage to orbit vehicle in the VentureStar.
Bob Clark
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AFRL Seeks Reusable Booster X-Plane Ideas. Posted by Graham Warwick at 5/12/2009 2:36 PM CDT "Guy Norris alerted us to it a couple of weeks ago in Aviation Week, but the Air Force Research Laboratory has finally released its request for information on concepts for a reusable launch vehicle. They are calling it the Reusable Booster System (RBS), because the focus is on a fly-back first stage carrying an extendable upper stage. "The RFI's stated objective "is to identify potential operational RBS concepts, including a family of expendable stage variants, and feasible system development approaches." And it is a step towards a potential subscale X-plane demonstrator that could fly in 2017-18." http://www.aviationweek.com/aw/blogs/defense/index.jsp?plckController=Blog&plckScript=blogScript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%3a27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3a3cea718e-0347-4d3d-bdfe-7a7a675f97de
Reusable Booster Integrated Demo – Concept Options Maturation Study. (RBID-COMS) Solicitation Number: RFI-PKV-09-01 Agency: Department of the Air Force Office: Air Force Materiel Command Location: AFRL - Wright Research Site https://www.fbo.gov/index?s=opportunity&mode=form&id=d3e825368b36760bf2ece9d87750ff98&tab=core&_cview=1&cck=1&au=&ck =
This is for proposals for a reusable vertical launch, horizontal landing unmanned booster to serve as the first stage of a two-stage-to- orbit (TSTO) system. The description seems to be tailored made for the X-33 suborbital system. I'm inclined to think intentionally so. Then this might open up funding for alternative methods for obtaining lightweight tanks for the X-33 such as the multiple cylindrical tanks method. It also would make possible a suborbital troop carrier or commercial transport system.
Bob Clark
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More on the Air Force's "Reusable Booster System":
USAF Seeks Reusable Booster Ideas. May 14, 2009 By Graham Warwick "The plan is to conduct an integrated demonstration of technologies and processes culminating in a subscale X-plane vehicle that would fly by 2017-18 and take the concept to a technology readiness level of 6, ready to enter full-scale development. "AFRL has several ground-based experiments already under way involving structures, controls and systems for an operationally responsive launch vehicle. The work is focused on a reference concept for an unmanned vertical takeoff and horizontal landing reusable booster capable of turnaround in 24-48 hours and launch within 4-8 hours of a request." http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/Reuse051409.xml
Spacelift Development Plan. http://www.acq.osd.mil/nsso/conference/briefs/HampstenSDP%20Public%20Release.ppt
Also from the Aviation Week article:
"AFRL's reference concept includes an integral all-composite airframe and tank structure that carries both internal pressure and external flight loads. The concept vehicle is powered by pump-fed liquid-oxygen/ hydrocarbon rocket engines."
The suggestion to limit the fuel to hydrocarbon rather than the higher energy liquid hydrogen probably stems from the fact that for this purpose the vehicle only needs to have a max speed in the range of Mach 3.5 to 7, as indicated by slide #5 in the "Spacelift Development Plan" powerpoint presentation. However, all three competing proposals for the X-33 liquid-hydrogen fueled vehicles, which needed to get to Mach 13+, probably could be adapted to use hydrocarbon fuel, for this Mach 7 max. proposal. This would be another method to effectively re-open the X-33 competition. The X-33. It's back on, baby!
X-33. http://www.astronautix.com/lvs/x33.htm
http://www.deskpicture.com/DPs/Astronomy/NASA/X33Concepts.jpg
Bob Clark
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