strap-on motors

scottb50 · Jul 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>What you are describing is known as a "cartridge loaded grain".  That concept is used in small motors like the Hydra 70 system where perfromance is not a major issue in order to have very low cost.  It hurts mass fraction and therefore only makes a bad situation worse for hybrids.  Isp for solids is not all that bad, about 264 sec (theoretical) and up to about 300 sec delivered, but the Isp density product is pretty good and the mass fraction for solids is very good.Hybrids tend to have a wee bit better Isp, but a lot more complexity and very poor mass fraction.  Shutting off the oxidizer is not much of an advantage.  If you do that you fall back to the ground.  Throttleability in rockets is useful for managing trajectory and managing dynamic pressure,  but doesn't do much for emergency situations.  You can't shut down and you can't fly back home.  Rockets are not airplanes. Posted by DrRocket</DIV> I agree with what you are saying, I just think the hyrid concept might offer benefits as compared to the classic solid motor. My point is a solid rocket offers a more compact and overall lighter pakage then liquid could. Look at the Buran, it used three Zenit strapons, a liquid only Shuttle would need 8-10 SSME's and they would be used once. That;s why the solid made sense. Re-using it cinched the deal. If it could have been built on site it might have worked, but politically it had to stay in Utah.Solids have the ability to provide large amounts of thrust when they are needed for a short time, once expended they are dead weight, unless they are dumped. If they don't have to get to orbit though they become dead weight for a short time with a TSTO vehicle, then it's a simple matter of aerodynamics and engines to return the burned out rockets as well as the empty propellant tanks. The ET doesn't go to orbit either, no use for it once it's empty either.The benefits I see with hybrids is more economical and environmentally friendly propellants as well as having the ability to shut the engine down if an impending failure is indicated. Ejecting the core could also be possible, reducing weight and allowing a return maneuver.If the intent is to fly people and engage in commerce it makes sense to make things as safe as they can possibly be. Reaching LEO obviously requires a huge amount of energy, I'm trying to think of the safest and most economical  way it can be done.My idea is a common Module, that can be assembled into various structures by adding more components. The common core of a vehicle would use liquid propellant Modules and solid propellant Modules, identical structures attached to one another, mounted on a subframe, to carry landing gear, and covered by a non-structural outer shell for aerodynamic needs. Modules would use two structures, a composite Cage that would use identical ends that could attach to any other identical end and a central band with four smaller attachment points, identical in concept to the ends. The second structure would be a single piece, filiment wound tube, defining the length of the Module.By modifying the inner section of the Module ends they could either pass over the Tube or contain the Tube. As an example a simple Module would use two Cages, both with one end open, to fit over the Tube and the other closed to hold the tube in place. Once the Cages are bolted in place you would have an extremely strong simple container. Put a third Cage, open at both ends and use a longer Tube and you have a bigger Module. Since each end of the Module is identical it can connect to any other Module as needed, having the smaller, but identical Adapters around the circumference of the Cage they can be attached side by side as needed also.http://www.photodump.com/Scottb50These are pretty simple examples. The TSTO stage would use 2-4 liquid propellant Modules and two solid propellant Modules. The Upper stage would use one propellant Module. The number of Cages and lengths of Tubes could be varied as needed, for this particular use we are discussing they are 15X30 feet Cages, but the same design could be made in infinite sizes, a .5X3inch Cage and a 6 inch Tube could be use as a power Module for an air/Hydrogen fuel cell, a 5x10 inch Cage could be used for an accumulator or hydraulic ram.     <div class="Discussion_UserSignature"> </div>

