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Some time ago, I posted a topic wherein I opined that given the very brief operational life of an EDL system, it might be possible to use an electric propulsion system to soft land. That system consisted of four RamGen technology supersonic (the inlet velocity is supersonic and can achieve 10 to 1 compression in a single stage, unlike ordinary turbines) CO2 compressors driven by high speed electric motors powered by high energy density primary batteries, effectively electric turbojets, to soft land and perhaps maneuver to an alternative landing spot if the landing area contains debris that could damage a pressure vessel landed on uneven terrain. Any rocket engines would provide mere seconds of thrust or require substantially more fuel as a function of their high fuel consumption. The specific impulse issue would be why we use turbines instead of rockets for normal powered flight. I wanted to revisit this topic because a company named InEnSto claims to have an Aluminum-Air primary battery with a gravimetric energy density of 2.3kWh/kg and a volumetric energy density of 6.2kWh/L.
At the time I posted the topic, Tadiran manufactured 700Wh/kg primary batteries for military and space applications. I was able to determine that using the Tadiran batteries, the mass differential between rocket engines using hypergolic propellants that provide little to no maneuvering capability and an electric propulsion system comprised of four vectoring thrust supersonic CO2 compressors affixed to the docking ring of a Cygnus PCM, the electric motors, and primary batteries was a wash, with respect to the mass required to achieve the desired outcome of permitting a surface exploration crew several minutes of maneuver to an alternative landing site. Since that time, substantially better batteries are now in advanced stages of development.
The desired landing area may be up to several tens of kilometers away from the actual landing area if the reentry isn't perfect due to a miscalculation or the desired landing area is an impassable debris field, upon close inspection. It would make a lot of sense to have the option to abort to an alternative landing area and an electric powered flight system not limited to precious seconds of operation lessens the consequences associated with imperfect reentries or poorly selected landing areas. The reentry maneuver and landing area selection would obviously be done as best as humanly possible, but a powered flight system subtracts out any "fudge factor" for a smooth EDL experience.
If this 2.3kWh/kg Al-Air battery becomes a viable technology for military applications, which is why I believe the Israelis originally developed it, then it could also be used for an electric powered flight system. The battery tech is ideal for this particular application. With more than three times the energy density of existing primary batteries, I think this application deserves renewed consideration.
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I do dimly remember something about a supersonic compressor. It was maybe a couple of years ago. I remember chasing down the documents on the research project. What jumped off the page at me was it had a very narrow band of operating conditions, something incompatible with operating over a wide range of speeds during a landing.
I'm not understanding how you want to employ this gadget. Or how you intend to mount it on the spacecraft. It won't really be an electric gas turbine engine, because you seem to be replacing the turbine and combustor with an electric motor and a fancy battery. It's more like an electric-driven ducted fan.
Entry speeds at Mars vary from 7.5 km/s for direct entry from an interplanetary trajectory, to about 5 km/s at Mars escape speed, to about 3.6 km/s from low Mars orbit. Using a very nominal figure of 230 m/s for speed of sound in a cold CO2 atmosphere, those speeds correspond to Mach numbers somewhere in the vicinity of 32, 21, and 15. Blunt objects come out of "hypersonics" at Mach 3, which is just about 0.7 km/s speed on Mars.
The only difference is for low ballistic coefficient (well under 100 kg/sq.m) you come out of hypersonics somewhere in the vicinity of 15-25 km above the surface. If high ballistic coefficient, this is more like 5 km above the surface and perhaps seconds from impact. We have ringsail chutes which can be deployed up to Mach 2.5, and whose terminal velocities when reasonably loaded are high subsonic on Mars.
It takes time on the order of a couple of minutes for the chute to decelerate from Mach 2.5 to high subsonic, which is why this technology cannot be used to land high ballistic coefficient items: there simply isn't time for it to work. But retropropulsion will work, which is why Musk's BFS vehicle does what it does for its proposed landing sequence. And it's only about a 0.7 km/s theoretical delta-vee to do it. Even if you double that to 1.4 km/s effective delta vee requirement, it's still not that much.
