11-28-07, 10:31 AM #21
"...why not simply avoid all the high cost of the magnetic accelerator and get your ram jet up to operational speed with space craft "piggy backed" on top of a more conventional airplane? Not too hard to get in going half the speed of sound essentially straigth up and a airplane is much cheaper than the magnetic accelerator that can achive v= 172m/s in 251meters. ..."
I do not know whether or not the "space craft" is multiple stage or not. Probably must be multiple stage if the package to be placed in orbit is more than a few hundred pounds.
Also if there is to be a ram jet first stage, it needs denser air so with an orbital package of 1000 pounds or so. I.e. a few tons for the total rocket/"space craft" the lifiting airplane has on its top, it may get the ram jet going at only 20,000 feet and headed nearly vertically upward just before separation. I do not know how high a ram jet can be used (perhaps that separation is at 15,000 feet?). The "ram jet" might actually be twins on each side of the rocket that takes over at the not functional altitude of the ram jet pair when they detach.
I am not sure there is any advantage to any of this compared to what is done - i.e. compared to just multiple stage rocket and fall away boosters strapped on to it for the initial phase of a strictly vertical launch.
Last edited by Billy T; 11-28-07 at 10:49 AM.
11-28-07, 10:44 AM #22
11-28-07, 10:46 AM #23
I did some simplified calculations.
Veho -impulse speed for H+O propellent = 13M (Mach)
Veb -impulse speed for solid state boosters used in SS = 8.8M
Vej -impulse speed for jet engine = 100M (because on oxygen on board)
Aef - aerodynamic fudge factor (to make Space Shuttle number more or less right).1 - no drag, the higher number the more drag. At higher altitude I assumed 1.
Based on Tsiolkovsky rocket equation, expanded mass fraction to gain dV speed using propellant impulse speed Ve: Nme = 1-e^(-dV/Ve). I multiplied Nme with Aef to include air drag
Ms - mass at start (T - tonns = 1000kg)
Me - mass at end (T) (after Md dropped)
Md - detached mass (T)
Me = Ms-Md-Aef*Ms*Nme
- We are attempting to deliver 100T to LEO orbit
- I was force to increase em sled speed to around 3M to allow scram to work
- I estimated carrier plane empty weight at 120T
1'st stage: dV= 0-5M, Veb=8.8M, Aef=1.45, Nme=0.63, Ms=2000T, Md = 150T, Me=597T,
2'nd stage: dV=5-28M, Veho=13M, Aef=1, Nme=0.83, Ms=597T, Me=102T
em-sled: dV=0-3M, Ms=Me=500T,
scram: dV=3-15M, Vj=100, Aef=1.8, Ms=500T, Md=120T, Me=280T
rocket: dV=15-28M, Veho=13M, Aef=1, Ms=280T, Me=102T
jet: dV=0-3M, Vj=100, Aef=1.8, Ms=530T,Me=500T
scram: dV=3-15M, Vj=100, Aef=1.8, Ms=500T, Md=120T, Me=280T
rocket: dV=15-28M, Veho=13M, Aef=1, Ms=280T, Me=102T
All this points roughly that em sled allow you to save 30T of fuel when attempting to deliver 100T to LEO.With this data, I'm convinced that jet-scram-rocket bits hands down em-sled.
Also, The saving using air breathing system is quite impressive
11-28-07, 10:50 AM #24
ael65...another problem with your suggestion is the fact that at high altitudes the friction caused by air molecules bombarding the surface of the air vehicle is much smaller...thus less thermal cover is needed.
Down at the surface level the air molecule concentration is much higher thus the air vehicle would heat up to very high temperatures.
11-28-07, 11:02 AM #25
If you want me (and probably others) to follow/check your analysis, then you will need to expand it - explain it more fully. For example, Ve is not defined (Ithink it is the set of values you give in terms of mach numbers, but at what temperature as speed of sound depends on that) and your assumption not clear etc. Is dV a differential increment of velocity of what? If it is, do you not need to integrate? As it stands, it is just a claim for what you started out believing.
