Exoscientist

There appears to be only two possible reasons for this: either the Raptor is not as reliable for reusability as thought or it was damaged during the landing burn. Bob Clark

That was for a crewed mission to another star system, tens of trillions of kilometers away. But the primary focus of this new research is for missions still in the Solar System, 1/10,000th the distance, with cubesat-like probes. Here’s the published report by Rene Heller et.al.: Low-cost precursor of an interstellar mission. René Heller1,2, Guillem Anglada-Escudé3,4, Michael Hippke5,6, and Pierre Kervella7 ABSTRACT The solar photon pressure provides a viable source of thrust for spacecraft in the solar system. Theoretically it could also enable inter- stellar missions, but an extremely small mass per cross section area is required to overcome the solar gravity. We identify aerographite, a synthetic carbon-based foam with a density of 0.18 kg m−3 (15 000 times more lightweight than aluminum) as a versatile material for highly efficient propulsion with sunlight. A hollow aerographite sphere with a shell thickness εshl = 1 mm could go interstellar upon submission to solar radiation in interplanetary space. Upon launch at 1 AU from the Sun, an aerographite shell with εshl = 0.5 mm arrives at the orbit of Mars in 60 d and at Pluto’s orbit in 4.3 yr. Release of an aerographite hollow sphere, whose shell is 1 μm thick, at 0.04 AU (the closest approach of the Parker Solar Probe) results in an escape speed of nearly 6900 km s−1 and 185 yr of travel to the distance of our nearest star, Proxima Centauri. The infrared signature of a meter-sized aerographite sail could be observed with JWST up to 2 AU from the Sun, beyond the orbit of Mars. An aerographite hollow sphere, whose shell is 100 μm thick, of 1 m (5 m) radius weighs 230 mg (5.7 g) and has a 2.2 g (55 g) mass margin to allow interstellar escape. The payload margin is ten times the mass of the spacecraft, whereas the payload on chemical interstellar rockets is typically a thousandth of the weight of the rocket. Using 1 g (10 g) of this margin (e.g., for miniature communication technology with Earth), it would reach the orbit of Pluto 4.7 yr (2.8 yr) after interplanetary launch at 1 AU. Simplistic communication would enable studies of the interplanetary medium and a search for the suspected Planet Nine, and would serve as a precursor mission to α Centauri. We estimate prototype developments costs of 1 million USD, a price of 1000 USD per sail, and a total of <10 million USD including launch for a piggyback concept with an interplanetary mission. https://www.aanda.org/articles/aa/pdf/2020/09/aa38687-20.pdf From the scaling indicated there, it appears the mass scales by the areal size of the sail, i.e., by the square of the diameter. So a 100 meter wide sail, would have a sail mass of 2.3 kg and a payload mass of 22 kg. Bob Clark

A good question. His idea needs to be tested in the lab. If I’m to make a guess, the side of the smaller sail facing the Sun is non-mirrored so has low light pressure pushing it away from the Sun, but the side facing the larger main sail is mirrored. And more importantly that larger main sail focuses all it’s light onto the smaller sail in concentrated fashion. Bob Clark

In that Robert L. Forward conception of using reflected light from the main sail to slow down a smaller sail at the destination, he also included the possibility of using the same method to actually *return* from the far destination. Imagine getting returned samples from ‘Oumuamua, the Jovian and Saturnian moons, and Pluto! However, there is a sticking point in using this method in the case we’re considering here. Forward was imagining it for laser propelled propulsion. In that case you can focus the reflected light that is coherent and collimated. But for our scenario we’re using solar light which will be non coherent and uncollimated. It may not be possible to get the highly focused light at long distances in this scenario. It might be we can carry a laser to do it but that may be too heavy. There may be other light weight methods to do it. An intriguing possibility: IF it did work, then could it be used to do staging by sending focused light from the main sail *forward* to a smaller sail ahead of it? Then we could increase the speed multiple times by doing multiple staging. Bob Clark

