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#11
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Air Breathing for VTVL
I have contemplated an air breathing SSTO vehicle. This hypothetical
vehicle would be shaped like the DC-X and use four of my induction jet engines. This is a cone with the four engines at the base. The induction jet engines are like ramjets except that they work at a standstill with an Isp of 4000. This Isp figure is only a measure of fuel consumption and not exhaust velocity. At higher velocities to about mach 6 they work like ramjets because they are exactly like ramjets. I guess they could be called a version of ejector ramjet. They have no moving parts just airflow ducting and a rocket-like regeneratively cooled nozzle. I would expect to employ a nose cone that can be moved forward to reduce the intake cross section and to eventually completely close the intake. I expect to achieve 50 T/W or better depending on the materials used and final configuration. I expect to have the Isp drop from 4000 at launch to 500 at mach 6 and have an average Isp of 1000 from 0 to mach 6. At this speed I close the intakes and work like a rocket until I reach orbit, at an Isp of 350. The mass ratio is 6 for the rocket mode and 1.3 for the air breathing mode, during which only fuel is used, no oxydizer. This includes a small fuel reserve for vertical landing. This vehicle is simple enough that a small team could build it with a low budget. I built my prototype engines by welding stainless steel sheets and turning steel parts on my lathe. I hope to build a prototype flying car first after which I can work on flying faster and higher. In this thread you guys have assumed that the air breathing engines are in addition to the rocket engine but they can be the same. You also assume that heavy and expensive turbine blades are needed to pump the air and they are not needed. The only weight penalty is the intake ducting and the variable cone. The rest of it is just like the rocket engine. You can see: http://www.vtol.net http://www.vtol.net/induction.htm http://www.vtol.net/air.htm Zoltan |
#12
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Air Breathing for VTVL
Sorry for the delay in responce. Ok, now we get down to some numbers. For a LOx/Kero VTVL SSTO using off the shelf engines you get a 14 ration between in orbit weight and GLOW. I.E. if you want 50 tons of loaded ship in orbit, your glow is about 700 tons. Using off the shelf (with water and LOx injection ) turbo-jets up to Mach 6 drop that to a 8 to 1 ratio, and your 50 ton in orbit in orbit craft has a glow of about 425 tons. Now with a 10-1 t/w ration jet engine set, you add 10 tons of jet engine but lose about 7 tons of rocket and tank dry weight. (If you assume 15-1 t/w engines their no weight gain.) As a nit the water and Lox injection also increases thrust in the same engine, so effective T/W improves, but I can't find out by how much. So I assume T/W stays at about 10. Now the weight per say isn't really interesting. The important thing is that you wind up with a smaller and more resilient craft. Its easier to a working structure for it and likely you get more reliability. The reason I pushed the point to a HTHL SSTO (aside from just seeing if it would work) is that it added operational flexibility, effectively lowered the necessary delta-v, lowered thrust needs on take off and low altitudes over a VTVL, and (since the LOx was added in flight) lightened the landing gear weight. The wings do double duty as fuel tanks and extra reentry surface to allow a cooler surface temp on reentry. Note the wing in my configuration are NOT scaled for take off with the full mass of 425 tons needed for a boost to orbit. Since it gets LOx in mid air (from mid air "refueling" or LOx mining from the air), its take off weight came to about 170 tons. The fact that the cost of the jets would buy a lot of LOx is REALLY irrelevant. Fuel is currently a virtually negligible fraction of launch costs. Servicing the craft (or building in the case of expendables) currently dominates launch costs, and a high dry mass fraction of structure and hull rather then bulky tanks is likely to dramatically lower servicing costs. Also of course the jet engines you add - replace nearly half of the Rocket engines you'd need for the far heavier (700 tons versus 425 tons) all rocket VTVL SSTO. Rocket engines are about as costly as, and far less durable then, jet engines. So you save money on the total engine system purchase and servicing costs. As to your statement that ==="I would like to find the point where the operational flexibilities possible = with an airbreathing engine do not cost performance in real terms."