Last edited by Billy T; 11-28-07 at 11:11 AM.
11-28-07, 11:49 AM #26
1) check http://en.wikipedia.org/wiki/Tsiolko...ocket_equation. This is a basis for my Nme formula. The Ve is engine impulse expressed in terms of speed. I'm using Veho,Vej,Veb in place of Ve for approperiate cases.
2) Go to http://en.wikipedia.org/wiki/Specific_impulse page where I took Veho,Vej,Veb values from. As a units I picked Mach numbers for speed and tonns for weight (just to keep them smaller)
3) Go to http://en.wikipedia.org/wiki/Space_shuttle to learn about SS parameters
4) dV is simple 'delta V' change of speed of the craft induceed by engine expelling exhaust at speed Ve and in a process using up Nme part of its weight. With Ms - liftoff mass, you can get Me mass at the end simpy: Me = Ms(1-Nme).
5) I new that at high altitude friction is much smaller, that is why I neglected it all together. I introduced Aef - aerodynamic fudge factor to make Space Shuttle numbers to agree. This factor turned out to be 1.45. I simply multiply Nme with Aef to get things right. I'm sure that this is heresy and must be better way to do it, but I just wanted to get things roughly there. For scram and jet phase lacking better idea I increased this number to 1.8 trying again to fudge things a little. You can notice that Aef is only used at the first stage and scram phase, latter on is 1 (no drag)
6) The whole puprpuse of this exercise was to relate the 3 propulsions in a bulk part. And for that I believe number shows clearly em-sled is not contributing much. 30T of jet fueal is one transatlatic trip, peanuts.
7) I can send an excel spreadsheet if someone is interested, but is so simple, you can write your own in no time.
11-28-07, 02:32 PM #27
One more thougt.
If you don't use jet/scram and just want to shoot single stage rocket to orbit with sm-sled:
em-sled: dV=0-3M, Ms=740T, Md=40T,Me=700T
rocket: dV=3-28M, Veho=13M, Nme=0.85,Aef=1, Ms=700T, Me=102T.
(I counted 40T for undercarrige and parachutes)
Here you save 2000T-740T = 1260T, almost 2/3th. This is an impressive saving as well. If scram jet is hard to develop technology, then rocket+em sleds is an option.
11-28-07, 03:06 PM #28
Thanks that helped a lot.
Your Veho = 13M does check out with your wiki link's 4400m/s as 4400/13 = 338m/s a very reasonable sound speed or M value.
Your Veb = 8.8 for solid state booster and wiki's 2500, however give 2500/8.8 = 284m/s. for that to be the spped of sound the air would need to be very cold (too lazy to see if it even exist at any altitude, but think not, certainly not where solid state boosters are used.
like wise your Vej =100M and wiki's 29,000 give M = 290m/s
Before I do more I will let you redo remainer as needed, if I have not misunderstood you.
I read wiki on scram jets also. APL/JHU where I worked had a scram jet project, but did not tell much about it (need to know etc. which I had none) APL/JHU also had at least one of the world's leading experts on combustion. The apl/JHU scramjet may have been unique in that it was not hollow, but used external combustion. I never saw any of the shapes tested, but think they must have been sort of tear drops (blunt end advancing first) with the fuel released in a ring near max diameter. The outer "walls" of this external combustion chamber were just the high Mach number air stream and the energy added by the fuel to the already very hot shock-heated gas expanded into the "tail space" of the tear drop yet it had more pressure times area = force (or at least that was the hope) pushing forward behind the max diameter than the stagnation pressure forces in the hypersonic flow around the blunt end was producing on the front side.
There was no need to cool the APL/JHU test shapes as the duration of the hypersonic flow was only a small fraction of a second. It was produced by a diaphram rupture shock wave generator with large high pressure tank bursting the diaphram, providing some duration to the post shock flow into the pre-evaccuated observation tube and it also had a large evacuated "dump tank" at the far end of the observation tube. I do not know if they ever got net thrust or not, but think they did as project continued funded for several years, perhaps more than a decade. It may still be going on as APL/JHU is the lead agency developing the ABM system deployed on AEGIS ships - no warhead - it collides with the incoming balistic missle!! - a "Kinetic Kill" and the closing on it is at very hypersonic speeds. An amazing guidance / controll problem has been solved - hitting a bullet with a bullet is a slow simple task by comparison.