The Starship HLS plan for Artemis may be even more complex than thought. It may require two refueling depots, the known one in low Earth orbit plus another one in an elliptical high Earth orbit: SpaceX seeks a single FCC license for multiple future Starship missions, including commercial/Starlink launches and Artemis. Filing shows some technical details about HLS lander, indicating it may require a 2nd refueling in an elliptical Earth orbit. [FCC filing link](https://licensing.fcc.gov/cgi-bin/ws.exe/prod/ib/forms/reports/swr031b.hts?q_set=V_SITE_ANTENNA_FREQ.file_numberC/File+Number/%3D/SATLOA2024121800288&prepare=&column=V_SITE_ANTENNA_FREQ.file_numberC/File+Number), most of the technical details is in the [Technical Annex](https://licensing.fcc.gov/myibfs/download.do?attachment_key=32702913) &nbsp; 1\. The filing covers launch, reentry and in-space operations in the following orbits: * LEO: circular orbit with altitude between 181km and 381 km, all inclinations. This would be the deployment orbit for Starlink and the orbit for HLS LEO depot. * Elliptical Earth Orbit: perigee is between 181km and 381 km, apogee is between 10,534km and 150,534km, inclination between 28 and 33 degrees. Filing refers to these as MEO/HEO but technically they're transfer orbit to circular MEO/HEO. This would cover GTO and transfer to MEO such as orbit of GPS satellites, although the filing didn't mention these. It did mention that this will be the Final Tanking Orbit (FTO) for crewed lunar mission where HLS lander will receive a 2nd propellant transfer. * Translunar Injection (TLI), Lunar orbits (NRHO, LLO) and lunar descent/ascent/surface: These would be for Artemis missions &nbsp; 2\. Communication bands used by Starship * UHF and IEEE 802.11ac 5.8 GHz band: Used for communication between HLS lander and EVA suits on the Moon. I believe these are required by NASA. Range is up to 2km. * S band: Most communication is in this band, including ship to Earth, ship to ship/depot, ship to Orion/Gateway, etc. HLS lander and depot will also use this band to communicate with NASA's TDRSS satellites in Final Tanking Orbit. * Ku band: This is used for radio communication between Starship and Starlink constellation, however it's only usable below 300km. * Ka band: Used by HLS lander for direct to Earth communication &nbsp; 3\. Technical details about HLS lander * As said above, a 2nd propellant transfer from depot to HLS lander may be required in an Elliptical Earth Orbit. Note that someone apparently with sources [mentioned this a few months ago on twitter](https://x.com/Jenakuns/status/1821914964220899650): "Starship HLS conducts 2 refuelling's; 1 in LEO, then a second one in an elliptical orbit to get the architecture delta v down. That's the reason why launch count doesn't line up with wet mass/payload ratio." * HLS lander will carry 4 dual-band (S/Ka) gimbaled parabolic reflector antennas, one in each quadrant. Exact location of these antennas is not disclosed. * HLS lander will carry 2 lunar landing radar in the 35.5-36 GHz band. It'll be activated 4km above the lunar surface and run for approximately 5 minutes until landing. There were [FCC Special Temporary Authority filings for testing this radar on an airplane as early as October 2021](https://apps.fcc.gov/oetcf/els/reports/STA_Print.cfm?mode=current&application_seq=111132&RequestTimeout=1000), call sign is WT9XBJ. https://www.reddit.com/r/SpaceXLounge/s/hEDBg96kBk Bob Clark

Robert Zubrin has made this point numerous times: https://x.com/robert_zubrin/status/1374861051490000896?s=61 https://x.com/robert_zubrin/status/1725747455247979000?s=61 https://x.com/robert_zubrin/status/1278300197664120833?s=61 https://x.com/robert_zubrin/status/1256718751619145728?s=61 https://x.com/robert_zubrin/status/1256574624202018819?s=61 https://x.com/robert_zubrin/status/1256571091100725249?s=61 https://x.com/robert_zubrin/status/1192796619894185987?s=61 https://x.com/robert_zubrin/status/1192785320011415552?s=61 https://x.com/robert_zubrin/status/1192444670191624192?s=61 https://x.com/robert_zubrin/status/1178265541342941184?s=61 https://x.com/robert_zubrin/status/1127195497964478464?s=61 https://x.com/robert_zubrin/status/1127283625366568961?s=61 And finally: Bob Clark

It might be Robert L Forwards idea of reflecting back the light from the main sail to a smaller sail would allow you to slow down the smaller sail to stop at the destination. A problem though is Dr. Forward intended this for the case of laser propulsion where the reflected light could be focused onto the receiving sail. This might not work for the non coherent light from the Sun. However, actually for the solar gravitational lens it still works as long as you are on a line extending out from the SGL so you may not need to slow down for that case. Bob Clark