DrRocket · Jul 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>... If they don't have to get to orbit though they become dead weight for a short time with a TSTO vehicle, then it's a simple matter of aerodynamics and engines to return the burned out rockets as well as the empty propellant tanks. The ET doesn't go to orbit either, no use for it once it's empty either.This is not at all a simple matter.  In fact it is terribly complicated.  In order to fly a component back you need propulsion, fuel, a control system, and some means of landing relatively softly.  All of that requires complexity and weight, and detracts a great deal from the primary mission which is to propel a payload into orbit.    The benefits I see with hybrids is more economical and environmentally friendly propellants as well as having the ability to shut the engine down if an impending failure is indicated.Hybrids require pumps and liquid storage, like liquid rockets plus a poorly loaded solid fuel component, plus associated plumbing and control systems and are far more complex and hence costly than conventional solids.  They are not particularly more environmentally friendly either.  The environmental impact of solids is sufficiently minor that the government declined to participate in the development of "clean" solid propellants -- in particular the elimination of ammonium perchlorate as an oxidizer.  They did that on the basis of lack of motivation.  I personally made the presentation on what might be done and was personally told that the DoD didn't care, by the guy in charge of making that decision.  It has become a non-issue.As I noted earlier, the ability to shut down an engine doesn't really do much for safety.  If you turn off the engine you crash.  And most rocket failures occur very rapidly.  The challenger accident was a bit strange in that regard as the leak occurred over a prolonged period of time, but had there been a means of sensing the leak and had there been some means of escape from the main vehicle, then an escape might well have saved the crew and would not have required shutting anything down.  But in most rocket failures the time between first indication of a problem and a catastrophic failure is only 1 to 100 milliseconds, and usually on the short side of that window.   Ejecting the core could also be possible, reducing weight and allowing a return maneuver.If the intent is to fly people and engage in commerce it makes sense to make things as safe as they can possibly be. Reaching LEO obviously requires a huge amount of energy, I'm trying to think of the safest and most economical  way it can be done.It is not so much a matter of energy per se as it is "delta V", and delta V is a matter of thrust and low weight.  That is why rockets place such a premium on Isp and on low inert weight.   My idea is a common Module, that can be assembled into various structures by adding more components. The basic idea of a modular design is attractive.  Where it usually runs into problems is with weight.  You need to design the modules so that they provide the absolute minimum in inert weight.  That takes a lot of engineering work.  It may work in the future, and perhaps you have a good approach, but it is a very difficult engineering task.  The only successful solutions of which I am aware are the ELV family of launchers, and they are modular largely in the use of variable strap-on booster configurations.The common core of a vehicle would use liquid propellant Modules and solid propellant Modules, identical structures attached to one another, mounted on a subframe, to carry landing gear, and covered by a non-structural outer shell for aerodynamic needs. Modules would use two structures, a composite Cage that would use identical ends that could attach to any other identical end and a central band with four smaller attachment points, identical in concept to the ends. The second structure would be a single piece, filiment wound tube, defining the length of the Module.By modifying the inner section of the Module ends they could either pass over the Tube or contain the Tube. As an example a simple Module would use two Cages, both with one end open, to fit over the Tube and the other closed to hold the tube in place. Once the Cages are bolted in place you would have an extremely strong simple container. Put a third Cage, open at both ends and use a longer Tube and you have a bigger Module. Since each end of the Module is identical it can connect to any other Module as needed, having the smaller, but identical Adapters around the circumference of the Cage they can be attached side by side as needed This might work but the devil is in the details and in this case in the ability to minimize weight, consistent with your modular structure and the need to sustain flight loads.  To really evaluate such a concept would require detailed analysis to define all loads, launch, aerodynamics, shock and vibration, thermal, etc, and then more detailed analysis to determine if the structure could handle the loads.  This sort of analytical work is usually well over half of the engineering involved in a rocket design.  And you get to do all of the structural design with a weight budget constraining the design.also.http://www.photodump.com/Scottb50These are pretty simple examples. The TSTO stage would use 2-4 liquid propellant Modules and two solid propellant Modules. The Upper stage would use one propellant Module. The number of Cages and lengths of Tubes could be varied as needed, for this particular use we are discussing they are 15X30 feet Cages, but the same design could be made in infinite sizes, a .5X3inch Cage and a 6 inch Tube could be use as a power Module for an air/Hydrogen fuel cell, a 5x10 inch Cage could be used for an accumulator or hydraulic ram.     Posted by scottb50</DIV> <div class="Discussion_UserSignature"> </div>

