Hybrid Rocket design

[mikkelrj]

Okay, im hoping some of you can help me out on this. <br /><br />If you were to built a hybrid rocket with the fuel stored in solid form, this would seem to me as the perfect solution for producing an effective and simple rocket that ultimately could use the atmospheric oxygen as oxidizer. A turbofan could deliever the intake for the first part of the uphill climb and then later it would be jettisoned to allow a scramjet like operation. Some LOX would be stored onboard the LV to allow for non atmospheric operation. This LOX could also provide some of the power in early flight to eliminate the need for moving parts. The LV would have a MECO capability by closing the inlets to the combustion chamber. Using solid fuel the complexity of the rocket would decrease greatly, but still maintain a high ISP due to the hybrid rocket design. A similar approach has been made with X43 and ATK's recent hydrocarbon scramjet test vehicles. But ultimately, by eliminating all the "plumbing" associated with liquid fueled rockets, the weight and effectivity would rise. <br />

 

[vogon13]

I don't understand the power source that will spin the turbofan assembly. The power to compress enough atmosphere to rocket chamber pressure levels will be quite large. Also, the oxidizer flow is 78% not oxidizer. Compressing and heating this material will be an enormous drain on the thermodynamic efficiency of your propulsion system.<br /><br /><br />Also, will there be a drive shaft through the rocket and a turbine in the exhaust of the rocket?<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[mlorrey]

Yes and no. Solid fuels are generally so low Isp that even eliminating most of their oxidizer in favor of solid fuel ram/scramjet designs (which have been tested) still results only in Isp averages below that of high performance liquid fuels, i.e. still too low for SSTO even without TPS. Adding turbo fans destroys mass fraction, and as soon as you talk about adding some oxidizer for performance above the atmosphere, you need pumps and tanks and so forth that also takes up mass fraction, so you lose that advantage too. Might as well go with a 2.5 stage design: airlaunch of an airbreathing solid fuel ramjet with a liquid/solid hybrid upper stage.

 

[scottb50]

It would be just as easy to use Oxygen. Carry it as LOX, the same LOX the liquid engines use, and inject it into SRB's. nearly 70% of the Shuttle SRB propellant is oxidizer.<br /><br />Turbofans only work at very low altitudes and airspeeds, turbofans don't work supersonic without complex inlets.<br /><br />The idea with SRB's is a lot of thrust, making up for low ISP with brute force.<br /><br />SSTO's are in the future, today we could do a two stage to orbit with a fly-back first stage rather easily.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[nacnud]

Why bother with a flyback stage, fishing the SRBs out of the Atlantic seems to work just fine.

 

[scottb50]

If all you are bringing back is empty Modules and high priced nozzles and engines, it doesn't have to be that complicated. <div class="Discussion_UserSignature"> </div>

 

[nacnud]

True, but it is hard to beat the simplicity of just trying to hit the ocean <img src="/images/icons/smile.gif" />

 

[scottb50]

If you want commercial service you can't throw stuff away. <div class="Discussion_UserSignature"> </div>

 

[mlorrey]

Well, obviously, Scott, if 70% of the SRB's propellant is oxidizer, then eliminating that mass is going to vastly improve your Isp and your mass fraction, and allow a smaller booster.<br /><br />Lots of thrust can't make up for low Isp. An SRB with no upper stage or payload can never exceed mach 18, even with a mass fraction of almost .86. Getting rid of the oxidant would improve things markedly: improve the Isp to about 600-650 secs. Reducing the dry mass by 70% means it now weighs about 28,000 lb, plus about 180,000 lb of fuel, for a mass fraction of about .86. The major problem is that without oxidant, it needs to reach mach 25 below 160,000 feet, which leads to a major TPS problem, though without wings or fins, it only needs an ablative cone, which isn't too bad, but that drops your mass fraction a bit, and also ups your delta-v requirements since you'll still need to make up for aerodynamic losses after burnout in the atmosphere to cruise into orbit.

 

[scottb50]

I didn't say anything about entirely eliminating that mass. To carry LOX and the required plumbing would add weight. Obviously you need the same volume of oxidizer for the same volume of fuel. <br /><br />Lots of thrust can't make up for low Isp...<br /><br />Lots of thrust can make up for low ISP, that's the point. Why do you thing Shuttle uses SRB's?<br /><br />The major problem is that without oxidant, it needs to reach mach 25 below 160,000 feet, which leads to a major TPS problem, though without wings or fins, it only needs an ablative cone, which isn't too bad, but that drops your mass fraction a bit, and also ups your delta-v requirements since you'll still need to make up for aerodynamic losses after burnout in the atmosphere to cruise into orbit....<br /><br />None of this makes much sense to me. If you figure a launch vehicle that gets a second stage to roughly the point the Shuttle gets to at SRB separation then sends a second stage into orbit you need little if any TPS on the first stage.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[gunsandrockets]

"If you were to built a hybrid rocket with the fuel stored in solid form, this would seem to me as the perfect solution for producing an effective and simple rocket that ultimately could use the atmospheric oxygen as oxidizer."<br /><br />That is an interesting idea for spaceflight applications. You may be surprised to know such a rocket with similar design has been used for decades! It's the Soviet SA-6 surface to air guided missile.<br /><br />The SA-6 uses mixed mode propulsion. Initial boost is from solid rocket propulsion, but once it gains some speed it converts to airbreathing ramjet propulsion using fuel rich solid rocket propellent.<br /><br />A hybrid rocket using an SA-6 style ramjet could have a lot of potential. Especially as the 1st stage of a two stage to orbit launch vehicle.