You can beat the Isp of a rocket with an airbreather, yes, but you cannot come close to the rocket's frontal thrust density. Nowhere near close. Doesn't matter much what kind of airbreather. By frontal thrust density, I mean thrust divided by vehicle frontal blockage area. My gut instinct says for high ballistic coefficient vehicles with short timelines between Mach 3 and 0, you will need as much thrust as you can get. Last time I looked at this, I got vehicle deceleration requirements on the order of 2-5 gees, more than an order of magnitude higher than anything achievable with any kind of an airbreather.
Now this may or may not apply to whatever it is you had in mind, I dunno. But be careful, you can't do this as some sort of nacelle outside the main body of the spacecraft. Above about Mach 5 to 6, there's no way to survive the shock-impingement aeroheating where the nacelle bow shock strikes the main body.
I first heard of the aluminum-air battery around 3 decades ago. I don't know how that's coming along, I haven't kept up with it. You might consider thermal (salt) batteries instead. They have very high energy density. One-shot devices, but they work pretty good. Commercially available. Mature technology.
GW
Last edited by GW Johnson (2018-03-19 10:28:32)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Now this may or may not apply to whatever it is you had in mind, I dunno. But be careful, you can't do this as some sort of nacelle outside the main body of the spacecraft. Above about Mach 5 to 6, there's no way to survive the shock-impingement aeroheating where the nacelle bow shock strikes the main body.
GW
So project like Skylon with the two nacelles at the tip of the wings are conceptually flawed and cannot survive an atmospheric entry?
Last edited by Quaoar (2018-03-20 06:57:44)
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GW,
Here's a 1,152 page report (61.3MB) on RamGen for your reading pleasure:
Design and Testing of CO2 Compression Using Supersonic Shock Wave Technology
If I've missed something about the fundamentals of how RamGen works, please let me know.
My understanding is that these things run at constant speed. Varying input power adjusts where the shock wave is located on the inlet ramp, producing different levels of compression and thus output power. The throttle response time is reportedly measured in milliseconds. It's not like a conventional turbine.
The basic vehicle configuration is HIAD on the bottom of Cygnus and RamGen on the top. The individual nacelles are mounted on struts that are folded along the sides of Cygnus during reentry. The HIAD covers pretty much everything, so I don't see how shock impingement heating would be much of a problem. Anyway, HIAD slows Cygnus to low supersonic speeds at roughly 20km in altitude prior to detaching from the module, the nacelles are deployed and pointed into the oncoming flow, the electric motors spool up, and then the module is propelled into the oncoming flow while a transition to a hover is made. I wasn't operating under the assumption that the frontal thrust density would ever match that of a rocket engine and that's not what the intent was. If you're already at Mach 1 roughly 20km above the ground, then I'd think you'd have enough time to slow your rate of descent to zero and spend a few minutes or less looking for a suitable landing spot using a distributed aperture system like that found on the F-35. It's a form of virtual reality VFR flight, but very brief.
I'm not fixated on getting the dV increment correct to merely land on the spot you'd find yourself over if you followed a ballistic trajectory into the ground. I want the astronauts to have a few minutes of powered flight. If the landing area looks like a debris field, then I want them to be able to "fly" to a better spot for touchdown. This system is intended to result in a better outcome in the event of "oops, we messed up", not "everything went perfectly". I think we need extraordinarily detailed surface maps and perfect reentries or powered flight.
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I flipped through some of this, trying to figure out what they were trying to do. It sort of looks like a centrifugal compressor, I presume operated at supersonic flow speeds. It seems coupled to some kind of combustor that makes use of sudden-dump vortex flameholding, although all the terminology looked different from what I saw in ramjet work. Got no clue yet what they were doing for a turbine, or how any of this couples to any sort of electric drive in a CO2 atmosphere concept. This is going to take a while to dope out. Reading giant pdf files in google docs is almost impossible. I did save the thing.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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GW,
Sorry to hear about your back troubles. I hope all is well. Hang in there. The pain becomes more tolerable with time.
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Kbd512:
Thanks. The surgery worked, and I didn't need the hardware, just a piece of bone removed. First few days out of surgery were rough, but I'm getting a lot better now. Staples come out next week.