spacy600 · Jul 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>  Hybrids require pumps and liquid storage, like liquid rockets plus a poorly loaded solid fuel component, plus associated plumbing and control systems and are far more complex and hence costly than conventional solids.  They are not particularly more environmentally friendly either.  The environmental impact of solids is sufficiently minor that the government declined to participate in the development of "clean" solid propellants -- in particular the elimination of ammonium perchlorate as an oxidizer.  They did that on the basis of lack of motivation.  I personally made the presentation on what might be done and was personally told that the DoD didn't care, by the guy in charge of making that decision.  It has become a non-issue.As I noted earlier, the ability to shut down an engine doesn't really do much for safety.  If you turn off the engine you crash.  And most rocket failures occur very rapidly.  The challenger accident was a bit strange in that regard as the leak occurred over a prolonged period of time, but had there been a means of sensing the leak and had there been some means of escape from the main vehicle, then an escape might well have saved the crew and would not have required shutting anything down.  But in most rocket failures the time between first indication of a problem and a catastrophic failure is only 1 to 100 milliseconds, and usually on the short side of that window.  Posted by DrRocket</DIV> Dr Rocket, Is this you?http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950025332_1995125332.pdf Also will you comment on this? http://www.dtic.mil/mctl/MCTL/Sec19MCTLg.pdf page 87, or just do a search on hybrid rocket in the document.Any comments on additives like IPDI, nano particles and the like would be helpful,Thank you.

DrRocket · Jul 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> Dr Rocket, Is this you?http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950025332_1995125332.pdfNo this is not me.  But I know both of the authors, and am familiar with the work described.  That work on clean propellants was interesting, but eventually went nowhere.  Ammonium nitrate as an oxidizer did not work out very well.  The lead propellant chemist at that time once described the problem as trying to replace the world's best oxidizer (ammonium perchlorate) with the world's worst oxidizer (ammonium nitrate) -- a bit of an exageration but actually rather close to the mark.  HTPE work went beyond clean propellant, but that was a different project, conducted elsewhere.Also will you comment on this? http://www.dtic.mil/mctl/MCTL/Sec19MCTLg.pdfpage 87, or just do a search on hybrid rocket in the document.Any comments on additives like IPDI, nano particles and the like would be helpful,Thank you.   This document is basically a DoD list of technologies that they would like to see protected, and despite disclaimers is mostly a list of things they would like to have controlled for purposes of export -- and it does have an influence on technologies controlled in commerce by the Department of Commerce and the Department of State.  The descriptions of the technologies are in some cases superficial, over-simplified, or inaccurate.  It appears that the descriptions of solids and hybrids were not written by experts, as they are quite over-simplified and inaccurate in some important aspects -- Isp estimates for one. I would read this document with some skepticism.  It provides some useful food for thought.  But there is a tendancy to make the list of critical technologies over long, and much over simplified.  There seems also to be a bit of over optimism and exageration.  Not surprising given the nature and purpose of the document.  There is no downside for the authors and DoD to taking an extremely optimistic view of emerging technologies, and no penalty for inaccuracy.I'm not sure where you ran across IPDI, but as I recall it is a urethane curative sometimes used in HTPB propellant formulations.  It can affect the ballistics slightly but the main role is in curing the HTPB and forming the cross-links that give structure to solid propellant.I'm not sure what you mean by nano particles with regard to rocket propellants.  Particle size for the solids in rocket propellants is important, and in fact particle size distribution for the aluminum and  ammonium perchlorate oxidizer is important in control of burn rate.  In many cases it is the factor used to adjust and tightly control burn rate.  The  normal partricles sizes are considerably larger than a nanometer, and particles that small would be difficult to produce and handle and would likely result in either very large burn rates or else might partially dissolve in the binder and result in erratic results.  I would also expect the explosive hazard to be quite severe with particles that small.I guess that one might put carbon nanotubes into propellant as a reinforcing agent, and perhaps a bit of a burn rate modifier.  Carbon black has been used for that purpose in some formulations.  But carbon black is relatively cheap and carbon nanotubes are quite expensive.  They are so expensive that I would think their use in propellant would be avoided.  My feel is that the benefits would be minor and the cost would be huge.If one were to use carbon nanotubes in the rocket business, I think the most likely application would be in the composites used for the rocket cases, and particularly in the resins used in combination with graphite fiber in the winding process.  Posted by spacy600</DIV> <div class="Discussion_UserSignature"> </div>