 

[gunsandrockets]

"Solid fuels are generally so low Isp that even eliminating most of their oxidizer in favor of solid fuel ram/scramjet designs (which have been tested) still results only in Isp averages below that of high performance liquid fuels,"<br /><br />Solid fuel rockets can have surprisingly high ISP, almost as good as LOX/kerosene. The solid propellent IUS which was used an an upper stage has an ISP of 296.<br /><br />http://www.astronautix.com/stages/ius1.htm

 

[mikkelrj]

I am aware of the "low" ISPs for solid fueled rockets, but if the oxidizer is some 70% of the weight, the ISP of a system that only had to carry the remaining fuel and use the atmospheric oxygen as oxidizer, would be very high indeed. Its actually the same thing as using Silane on a Mars LV, which ignites spontaneously with CO2, the ISP for such a system is very low, but when the LV do not have to carry the oxidizer, the effectiveness of such a system is huuge. <br />I know many of you have much more knowledge on this subject than i, and therefore i am just thinking out loud, to see if this rocket profile could have any real world applications.<br /><br />NB If the rocket were implemented with shafts to provide oxygen for the solid fuel, would that not eliminate the need for a moving air intake? The fuel is ignited and the oxygen is automatically sucked in for combustion. If you were to stop the process it would simply include a closure of the shafts. <br />Obviously such a rocket would have a limited operating range within the atmosphere, and the most promising application would be an effective "magnum" first stage, pushing a large payload up to a certain height and handling over operations to an upper stage, most likely cryogenic.

 

[mlorrey]

Scott, the shuttle uses SRBs because it can't even get off the ground without them:<br /><br />SSME thrust: 3 x 469,000 lb thrust = 1.407 million lb<br />Mass of shuttle = 239,000 lb<br />mass of ET, full = 1.68 million lb<br />total mass, w/out SRBs = 1.9+ million lb<br />Even then, the Shuttle's mass fraction isn't much over .87, which isn't enough for SSTO at 450 sec Isp.

 

[mlorrey]

I would stay away from cryogenics entirely if you want to operate this as a totally reusable launch system.<br /><br />Now, layering the fuel in the booster, so that the innermost layer has oxidizer in it, which will provide thrust with intakes closed at launch, until you get up to speed, then the oxidant layer will be burned out and you'd be operating on pure fuel. Then, when you've reached a certain speed, the pure fuel layer will be burned out and you'd go back to a fuel and oxidizer layer again, shut the intakes, and go back to rocket mode. This might work as a usable solid rocket based combined cycle propulsion system.

 

[mikkelrj]

What speed are we looking at before the shafts could be opened?

 

[josh_simonson]

In a rocket, your fuel grows more expensive per pound the higher you get. SRBs are used on many rockets to help the first stage, dispite the low ISP, because the first stage fuel is the cheapest. <br /><br />I don't think solid fueled boosters really lend themselves to re-useability very well anyway. You need to cast a new core inside the casing with every re-use, then analize it carefully to make sure it has good adhesion to the walls, no cracks, ect.<br /><br />Liquid fuels do require more hardware, but if you're building a re-useable vehicle and the hardware is sufficiently rugged and redundant, it doesn't matter that the system is more complex because it's amortized over many launches.

 

[mlorrey]

Given todays materials technology, I wouldn't waste time trying to breath air any faster than mach 12. A problem you have with solid fuels lower Isp is that it also has a much lower stagnation temperature than kerosene or hydrogen. This means that the thermal effects of hypersonic air travel will dampen out the thrust of your solid rocket motor, so at some speed, likely around mach 6 or less, your solid rocket motor would add no extra heat to the superheated intake air if it is rammed to subsonic speeds. If you could make it operate in supersonic combustion, it could potentially reach mach 8-10 before this also dampens out, but this is guestimating only, and may be overshooting the mark. I would suggest being conservative and aim to shut down air breathing at mach 6-8. This should mean that you'll need to keep 30-50% of your original oxidizer for launching and post-airbreathing propulsion. <br /><br />The reason the SRB can go faster than that in rocket mode is that it operates at higher combustion chamber pressures in that mode than it could in airbreathing mode.

 

[gunsandrockets]

I found something almost exactly like the hybrid rocket you are interested in. It was a Soviet mobile ICBM design which used an airbreathing 1st stage, and a solid propellent 2nd and 3rd stage. Because the airbreathing stage had an ISP of 550, the total mass of the ICBM was only 30 tonnes, half what a conventional rocket would mass.<br /><br />Adapted to a modern launch vehicle design, equally great mass savings could be expected.<br /><br /><br />http://www.astronautix.com/lvs/gnom.htm

 

[mikkelrj]

Thank you, that is really interesting information. It probably would be possible to increase the ISP of such a system even further then what they achieved some 40 years ago.

 

[bwhite]

Paraffin wax hybrids. Candle wax, believe it or not. <br /><br />http://news-service.stanford.edu/news/2003/november5/rocketwax-115.html

 

[mikejz]

There is some research into paraffin hybrids. Some seem to suggest thatwith the proper addiative a lox/paraffin moter could have an ISP of an average RP-1 Lox sort.

 

[scottb50]

It might even sound like Buck Rogers ship. <div class="Discussion_UserSignature"> </div>