== Depends on what performance your concerned with, and how it works in your design. A 15-1 T/W ration jet engine in my configuration will allow you to boost to orbit with no added dry mass since the added weight of the jet will be offset by the lower weight of tanks and rocket engines needed. But is that the critical performance number? Is operational flexibility, operational costs, development cost, etc more critical? Also how the craft ascends effects your numbers. My HTVL configuration needs a lot less take off thrust, hence engines, then a VTVL configuration. So the trade off in weight and engines would be very different for VTVL jet/rocket SSTO. Even a HTVL that does a fast acceleration up into vacuum like Rascal would need a lot more jet engine thrust (hence weight), vrs a HTHL that flies a more moderate ascent to max airbreathing altitude and speed. So at what point a given engine is acceptable, depends on the details of the craft that needs it, and what its design priorities are? If your question is at what T/W can a jet engines weight in a VTVL SSTO pay for itself in dry weight. That depends on how much of the total Delta-V it can deliver. For example. If your total delta-V to orbit (including virtical assent) is 8730 m/s (which is a standard LOx/Kero delta-v to orbit number). If the jet can deliver 3670 m/s it would need a T/W of about 23 to break even on the dry weight, if you ignore lower delt-v requirements due to a lower airdrag on the smaller craft. If the jet can deliver 1835 m/s it would need a T/W of about 40 to break even on the dry weight. Kelly Starks "Humans are a race of compassionate predators." |
#13
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Air Breathing for VTVL
"Kelly St" wrote in message ... Sorry for the delay in responce. Ok, now we get down to some numbers. For a LOx/Kero VTVL SSTO using off the shelf engines you get a 14 ration between in orbit weight and GLOW. I.E. if you want 50 tons of loaded ship in orbit, your glow is about 700 tons. Using off the shelf (with water and LOx injection ) turbo-jets up to Mach 6 drop that to a 8 to 1 ratio, and your 50 ton in orbit in orbit craft has a glow of about 425 tons. I have been using 16 ratio to LEO with 9,000 m/s required. From Mach 6 there is about 6,000 m/s remaining to orbit plus fairly minor gravity and drag losses. At 3,500 m/s exhaust velocity of a vacuum optimised engine, you are looking at a rocket ratio of 6. I assume you are figuring the remaining ratio is feeding the jets? Now with a 10-1 t/w ration jet engine set, you add 10 tons of jet engine but lose about 7 tons of rocket and tank dry weight. (If you assume 15-1 t/w engines their no weight gain.) As a nit the water and Lox injection also increases thrust in the same engine, so effective T/W improves, but I can't find out by how much. So I assume T/W stays at about 10. High Mach jets are a bit more complex. The 10/1 jet also has an intake system of similar mass. The variable ramps, boundary layer bleed, subsonic diffuser, and various Mach scheduled actuators are just as heavy as your base engine according to my references. The afterburner with variable throat and nozzle geometries that vary with Mach, dynamic pressure, and temperature also masses about half as much as the dry turbojet. The result tends toward a 1,000 lb engine producing 10,000 lbs of thrust dry. Add 1,000 lbs for the intake system, and 500 lbs for the afterburner/ nozzle system, and you end up with a (generic) 2,500 lb system producing 15,000 lbs of thrust with afterburner. This would net a T/W ratio of 6. Mass injection increases T/W at the expense of Isp. It quickly becomes a trade study beyond the scope of usenet engineering. Now the weight per say isn't really interesting. The important thing is that you wind up with a smaller and more resilient craft. Its easier to a working structure for it and likely you get more reliability. We probably disagree here. The reason I pushed the point to a HTHL SSTO (aside from just seeing if it would work) is that it added operational flexibility, effectively lowered the necessary delta-v, lowered thrust needs on take off and low altitudes over a VTVL, and (since the LOx was added in flight) lightened the landing gear weight. The wings do double duty as fuel tanks and extra reentry surface to allow a cooler surface temp on reentry. Note the wing in my configuration are NOT scaled for take off with the full mass of 425 tons needed for a boost to orbit. Since it gets