If you read the wiki scram jet article, you will see there is a long way to go in development before anything even vaguely resembling a scram jet will really fly, perhaps never will. Some of the more candid expertswith no funded project call it the "scamjet" (gets $ from military in a technical scam) and other say it is the least efficient way to orbit anything. Thus, for at least several decades still I would forget about scram jets.
You were wise to recognize the need to increase to about M=3 the terminal speed of the EM rail gun as that is where the ram jet does well, OK from M=2 to 5. Thus I think if you want to continue looking at alternatives, you need to compare the EM railgun with the cost of a "large fighter" airplane.
I.e. an airplane that can lift subsonically the ram jet on its back at least to above 30,000 feet (40,000 would be good target) and then go sonic to mach of at least 2.5 in a dive of less than 10,000 feet where if pulls a lot of "Gs" to level out and turn up to at least a 45 degree angle just before ignition and then release of the ram jet.
As I suggested before, this "ram jet" is probably a pair strapped on to the sides of the rocket (oxidizer inside) which takes over when the ram jet is running out of air and falls away from the "third stage" (first rocket stage)
I suspect that this type of approach which uses the air for the first two stages (airplane and ram jet) may have a change of being attractive, but still doubt that as we surely are reinventing an idea already well analized and rejected by a dozen or more space interested organizations 30 or more years ago. I.e. a week after the first time a "buz bomb" flew over England in WWII (or even Von Braum earlier in German) must have had idea of not lifting all the oxidizer for trip into space.
Last edited by Billy T; 11-28-07 at 03:41 PM.
11-28-07, 03:56 PM #29
I may be wrong and the real data on the super sonic transport's "take off weight" to after "passengers leave landed weight" might be a fair test of my hunch. Note I am letting you call all of the fuel except the required safety margin at landing as "lifted weight" since the EM rail gun does not need to lift or accelerate any of its opperational fuel.
*Perhaps a wide horizontal sheet of stiff aluminium alloy and eddy currents can be used. That will be more disipative that iron magnet sled and at these speeds, the effective frequency for the skin depth of the aluminum may be a problem (Why I said "wide sheet")
Note a "magnetic rail gun" is just an electric motor in linear, rather than circular form. Both the "rotor" and the stator" of all electric motors I know of are heavy**. When made linear, each part of the stator is used only briefly. Even the price of magnetic laminated iron (much less copper) per pound of the stator long enough for Mach 3 is scary, I think.
**We are not talking about a clock motor here - this may be the world's most powerful electric motor, by far. Again you are not, I think, fully appreciating what it costs for that peak power capacity. Accelerating 740 tons to Mach 3 in 15 or so seconds is power demanding, no matter how you do it.
PS if you have not already realized it, I am too lazy to do much searching.
Last edited by Billy T; 11-28-07 at 04:31 PM.
11-28-07, 10:45 PM #30
I used Mach numbers casually assuming 1M=1200km/h for simplicity, I paid no attention to sound speed change versus altitude since Tsiolkovsky formula needs speed ratio relative to inertial observer. I found 28M somewhere as LEO, and now I see that at high altitude it is really 25.4.
Veb=8.8 comes from (7.8*4+13)/5 which represents 4/5 of thrust achieved by boosters at impulse 269s and 1/5 at 455s main engine. This is again a fudging around.
As to Vej=100M I had to make a mistake, since I remembered somewhere 30k-40k and pick it in the middle despite 29k as you pointed out.
11-28-07, 11:08 PM #31
11-29-07, 08:29 AM #32
You really need to look at the power required by the EM rail gun that can launch 740 TONS to Mach = 3 in about 16 seconds. You seem to be ignoring this problem. I would not be extremely surprised if all the electric power generation capacity in the US was inadequate! If guessing, I would bet it is more than the grid serving Disneyland (your prefered site) can make.