The Parker Solar Probe recently survived its closest flyby of the Sun at only 0.04 AU. This gives confidence that the proposal to achieve high speed of a solar sail using a close flyby of the Sun using the ultralight, but high temperature material aerographite can work: Interstellar Sails: A New Analysis of Aerographite by Paul Gilster | Sep 27, 2023 | Sail Concepts | https://www.centauri-dreams.org/2023/09/27/interstellar-sails-a-new-analysis-of-aerographite Such a solar sail could reach a speed of 2%c, 6,000 km/s, using this close flyby. At this speed it could reach the solar gravitational lens(SGL) at 550 AU in only 6 months, and ‘Oumuamua in only 11 days(!) The implications are stunning. Aerographite is an existing material. Then this means we currently have this capability. Telescopes placed at the solar gravitational lens(SGL) would have the ability to amplify the images of an Earth-sized exoplanet by 100 billion times. It could resolve continent-sized features on such a planet. ‘Oumuamua is an interstellar object whose unusual motions led some to speculate it could be of artificial origin. Then we now have the capability to directly observe Earth-sized exoplanets in other star systems and to determine features on an interstellar object that came into our solar system which may have been artificially produced. Bob Clark

The new era of heavy launch. By Gary Oleson The Space Review July 24, 2023 https://www.thespacereview.com/article/4626/1 The author Gary Oleson discusses the implications of SpaceX achieving their goal of cutting the costs to orbit to the $100 per kilo range. His key point was costs to orbit in the $100 per kilo range will be transformative not just for spaceflight but because of what capabilities it will unlock, actually transformative for society as a whole. For instance, arguments against space solar power note how expensive it is transporting large mass to orbit. But at $100/kg launch rates, gigawatt scale space solar plants could be launched for less than a billion dollars. This is notable because gigawatt scale nuclear power plants cost multiple billions of dollars. Space solar power plants would literally be cheaper than nuclear power plants. Oleson makes other key points in his article. For instance: The Starship cost per kilogram is so low that it is likely to enable large-scale expansion of industries in space. For perspective, compare the cost of Starship launches to shipping with FedEx. If most of Starship’s huge capacity was used, costs to orbit that start around $200 per kilogram might trend toward $100 per kilogram and below. A recent price for shipping a 10-kilogram package from Washington, DC, to Sydney, Australia, was $69 per kilogram. The price for a 100-kilogram package was $122 per kilogram. It’s hard to imagine the impact of shipping to LEO for FedEx prices. Sending a package via orbit transpacific flight would not only take less than an hour compared to a full day via aircraft, it would actually be *cheaper*. Note this also applies to passenger flights: anywhere in the world at less than an hour, compared to a full day travel time for the longer transpacific flights, and at lower cost for those longer transpacific flights. Oleson Concludes: What could you do with 150 metric tons in LEO for $10 million? The new heavy launchers will relax mass, volume, and launch cost as constraints for many projects. Everyone who is concerned with future space projects should begin asking what will be possible. Given the time it will take to develop projects large enough to take advantage of the new capabilities, there could be huge first mover advantages. If you don’t seize the opportunity, your competitors or adversaries might. Space launch at FedEx prices will change the world. These are the implications of SpaceX succeeding at this goal. However, a surprising fact is SpaceX already has this *capability* now! They only need to implement it: SpaceX routine orbital passenger flights imminent. http://exoscientist.blogspot.com/2024/11/spacex-routine-orbital-passenger.html Bob Clark

Actually, at least for the reusable Starship V3, using the small 3rd stage/lander is cheaper on that measure as well. The current Artemis plan would make the Starship HLS expendable. But instead with a 3rd stage/lander only the cheaper, smaller lander would be expendable. It might be even the lander can be reusable as well. Zubrin has made this point in regard to Mars missions as well. By sending the entire Starship to Mars it is out of use for two years. While using the smaller lander as the stage that goes to Mars, Starship can be returned to Earth for its many reuses after launch, and only the smaller lander is out of use for two years. There really is no logical reason to do multiple refueling, multiple launches for a single Moon or Mars mission when it can be done in a single launch format. Bob Clark