LOx in mid air (from mid air "refueling" or LOx mining from the air), its take off weight came to about 170 tons. Your article was not online when you posted the link last time. Just going from your posts. It would seem to me that you need at least 10% of take off mass for airframe and landing gear. That would be 17 tons of airframe plus 10 tons of jet engines. ( I think jet mass is light) 3 tons of rocket engines and 7.5 tons of various tankage for another 10.5 tons. I see a minimum 75% of your mass in orbit being airframe, tanks, and engine systems. Not including a possible ACES or LOX refueling mass penalty. By the time you add TPS, cargo handling, and control systems, you could reach negative payload. The fact that the cost of the jets would buy a lot of LOx is REALLY irrelevant. Fuel is currently a virtually negligible fraction of launch costs. Full agreement. Servicing the craft (or building in the case of expendables) currently dominates launch costs, and a high dry mass fraction of structure and hull rather then bulky tanks is likely to dramatically lower servicing costs. Agreement would be conditional on actually achieving the goals you suggest. Also of course the jet engines you add - replace nearly half of the Rocket engines you'd need for the far heavier (700 tons versus 425 tons) all rocket VTVL SSTO. Rocket engines are about as costly as, and far less durable then, jet engines. So you save money on the total engine system purchase and servicing costs. Currently your arguement seems to make sense. If anyone produces a reasonable RLV rocket engine in some quantity, then no. Rockets are a relatively immature propulsion compared to jets. I can buy a well used 5k jet engine for ~$40k. If I were to place a quantity order (100 units?) with XCOR, I could beat that price for new 5k rockets. The guts of a pump fed rocket total an easier problem than a high Mach turbojet, if a mature RLV unit is produced. This includes servicing access. The durability an cost problems can be made to go away. As to your statement that ==="I would like to find the point where the operational flexibilities possible = with an airbreathing engine do not cost performance in real terms."== Depends on what performance your concerned with, and how it works in your design. A 15-1 T/W ration jet engine in my configuration will allow you to boost to orbit with no added dry mass since the added weight of the jet will be offset by the lower weight of tanks and rocket engines needed. But is that the critical performance number? Is operational flexibility, operational costs, development cost, etc more critical? Your last sentence describes the critical areas. Our disagreement is on whether *current* jet engines are capable of doing what you suggest. Also how the craft ascends effects your numbers. My HTVL configuration needs a lot less take off thrust, hence engines, then a VTVL configuration. So the trade off in weight and engines would be very different for VTVL jet/rocket SSTO. Even a HTVL that does a fast acceleration up into vacuum like Rascal would need a lot more jet engine thrust (hence weight), vrs a HTHL that flies a more moderate ascent to max airbreathing altitude and speed. So at what point a given engine is acceptable, depends on the details of the craft that needs it, and what its design priorities are? Details are the thing.:-) I feel that the mass of airframe more than offsets the engine thrust requirement. I don't like VTVL myself. It is however, a useful method of determining relative penalties of equipment. If your question is at what T/W can a jet engines weight in a VTVL SSTO pay for itself in dry weight. That depends on how much of the total Delta-V it can deliver. For example. If your total delta-V to orbit (including virtical assent) is 8730 m/s (which is a standard LOx/Kero delta-v to orbit number). I use 9,000, which I got from people on this group a few years back. If the jet can deliver 3670 m/s it would need a T/W of about 23 to break even on the dry weight, if you ignore lower delt-v requirements due to a lower airdrag on the smaller craft. If the jet can deliver 1835 m/s it would need a T/W of about 40 to break even on the dry weight. Another set of numbers to plug in. Thanks. My personal opinion is that the most profitable launch system in the near term would be based on a HTHL launch assist platform capable of Mach 3-6 with staging at very high altitude and low dynamic pressure. Upper stage, dunno. Basically Len Cormiers earlier Space Van concepts. The VTVL in the subject is to find performance levels required that are clearly not contrived to make my airbreathing concept look good. For what I have in mind, I need to be able to clearly state the performance penalty to be paid for operational flexibility and somewhat easier development/testing environment. Kelly Starks "Humans are a race of compassionate predators." |