spacy600 · Jul 13, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>  I'm not sure where you ran across IPDI, but as I recall it is a urethane curative sometimes used in HTPB propellant formulations.  It can affect the ballistics slightly but the main role is in curing the HTPB and forming the cross-links that give structure to solid propellant.I'm not sure what you mean by nano particles with regard to rocket propellants.  Particle size for the solids in rocket propellants is important, and in fact particle size distribution for the aluminum and  ammonium perchlorate oxidizer is important in control of burn rate.  In many cases it is the factor used to adjust and tightly control burn rate.  The  normal partricles sizes are considerably larger than a nanometer, and particles that small would be difficult to produce and handle and would likely result in either very large burn rates or else might partially dissolve in the binder and result in erratic results.  I would also expect the explosive hazard to be quite severe with particles that small. Posted by DrRocket</DIV> I am not a member of AIAA so I can not get the whole thing:http://pdf.aiaa.org/preview/CDReadyMJPC2003_775/PV2003_4593.pdfAlso :These aluminium powders were used in manufacturing several laboratory composite solid rocket propellants, based on ammonium perchlorate (AP) as oxidizer and hydroxil-terminated polybutadiene (HTPB) as binder. The reference formulation was an AP/HTPB/Al composition with 68/17/15% mass fractions respectively. The ballistic characterization of the propellants, in terms of steady burning rates, shows better performance for propellant compositions employing nano-aluminium when compared to micro-aluminium. Results obtained in the pressure range 1–70 bar show that by increasing the nano-Al mass fraction or decreasing the nano-Al size, larger steady burning rates are measured with essentially the same pressure sensitivity. Print publication: Issue 33 (23 August 2006) Received 21 April 2006, in final form 12 July 2006 Published 4 August 2006 http://www.iop.org/EJ/abstract/0953-8984/18/33/S15  Regarding IPDI,  http://www.colorado.edu/ASEN/SrProjects/Archive/AIAA_Paper_Awards/MaCH-SR1_JANNAF.pdf Just trying to get a feel for the state of the art.Thanks for your thoughts.

DrRocket · Jul 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>http://www.itam.nsc.ru/users/libr/eLib/confer/ICMAR/2002/part_1/leets.pdf Posted by spacy600</DIV>The key item in that report is the regression rate that was measured -- on the order of 1 mm/s.  That is quite low and that is a problem. It means that to develop large thrust you need very large surface area and that dramatically limits the propellant mass fraction that can be achieved.  <div class="Discussion_UserSignature"> </div>

spacy600 · Jul 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>  I'm not sure what you mean by nano particles with regard to rocket propellants.  Particle size for the solids in rocket propellants is important, and in fact particle size distribution for the aluminum and  ammonium perchlorate oxidizer is important in control of burn rate.  In many cases it is the factor used to adjust and tightly control burn rate.  The  normal partricles sizes are considerably larger than a nanometer, and particles that small would be difficult to produce and handle and would likely result in either very large burn rates or else might partially dissolve in the binder and result in erratic results.  I would also expect the explosive hazard to be quite severe with particles that small. Posted by DrRocket</DIV> nano, micro, energetic particles all seems to improve burn times and improve performance.It is just a question of how small is practical. And if these particals are explosive when bound in htpb.  http://64.233.169.104/search?q=cache:gTVixx5cnrIJ:www.alpoco.co.uk/library/Ultrafine%2520Al%2520Powders.pdf+%22Alex%C2%AE+powder%22+cost&hl=en&ct=clnk&cd=4&gl=us  http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2642&DID=115851&action=detail Maybe even a turbo function  <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" width="18" height="18" /> nano-sized particles can also be dispersed into high-temperature zone for direct oxidation reaction and rapid energy release  <div style=""> </div>