Thanks for that discussion but a key option was missed: using a 3rd stage/lander. Robert Zubrin has made this argument numerous times that addition of a 3rd stage results in a more efficient architecture for the Moon or Mars. In fact it would result in single launch missions for both the Moon and Mars, no orbital refueling flights required at all. The expendable Starship at 250-ton capability and the reusable Starship V3 at 200-ton capacity have about twice the capacity of the Saturn V so would have about twice the capacity for single flight, round-trip missions to the Moon when using additional stage(s). And Zubrin’s Mars Direct approach could do Mars missions using two Saturn V class launches. So expendable Starship or Starship V3 could also do that in single launch format when using additional stage(s) Bob Clark

Yes. In that SpaceWatch.Global interview Zubrin makes clear the Starship upper stage is what Elon says could be made for $10 million, not the Superheavy. But a key point is you can make smaller launcher using the Starship itself as the booster, with a smaller “mini-Starship”, if you will, as the upper stage. At least I assume that is what Zubrin is arguing. I don’t think he would be arguing in favor of a SSTO. Then, if you run the numbers such a smaller two-stage vehicle could be a 100-ton class launcher as an expendable. Bob Clark

Actually, it doesn’t even give any plans on what such an architecture should look like, i.e., what launchers to use. The above graphic is what a reader graphed out based on the description in an earlier version NASA published on line. But that report is now deleted so no description is now available online. It looked like 16 launches of SLS-like vehicle over 9 years. Untenable on cost grounds. Bob Clark

Robert Zubrin’s made a key statement in this SpaceWatch.Global interview that Elon told him SpaceX could build Starship for $10 million. This leads to a surprising conclusion: SpaceX can build a Moon or Mars rocket for ca. $10 million. Now. Such a rocket could offer costs of $100/kilo to orbit. Now: SpaceX routine orbital passenger flights imminent. http://exoscientist.blogspot.com/2024/11/spacex-routine-orbital-passenger.html Bob Clark

The launch video prior to hotstaging says the tower was go for catch. Then later they decided to divert. The tower issue wasn’t known immediately? Also, The voiceover says the boostback back burn should last another approximately 30 seconds, but then it ends immediately after she says this. Did the boostback end early? Bob Clark

For my cost estimates I was taking cost reductions by a factor of 10 in both scenarios by reusability. If for Superheavy/Starship you take it to be a reduction by a factor 100 in cost by reusability then the same would be true for Starship/mini-Starship and costs would be ca. $1 million per launch, still much lower than a supposed $18 million for 18 launches with reusability of the full Superheavy/Starship. There really is no logical reason to use the multiple launch approach with 18 SuperHeavy/Starship launches to accomplish a single mission compared to a single launch approach of a rocket at only 1/3rd the size. It would be like the Apollo engineers running the numbers on the Saturn V and realizing it could do a Moon mission in a single launch, decided to do it instead with 50 launches of the Saturn V for that one single Moon mission. The argument of the greater payload of the multiple launch approach doesn’t hold water either. SpaceX said their multiple launch, multiple refueling approach can get 100 tons cargo to the lunar surface. The Starship/mini-Starship using an existing Falcon 9 upper stage or Centaur V upper stage as the lander can get 20+ tons cargo to the lunar surface. Then the equivalent of 50 launches of this launcher can actually get 1,000 tons to the lunar surface. Bob Clark