#14
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Air Breathing for VTVL
johnhare wrote
My personal opinion is that the most profitable launch system in the near term would be based on a HTHL launch assist platform capable of Mach 3-6 with staging at very high altitude and low dynamic pressure. Upper stage, dunno. Let's consider the upper stage first, and work backwards. Say we want 10 tons in LEO. A mass ratio of around 4 is good, because we can get dry mass to around 10% without expensive scrimping, leaving 15% of the gross mass for payload. So the gross mass is 65 tons for our 10 tons of payload. Nice'n'easy. If a LOX/LH2 stage is used, with an isp of 440 and a mass ratio of 4, that's 6,000 m/s delta V. To get somewhere interesting you need to start at about 1600 m/s horizontal velocity and 100 km of height (second stage start at 100 km is good, no Q to worry about so you can leave the aerodynamic structure mass in the booster, you get vacuum isp's, plenty of coast time for separation, and so on). So that's the target. 65 tons to 100 km altitude and 1600 m/s horizontal. You can mess with that a bit, but somewhere in there is what's needed. I don't think a pure airbreather will do that, at least not easily. But I do think a HTHL airbreather with LOX/kero rocket boost could do it. Preferably piloted, though the second stage needn't be man-rated. Something like: jet fuel 70 tons rocket fuel/oxidiser 190 tons 2nd stage gross mass 65 tons 1st stage dry weight 110 tons flyback fuel 15 tons for a 450 ton GLOW, suitable for standard aviation-type runways. No pads, just an extra hanger with LOX/LH2 supplies at an airport. You can maybe even use the jet turbines as pumps for the rocket fuel. Or part-burn fuel to run the jet engines when there is no air. Use the rocket engines as RATO's. And so on. -- Peter Fairbrother BTW - I was looking at teeny jets recently, around the 100N size. They have a T/W around 11, and exhaust velocities around 1150 m/s. Cost a couple of kilobucks. But, they have radial compressors . |
#15
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Air Breathing for VTVL
"Peter Fairbrother" wrote in message ... johnhare wrote My personal opinion is that the most profitable launch system in the near term would be based on a HTHL launch assist platform capable of Mach 3-6 with staging at very high altitude and low dynamic pressure. Upper stage, dunno. Let's consider the upper stage first, and work backwards. Say we want 10 tons in LEO. A mass ratio of around 4 is good, because we can get dry mass to around 10% without expensive scrimping, leaving 15% of the gross mass for payload. So the gross mass is 65 tons for our 10 tons of payload. Nice'n'easy. This is one of the reasons I suggested on SSP that a healthy suborbital industry* is more than halfway to a healthy orbital industry. It was fairly easy for you to figure the numbers on an upper stage. It will be easy for others to do the same. If a LOX/LH2 stage is used, with an isp of 440 and a mass ratio of 4, that's 6,000 m/s delta V. To get somewhere interesting you need to start at about 1600 m/s horizontal velocity and 100 km of height (second stage start at 100 km is good, no Q to worry about so you can leave the aerodynamic structure mass in the booster, you get vacuum isp's, plenty of coast time for separation, and so on). So that's the target. 65 tons to 100 km altitude and 1600 m/s horizontal. You can mess with that a bit, but somewhere in there is what's needed. I'm more in favor of flexible performance targeting early on. This involves finding the break points of various systems and staying well under them. If, for instance, the prefered TPS becomes a problem after Mach 5.5 on reentry, then it might be more profitable to stage at a velocity and altitude that gives less than that. I don't think a pure airbreather will do that, at least not easily. But I do think a HTHL airbreather with LOX/kero rocket boost could do it. Preferably piloted, though the second stage needn't be man-rated. I'm quite sure that airbreathing will not do the whole booster job. It will take rockets to get the Mach numbers/altitude I suggested. OTOH, if it turns out that airbreathers can economically provide mach 2.5 at 40 km, then it might be more profitable to use the single system on boost. Understanding