I did figure out that the Ramgen guys are mixing and matching components for these various tests and applications. Some of them use a more-or-less conventional centrifugal compressor (just with transonic flow inside parts of it), and they are working on something highly experimental that they call a "rampressor", which uses supersonic shock compression in a geometry more-or-less like a Mazda rotary engine. Both conventional centrifugal compressors and this rampressor thing are capable of single-stage compression ratios in the 5-10 range. Centrifugal compressors are inherently one-stage, but are also inherently voluminous. The rampressor offers compactness.
I'm just guessing that your idea is to put these components together into a nacelle (or "nacelles") as an electric-driven source of high pressure air to be expanded for thrust production. The nacelle is (or "nacelles are") hidden in the wake behind the aft end of your Cygnus-as-airframe module for entry, and extended for operation afterward.
Instead of worrying about capturing supersonic slipstream air, let it draw air from the wake zone, and just put the exit out into the slipstream for retropropulsion. The air draw will reduce the wake zone pressure, increasing drag, and thereby amplifying the retro thrust effect that you want.
The only problem I forsee with a scheme like this for Mars is the extreme low pressure of the atmosphere (about 0.6% Earth-normal at the surface). Compression-wise, 10 times nothing is still next-to-nothing (~6% of Earth normal). I doubt you'll get much thrust relative to the weight, even reduced to 38% Earth-normal weight. At the low ambient pressure, the dynamic pressure of the supersonic slipstream air really won't change that picture, even if you found some way to recover it, with a supersonic inlet of some kind.
The application for this is more likely Earth than Mars, because of that incredibly-thin air on Mars. If you wanted to test such a thing here, you go to high altitude like they did with the Viking probe chutes. You get Martian ambient pressure at about 110,000 feet on a standard day. To demonstrate hover feasibility on Mars, you must offset 38% of the test vehicle's weight with your thruster, operating at 110,000 feet.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Glad to hear about successful surgery, GW.
As to drawing air from the wake zone of a body, isn't that what a U-tube pulse jet does?
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Hi Elderflower:
Absolutely right that a U-tube pulse jet draws wake air the best. It affects the drag of the body creating the wake by a tiny amount, because wake zone pressure is reduced when you eject some of its mass. That acts to increase the drag.
Doing what Kbd512 suggests to produce a retropropulsion stream also affects the basic body drag. There were some supersonic wind tunnel studies done under NASA funding in the late 1950's with a retro jet and a Mercury capsule shape. That one was right on vehicle centerline, and reduced drag slightly, with a correlatable mass-ejected effect.
This effect was published in 1961, and appears in a figure as a cited item in my copy of Sighard Hoerner's "Drag Bible". When Musk started his efforts toward recovering Falcon-9 stages, I saw lots of talk about how supersonic retropropulsion was supposedly impossible, conveniently forgetting this long-published study showing that it really works, and with a certain quantifiable effect on drag.
It's really nice to see how modern Spacex efforts and those early NASA results agree so well. Sort of restores faith in physics!
If you put the thruster device at the rear of the body, and let it suck wake air, the way I suggested to Kbd512, the two effects upon drag ought to roughly cancel each other. That makes for fewer potential errors in predicting trajectory before you actually attempt to fly.
GW
Last edited by GW Johnson (2018-03-25 11:31:26)
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Air-fueled Battery for Electric Cars
Researchers at the Scotland’s University of St. Andrews are working on a project on the air-powered battery. If successful they will replace the lithium cobalt oxide electrode in the fuel cell. The “STAIR” (St. Andres Air) battery will be compatible on all renewable energy resources such as solar, wind, and oxygen. Professor Peter Bruce who is leading his team for this project, is of the opinion, “Our target is to get a five to ten fold increase in storage capacity, which is beyond the horizon of current lithium batteries. The key is to use oxygen in the air as a re-agent, rather than carry the necessary chemicals around inside the battery.”
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Since reading RamGen links from another topic I am wondering if the aeroshell with a canted entrance for the inlet would work to capture then to be stored CO2 in internal tanks on the way down.
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