DrRocket · Jul 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> nano, micro, energetic particles all seems to improve burn times and improve performance.It is just a question of how small is practical. And if these particals are explosive when bound in htpb.  http://64.233.169.104/search?q=cache:gTVixx5cnrIJ:www.alpoco.co.uk/library/Ultrafine%2520Al%2520Powders.pdf+%22Alex%C2%AE+powder%22+cost&hl=en&ct=clnk&cd=4&gl=ushttp://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2642&DID=115851&action=detailMaybe even a turbo function  nano-sized particles can also be dispersed into high-temperature zone for direct oxidation reaction and rapid energy release   Posted by spacy600</DIV>I don't know what you mean by improve burn times.  Burn rates are targeted to the application, and there is no "best" burn rate.  Smaller particle sizes generally result in higher burn rates.  If you want higher burn rates then that is good.  If you don't then it is not so good.  Some old interceptor designs, Sprint in particular, had very high burn rates compared to what is normal today, and that provided extremely high acceleration.  The particles are probably not especially explosive when in a cured binder.  However, all solid propellants are somewhat explosive.  That in and of itself is not always a problem.  There are issues with sensitivity in not only the final cured state but also in intermediate states during production.  Some propellants are very sensitive prior to cure, and that can be a problem.  One also has to be a bit careful about extremely small particle sizes of aluminum.  Aluminum reacts very rapidly with oxygen, and that is why it is such a good fuel.  But aluminum naturally forms a protective oxide coating and if the particle size is so small that the surface area to volume ratio is small then a significant proportion of the aluminum may already be oxidized and therefore not a good fuel.As I recall the research does show some Isp improvement with small aluminum particle sizes, I suspect due to reduced two-phase losses.  So long as there is sufficient residence time for complete combustion I would not expect much benefit from chemistry alone.I would expect that if one has an application for "nano" sized particles that the application might be sufficiently demanding of performance that some binder other than HTPB would be used.  HTPB binder is common, though not universal, in space launchers, but for higher performance applications more energetic binders are often used.  There are other applications for small particle size metals, in incendiary compounds for instance, where they ought to really shine.Injection of aluminum directly is not an option with conventional solid rockets since there is no good means of injecting anything into the case. You might do something like that with an alternate design, and yes you might get VERY rapid release of energy.  The trick is to control it and not simply have a dust explosion.  And the trick would also be to be able to control the material flow since powders are harder to deal with than are liquids.  To the best of my knowledge this sort of thing has not been done.  There has been some experimental work done with gel propellants, but that was pretty nasty.Small particle size metals are extremely hazardous in a manufacturing environment.  And accidents with them can lead to truly horrendous burn injuries. It doesn't take "nano" size particles to be a problem, but the smalle the particle size generally the greater the hazard, due to increased surface area. <div class="Discussion_UserSignature"> </div>

spacy600 · Jul 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I don't know what you mean by improve burn times.  Burn rates are targeted to the application, and there is no "best" burn rate.  Smaller particle sizes generally result in higher burn rates.  If you want higher burn rates then that is good.  Posted by DrRocket</DIV> I should have said  regression rate.BTW above you said 1mm/s  regression rate improvement was not so good, for a launch vehiclewhat would be a good regression rate?Thanks again

DrRocket · Jul 14, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> I should have said  regression rate.BTW above you said 1mm/s  regression rate improvement was not so good, for a launch vehiclewhat would be a good regression rate?Thanks again  Posted by spacy600</DIV>  It is usually called a regression rate for hybrids and a burn rate for conventinal solids,m but either term will do.  A typical burn rate for a solid propellant is 0.26-0.60 inches per second at 1000 psi, 0.26-0.50 for an HTPB is pretty normal.  0.33 ips or so would be typical of a space booster.  It can go higher with burn rate catalysts and lower with loss of performance.  <div class="Discussion_UserSignature"> </div>

CalliArcale · Jul 15, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>Rockets are not airplanes. Posted by DrRocket</DIV>I know what you mean, but I absolutely cannot resist replying with this picture: <img src="http://sitelife.space.com/ver1.0/Content/images/store/12/11/5c043629-55e8-412a-9b29-82ba1ca638ed.Medium.jpg" alt="" />   <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