that no current airbreather will do this. Something like: jet fuel 70 tons rocket fuel/oxidiser 190 tons 2nd stage gross mass 65 tons 1st stage dry weight 110 tons flyback fuel 15 tons for a 450 ton GLOW, suitable for standard aviation-type runways. No pads, just an extra hanger with LOX/LH2 supplies at an airport. And smaller. There is some market for smallsats if you get the price down. You can maybe even use the jet turbines as pumps for the rocket fuel. Or part-burn fuel to run the jet engines when there is no air. Use the rocket engines as RATO's. And so on. Rocket pumps are a severe mismatch to jet turbines. Your point on looking at all possibilities is right though, as long as you keep using numbers and letting them do their job. -- Peter Fairbrother BTW - I was looking at teeny jets recently, around the 100N size. They have a T/W around 11, and exhaust velocities around 1150 m/s. Cost a couple of kilobucks. But, they have radial compressors . What is wrong with radial compressors? Any idea on the turbine inlet temperatures? I should mention that teeny compressors can get into trouble at altitude due to the drop in Renolds numbers. *A healthy suborbital industry being 2 or more vehicle types being flown daily by 3 or more companies. If the operating companies have to look through their records to figure out how many flights since the last abort, we are on the way. |
#16
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Air Breathing for VTVLo
johnhare wrote
"Peter Fairbrother" wrote BTW - I was looking at teeny jets recently, around the 100N size. They have a T/W around 11, and exhaust velocities around 1150 m/s. Cost a couple of kilobucks. But, they have radial compressors . What is wrong with radial compressors? They don't accept high inlet velocities as well as axial ones. ? Am I right? No-one since Whittle has used radials by choice, but my understanding may be flawed. I think teeny jets have to use radials, not axials, for Reynold-number type reasons. Any idea on the turbine inlet temperatures? No, sorry. EGT max is only 700 C tho'. Talking of models, I used to use Jetex "augmenter tubes" when I werre a lad. They "captured and entrained" air with the the Jetex exhaust, and heated and accelerated it (supposedly!. Long story, but they did actually work, and far better than the static figures show when at speed and tuned up). Here's a link: http://jetex.org/motors/motors-accessories.html The math possibilities are indubitable for an air-breather (tho' it need not even "breathe" the air, just heat it and expel it, like an aircraft uses the air all around for lift). (Jetex motors were refuellable solid-fuel rocket motors, used mostly to power model aircraft as opposed to rockets. They were fairly long-duration, about 10 seconds or more. I've been told they are still made.) -- Peter Fairbrother |
#17
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Air Breathing for VTVLo
Peter Fairbrother wrote in message
They don't accept high inlet velocities as well as axial ones. Thats what a complicated and heavy intake is for. But for a given tip velocity the inlect velovity is lower on a radial compressor ? Am I right? No-one since Whittle has used radials by choice, but my understanding may be flawed. The liquid air plants I have seen use large radial compressor and expanders. For flight engines i belive thay are still used in turboshafts IIRC. The main reason thay are avoided in aircraft is their very large frontal area compared to axial flow types. I think teeny jets have to use radials, not axials, for Reynold-number type reasons. There are blade tip clearance losses that dominate in small turbines also. Radials don't suffer as badly from that esp. if there are of the closed impeller type. Greg |
#18
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Air Breathing for VTVLo
Greg wrote:
There are blade tip clearance losses that dominate in small turbines also. Radials don't suffer as badly from that esp. if there are of the closed impeller type. Are radial compressors more subject to FOD? Paul |
#20
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Air Breathing for VTVLo
"Paul F. Dietz" wrote in message ...
Greg wrote: There are blade tip clearance losses that dominate in small turbines also. Radials don't suffer as badly from that esp. if there are of the closed impeller type. Are radial compressors more subject to FOD? Paul I think so. Its more a function of tip velcity which is very high on both radial and axil flow compressors. But i could be wrong. Greg |
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