You might consider in what time it gets the 740 tons up to 60mph just for a fun comparison to a "mussle car" accelerating only a ton. Also don't forget the rail gun is only inches from a very powerful, dense-air, shock wave. We are neglecting the power put into that shock wave and the losses in the energy storage system that is required and the magnet current windings. Perhaps the stresses on the magnets (the reaction force you once forgot) may cause them to be very expensive. I also note the shockwave will bounce off the ground probably still with considerable destructive force in a "whip/saw" 1/2 punch added to these reaction forces.
Summary: If you are serious about use of the rail gun (even neglecting huge cost) you need to consider these questions.
Last edited by Billy T; 11-29-07 at 09:32 AM.
11-29-07, 03:23 PM #33
Well not a small thing. Nasa in 2002 was hoping to use it to bring launch costs down 10 times. Check: http://archives.cnn.com/2002/TECH/sp...hes/index.html
11-29-07, 06:56 PM #34
I doubt if it is a good idea (for reasons already given) but think someone might look more seriously at my idea which evolved during our exchanges. I.e. why not a "large fighter" airplane with twin ram jets explosive bolt strapped to sides of space rocket "third stage" mounted on top? There is essentially zero risk in making a "large fighter" which could take the ramjet & rocket to about 40,000 feet efficiently sub sonically then power dive straight down to get to about M=3 certainly above 30,000 feet anyway.
As it is strong (fighter like) it then pulls a human-limited 5 or 6 G turn of about 135 degrees so it is headed up at about 45 degrees when the ram jets start thrusting. They (and still joined space rocket) separate from the "large fighter," perhaps just by accelerating off short rails. Surely the ram jets & and rocket could still be doing M=2.5 in that cold air and the ram jet "second stage" would function ok. The ram jets would be getting better as it climbs and speeds up to about M = 4. I do not know at what altitude the ram jet "flames out," but it intentionally does not climb to that altutide until going at about M = 6 (or 7+ if well designed for thinner air.)
Thus, incontrast to the typically multi-stage space rocket, which may have 2/3 of its initial weight as oxygen, only the third stage's oxygen is not taken from the air. Seems like a great economy with both first and second stages reusable and carrying no oxygen, plus the third stage turns on only at high altitude when already going at M > 6.
I am too lazy and too old to find the flaw in this idea, if there is one, but you might be interested in "shooting it down" (Part of why I have been hard on your EM rail gun launch idea is to help motivate your revenge. - I think you are good enough with physic to kill my idea and as it surely has occured to some others, there probably is something wrong with it, but I do not see where that problem can be hidden.)
I am open to (and hoping for) a good attack on my suggested alternative launch system, by anyone.
Last edited by Billy T; 11-29-07 at 07:27 PM.
11-30-07, 12:23 AM #35
The other question to ask: is it better to get oxygen from the ground or collect it at high altitude/high speed and liquify to fill rocket tank ? Liquify means to take away kinetic energy, let say from 300K to 90K for O2, that means lower molecule speed from 480m/s to 268m/s. This sound easier then accelarate from 0 to 2400m/s.
11-30-07, 09:43 AM #36
I do not know the optimium profile for ram jet to fly, but bet the angle with the horizontal should be something like: 45 + 2t where t is the seconds lapsed from separation from stage 1. E.g. after 20 seconds, then the ram jet's angle wrt horizontal would be 85 degrees and then remains at 85 degrees for t > 20 until flame out / space rocket separation occurs. (90 degrees, "straight up" is surely a bad idea as after ram jets separate for space rocket it would be hard to prevent a "tumbling re-entry" back into denser air, which would probably destroy them. Even 85 may be too vertical as you want there to be a well defined "down" on some side, not the tail, of the ram jets.)