spacy600 · Jul 15, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'>I don't know what you mean by improve burn times.  Burn rates are targeted to the application, and there is no "best" burn rate.  I agree rockets have to be tailored to the job at hand. Are hybrids good atany of them? Heavy, medium, micro/ nano or human launch vehicles?  (I am making the distinction between the "Classical hybrid" and the ones made with modern "additives"  The particles are probably not especially explosive when in a cured binder.  However, all solid propellants are somewhat explosive.  That in and of itself is not always a problem.  There are issues with sensitivity in not only the final cured state but also in intermediate states during production.  Some propellants are very sensitive prior to cure, and that can be a problem.  I am looking at after they are cured. Storage and on the pad. One also has to be a bit careful about extremely small particle sizes of aluminum.  Aluminum reacts very rapidly with oxygen, and that is why it is such a good fuel.  But aluminum naturally forms a protective oxide coating and if the particle size is so small that the surface area to volume ratio is small then a significant proportion of the aluminum may already be oxidized and therefore not a good fuel.As I recall the research does show some Isp improvement with small aluminum particle sizes, I suspect due to reduced two-phase losses.  In the reports that I have been reading, I have seen the terms "Flakes, coated flakes, nano, micro, smallenergetic particles" so it seems research is ongoing.   So long as there is sufficient residence time for complete combustion I would not expect much benefit from chemistry alone.Swirling and vortex of the oxidizer  seems to increase "contact time" (I have to go back and reread the reports)I have not seen experiments done with this and "particles" I wonder what the outcome would be? I would expect that if one has an application for "nano" sized particles that the application might be sufficiently demanding of performance that some binder other than HTPB would be used.  HTPB binder is common, though not universal, in space launchers, but for higher performance applications more energetic binders are often used.  There are other applications for small particle size metals, in incendiary compounds for instance, where they ought to really shine.Injection of aluminum directly is not an option with conventional solid rockets since there is no good means of injecting anything into the case. You might do something like that with an alternate design, and yes you might get VERY rapid release of energy. Yes I was half joking, but it would be cool if they could do it  <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-smile.gif" border="0" alt="Smile" title="Smile" width="18" height="18" /> The trick is to control it and not simply have a dust explosion. Grain silo's and sugar factories.  And the trick would also be to be able to control the material flow since powders are harder to deal with than are liquids.  To the best of my knowledge this sort of thing has not been done.  There has been some experimental work done with gel propellants, but that was pretty nasty.Small particle size metals are extremely hazardous in a manufacturing environment.  And accidents with them can lead to truly horrendous burn injuries. It doesn't take "nano" size particles to be a problem, but the smalle the particle size generally the greater the hazard, due to increased surface area. Posted by DrRocket</DIV>

DrRocket · Jul 15, 2008

Replying to: <DIV CLASS='Discussion_PostQuote'> Posted by spacy600</DIV>I'm not sure what you are reading.  There is probably some research going on in universities, Standford and U of Alabama Huntsville leap to mind.  But industry has generally not been chasing hybrids recently, although there was some interest 10 or 15 years ago -- during the time frame of that paper that you posted.  The problem is that hybrids basically did not work out very well, although Ruttan and Lockheed have used one or two in the past 5 years or so.The basic problem with hybrids is that they don't really do anything that can't be done as well or better with either liquids or solids.  In my opinion they combine the worst features of both.  They have all the complexity of liquids, having to deliver an oxidizer to a combustion chamber, the poor mass fraction of liquids, and only a little improvement in Isp over conventional solids, pluse some serious challenges with combustion and pressure oscillation.  If you have to go to additives and sophisticated schemes to get decent regression rate and eliminate pressure oscillations you have lost the one attraction of hybrids which is the chemical simplicity of the fuel.But they are nice for academic research because the fuels are simple and they don't have to work with explosives.  It is something that universities can do relatively easily.  They make fire and smoke and therefore have a bit of "gee whiz" appeal.  Unfortunately I have seen them used in settings that were totally inappropriate from a safety perspective, because of the ill-informed notion that they were totally safe.  I have also seen the people who thought they were totally safe blow them up.If you are really interested in advanced propulsion I suggest that you hitch your wagon to different technology.  I think in the future that nuclear thermal propulsion will prove to be a winner for deep space missions.  Plasma and ion drives also hold promise for deep space. The earth to LEO mission is more of a challenge, and I'm not sure what willl happen there.  I think we have about reached the limits of chemical propulsion.  There has been some research in exotic compounds for high performance, but the benefits are marginal and the costs and hazards are high -- I used to kid the chemists that what they really needed was a new periodic table.Nuclear propulsion might make it for the earth to LEO mission.  Electromagnetic rail type systems might be a partial solution.  But in the meantime I think we will see refinements of current chemical systems, perhaps improving reliability and reducing costs, but only at the margins.If you are still in school and want to study this stuff you might take a look at the University of Alabama, Huntsville  and in particular at Clark Hawke's Propulsion Research Center. <div class="Discussion_UserSignature"> </div>

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