The renewed discussions on the possibility SLS might be cancelled have renewed discussion of replacement architectures for returning us to the Moon. I mentioned in the thread: that because of the low production cost of the Starship, SpaceX should investigate a smaller system with the Starship as the 1st stage and a "mini-starship" as an upper stage. A key cost fact is SpaceX amply demonstrated with the Falcon 9 that the first stage is much easier to reuse than the upper stage. Then I estimated a $11.7 million cost for such a Starship/mini-Starship launcher as partially reusable. I calculated the expendable version might have an LEO payload of 120 tons. Then based on the Falcon 9, if the Starship as 1st stage landed downrange this launcher might still get 80% payload of the expendable version. That would mean still 100 ton payload as partially reusable at a $11.7 million cost. That's a cost per kilo of ~$120 per kilo. That's nearly two orders of magntitude cheaper that the going rate just a few years ago, before SpaceX, of $10,000 per kilo. And even with the partially reusable Falcon 9, the price per kilo is still $3,000 per kilo. Quite importantly, because it doesn't need orbital TPS, orbital refueling, tank stretch or upgraded Raptors, this is a capability we have now. I argue that this is better than the current approach. For instance the fully reusable Superheavy/Starship V2 will have payload at 100+ tons, and still need all of orbit capable TPS, orbital refueling, tank stretch and upgraded raptors. And it will need all of these to get a ca. $10 million reusable launch cost. But the SS/mini-SS will reach the same payload capability, at only be 1/3rd the size of the SH/SS, without the difficult technical advances. The advantage of this approach extends also to lunar missions. SpaceX wants a multiple refueling approach of SH/SS for lunar missions. This will be in the range of 18 total flights with refuleings, the orbital depot, and the Starship HLS itself. In contrast the SS/mini-SS can do it in a single launch. And the current plan needing 18 launches will actually be 18*4 = 72 times bigger than using a single Starship. Taking into account the size also of the mini-Starship the current SH/SS lunar plan will be about 50 times the size of just a single SS/mini-SS. A comparison of the relatively sizes of the two approaches for getting to the Moon: Compared to: Bob Clark

The conclusion you could get a Saturn V-class launcher, i.e., 100+ ton payload to LEO, using the Starship as a 1st stage and a “mini-Starship” as an upper stage is based on this estimate Elon once made for an expendable Starship dry mass. But that is for an upper stage use where it did not have enough engines for liftoff from ground. Assume for 1st stage use it needs 9 engines. Increase the dry mass now to 50 tons for the greater engine mass. For the mini-Starship, an upper stage commonly is 1/3rd to 1/4th the size of the lower stage, so call it 420 tons propellant mass. As an upper stage it doesn’t need high engine thrust so assume same mass ratio of ~30 to 1 as for Elon’s expendable Starship estimate, giving it a dry mass of 14 tons. Take Starship exhaust velocity as ground launched as comparable to that of the Superheavy, 3,500 m/s. And take the upper stage’s vacuum exhaust velocity as 3,800 m/s. Then we could get ~120 tons to LEO: 3,500Ln(1 + 1,200/(50 + 434 + 120)) + 3,800Ln(1 + 420/(14 + 120)) = 9,200 m/s. In that last post I cited Zubrin’s comment that Elon’s says the Starship production cost could be brought down to ~$10 million. But that undoubtedly is for high production rates. But the current production cost for Superheavy/Starship is estimated to be ~$90 million, with about 30%, $27 million, for Starship: STARSHIP COST ANALYSIS OVERVIEW Note: This is Payload's current estimate and not based on access to any internal Space data or proprietary information. Current Estimated Starship & Booster Full Stack Cost (S in thousands) 39 Raptor Engines 39,000 Labor. 35,000 Structure, plumbing, tiles, parts 13,000 Avionics 3.000 Total 90,000 *Payload costs estimates are based on a post-R&D 1-2 year forward-looking model. This is an educated best estimate and not based on Space internal data. Further cost reductions are expected in the long-run. $90M cost: Payload estimates it costs $90M to manufacture a fully integrated Starship based on a post-R&D/test production phase near-term model. The go-forward cost does not factor in the near $5B SpaceX has spent on R&D to date. ~70% of costs accrue to Super Heavy and ~30% to Starship upper stage. Future Starship (upper stage) cost reductions: As Starfactory comes online and Raptor production is refined, Space aims to reduce costs even further. A focus on Starship's upper stage: When SpaceX achieves full reusability, production of Starship second stage vehicles will be an order of magnitude higher than booster production. • The company plans to eventually build multiple second stage Starships per week and reduce Raptor engine's production cost to $250K a pop. If successful, the long-term cost to mass produce second-stage Starships could drop to $10M to $15M a vehicle. However, for purposes of this report, we will analyze costs as they are today. Raptor 2 engines ($39M) Payload estimates each Raptor 2 engine costs ~$1M to build. The 39 engines-which include three additional upper-stage engines that will be added in the future-are by far the biggest Starship cost, adding $39M to total cost. SIM per Raptor 2 engine is half as expensive as its $2M+ Raptor 1 predecessor. 20 SpaceX hopes to eventually bring the cost per engine down to ~$250K. Payload Research 18. Elon Musk on X 19. Space 20.Elon Musk on X 21. Elon Musk on X https://docsend.com/view/fi9wuazzeex57iig Say, for a mini-Starship upper stage its cost would be a 3rd of the $27 million production cost of the Starship, so $9 million. So the full vehicle production cost at $36 million. So a Saturn V-class launcher capable of 100+ tons to LEO at ca. $36 million. Note also SpaceX demonstrated with the Falcon 9 landing just the first stage, i.e., partial reusability, much easier than full reusability. Then this Starship now as first stage could be reused cutting its cost to, say, $2.7 million per launch, for the total partial reuse cost of $11.7 million per launch. This is a Saturn V-class vehicle capable of single launch Mars or Moon missions we could launch now. No thermal tile problems, or needing to master orbital refueling, or stretching tanks, or increasing Raptor thrust. We have this capability now. Bob Clark