All this would take place over the ocean, south east of Miami, Florida, and small foldout wings, initially curved around the ram jet body into conformal recesses, would guide it to near a waiting recovery ship. Perhaps it flies into balloon supported capture net to avoid even the weight of any parachute. (Such nets were used to capture spy sattelite film packs back in the days when high resolution digital images were not possible, I am almost sure.)
As ram jet must be strong to also survive the 5 or 6 Gs produced when the first stage turns upwards after its straight down power dive, perhaps on relatively calm sea (that is predictable) it simply plows thru some wave crests after a near surface "almost stall manuever" and floats while recovery ship comes to collect it.
There are a lot of options, many of which have been used before. That is also why I like my idea - everything is "old hat" with low development risk.
On your idea of liquifying O2 that is already at altitiude for the space rocket, instead of transporting it up from the ground, I do not see any hope for that for two reasons, either of which I think is completely lethal to the idea:
First a quite heavy compressor would be required to fill the space rocket's tanks in a short time, even if the extra O2 desired could be scooped up without such excessive drag added on the ram jets that their thrust would be totally cancelled. ("Short time" is required as the ram jet is burning fuel while the O2 is being collected from the air. If collection is done too slowly, the weight of this extra fuel will be greater than the weight of the O2 required for the space rocket.) I.e. extra drag, extra ram jet fuel weight, and the compressor weight easily kills this idea.
Secondly, you already have some heating problems to worry about at M = 3 or greater in the atmosphere. How in the world do you propose to dump the heat produced when atmosphereic O2 is compressed to high density? Without dumping this heat, the O2 will be a very high pressure gas, reqiring storage tanks with steel walls several inches thick. -i.e. much more weight to lift than just taking the liquid O2 the space rocket requires up from the surface. If you try to dump the heat with some heat exchanger in the air stream, then I am sure the ram jets's thrust is cancelled several times over! Again idea is dead. Lets just keep it as simple as possible, with zero new technology required. (See foot note on this also.)
*People in APL/JHU's space department were always trying to combine several functions into one new insturment to save weight. The head of the department always like to reuse the "tried and proven" separate insturment again. He had a great way of re-inforcing the fact that combining functions in one device is seldom optimal for either. He would say: "I only know of one "tool" which is ideal for two different tasks, and God made that one."
Thus, like him, I prefer separate, well proven concepts for the air breathing stages, which can be optimized for their different tasks.
Last edited by Billy T; 11-30-07 at 10:18 AM.
12-01-07, 01:22 PM #37
I recommend looking at http://www.onera.fr/conferences/ramj...amjet-pde/#pde
Toward the end the PDE concept is discussed. I particularly like the theoretical graphs showing unheard of specific Impulse for PDE in a range of 9K, particularly important that it goes from 0 to 5M. This is very interesting because if someone managed to marry PDE with scramjets in a single unit the whole speed range 0-15M can be covered very efficiency. This gives more validation to this thread name pulse-ram-scram. There is also talk about pulse scram concept.
12-01-07, 06:07 PM #38
12-01-07, 06:10 PM #39
12-01-07, 10:05 PM #40
To dragon & psycha:
Either or both of you can be correct. It all depends on how you define "friction" and whether or not you are going super sonically thru the air:
I think you both probably know what follows:
Always there is little more than a few molecules thick transition region next to the skin of the moving craft, in which wrt to it the air stream speed increases from zero to nearly the flow velocity an inch or so distant from the skin.
If the speed is sub sonic that is about the "whole story" and air can be treated (and is) as an incompressible fluid.
If the speed is super sonic, then the air flow has two very distinct flow regions separated by a quite thin "shock" layer. The inner region (next to the moving craft) is basically like sub sonic flow but thru denser air, as the air density is increased behind the shock transition layer. (This increase in density, or compression as molecules transit the shock layer, is the main cause of the heating and disipation.)
Thus, if "friction" means "rubbing against the side of the craft", there is little friction at work in super-sonic flight. If however, "friction" means "motion induced collisonal disipative processes at the atomic level in a thin layer" that is in fact what is happening mainly in the shock transition layer.
Last edited by Billy T; 12-01-07 at 10:11 PM.
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