How are you getting twitter links to embed? Bob Clark

Nice calculation. I’d like to get some feedback on these estimates I made after I heard Robert Zubrin say Elon told him the Starship, i.e., the upper stage only, could be made for ~$10 million production cost: https://twitter.com/spacewatchgl/status/1855925836932841756 I was surprised that Elon estimates a Starship only cost of ~$10 million. At a ~$10 million Starship cost, SpaceX should investigate it as 1st stage of a smaller system, with a mini-Starship as the 2nd stage at perhaps only ~$2 million additional cost, due to its proportionally smaller size. Get ~100 ton to LEO Saturn V-class launcher at only ~$12 million cost(!) As the first stage now, Starship loses only a proportionally small payload by reusing if you land it down range. Then close to a 100 ton partially reusable launcher for only ~$3 million(!) Say, payload reduced to 80 tons with partial reusability. Then price per kilo only $3 million/80,000kg = $37.5 per kilo(!) Bob Clark

I haven’t seen the discussion on the best payload bay doors to use. Is there some reason why the SpaceX “clamshell” version is non-optimal: https://i.sstatic.net/D3MYv.png Bob Clark

I don’t agree with that. Note that SpaceX also wants to use his idea of using ISRU on Mars to generate the propellant for the return trip. Zubrin is just using standard spaceflight engineering practice that for high delta- v missions like to the Moon or Mars you use progressively smaller additional stages. Instead of using additional stages, SpaceX wants to use that same single large size Starship to go all the way to the Moon or Mars. This requires multiple refuelings. Plus, because of its large amount of propellant the ISRU requirements on Mars become large also. Zubrin estimates 10 football fields worth of solar cells for the power to run the ISRU. Bob Clark

The glowing base on the booster during reentry prior to the landing burn has been described as reentry heating. But closer examination suggests fire in the engine compartment: https://youtube.com/clip/UgkxfnFwF67GBwv4ItsloQPyL-XOTQSYz723?si=iOlffJRajEGCMDE8 This may have been due to the leaks that have been a recurring problem for the Raptor. Bob Clark

Commercial Starship Super Heavy booster came within one second of aborting first “catch” landing. Jeff Foust October 25, 2024 https://spacenews.com/starship-super-heavy-booster-came-within-one-second-of-aborting-first-catch-landing/ This illuminates why I argue it’s likely SpaceX will ultimately decide to go with landing legs and landing on a landing pad anyway. Landing legs can be optimally designed to be lightweight to add only a small amount to the dry mass. Then it won’t be worth risking a launch tower worth hundreds of millions of dollars for such a small loss in payload. The loss of revenue from missed launches of the operational Starship over the months when the tower is being rebuilt can amount to billions in lost revenue. Also it’s mentioned there wasn’t the expected amount of pressure in the Raptor. This could have been due to a fuel leak. Fuel leaks and the resulting fires have been a recurring problem with the Raptor: What Happened to Starship SN11? | SpaceX Starship SN11 Test Flight & Explosion Cause Analysis. https://youtube.com/clip/UgkxziHi2GsiVoF9v_DAa1YYQuU6fDskrWQU?si=qx0I7p_ussOY9bhT Then the giant plume of flame seen shooting up the side of the booster may in fact have been due to a fuel leak. Additionally, its mentioned one of the chine covers blew off exposing valves that are sources of single-point failure if they had been damaged. Notably the landing video shows the cover blew off before the engine restart began. It was speculated it was due to atmospheric forces, but it may have been due to a fire within the engine compartment releasing pressurized gas. Bob Clark