"SpaceShipTwo could be single stage to SUBorbit."

[exoscientist]

300kg would have to include everything about the engine , all piping from the tanks, the mounting structure , control valves , you name it and it's included in that weight , the engine by itself is nowhere near that heavy , even with both ends on it I could pick it up by myself no problem (and I'm not a muscle builder) . Wish I could just kick you a drawing but I don't work at that particular shop anymore and I'm sure it was proprietary information anyway . How it worked is no secret but the exact specs I couldn't give them even if I had them .

Do you know if the oxidizer was pressure-fed of turbopump-fed? That would affect how heavy the oxidizer tanks were since pressure-fed tanks are thicker and heavier.


Bob Clark

 

[vulture4]

Do you know how hazardous Hydrazine is and unstable ? Not a substance I would choose to mess with if I have a choice .

This is a great thread! Real rocket science!

As to hypergols, I have actually smelled both hydrazine and N2O4, albeit in (hopefully) low concentrations. They, and closely related hypergols, like UDMH, aerozine-50 and nitric acid, are used as primary propellants in a number of rockets, including the Titan, Proton, and Delta II upper stage, as well as for RCS on Shuttle. Though there have been some close calls hypergols have never caused an actual fatality in US aerospace, but because of their hazards they are very expensive to utilize; the TLV (i.e. routinely allowable exposure) for monomethyl hydrazine is only 10 parts per billion. You need personal protective equipment, training on the PPE, medical coverage, contingency planning, training for the people who do the training, and tons of paperwork. The simple spark-ignition NO2/ethane thruster recently demoed by Orion shows clearly that storable nontoxics can be used for RCS, and many new programs say they will use nontoxics, but when push comes to shove they want to cut development cost. So they grab existing hypergol tech off the shelf, save on development, and greatly increase operational costs.

Congrats to SteveCNC and everyone involved in the SpaceShip2 design. Creating the first commercially feasible engine to carry tourists into space is fantastic, and it has a hybrid engine. However all-liquid designs have advantages as well. Liquid propellants lend themselves to rapid refueling of reusable vehicles as shown in the suborbital X-15 which utilized the Reaction Engines XLR-99, and the Rocket Racing League Velocity XL with its Armadillo engine, both using liquid oxygen. The LOX/LH2 RL-10, used for decades in ELV upper stages, weighs 298 lb complete with turbopumps and thrust vector control, and has a thrust of 15,000 lb. Hybrids appear to have significantly lower development cost then liquid propulsion of similar thrust. Nevertheless, I think had NASA not stupidly canceled the X-34 program, several of the really smart people now involved with SpaceShip2 would have had the chance to get comfortable with air-launched LOX/kerosine suborbital reusables and might have chosen a different path. LOX, delivered to KSC, costs about 60 cents a gallon; it's cheaper than gasoline! LH2 is about 98 cents, RP-1 about $1.50 a gallon. Fueling with LH2 is a bit tricky but not dangerous with the proper umbilicals, and LOX is relatively easy to handle, as is RP-1. When frequent turnaround, when dozens or hunderds of flight cycles per vehicle are considered, liquid propulsion probably has a significant advantage.

Finally, the genius of the Virgin Galactic approach is partly in the air launch concept; you certainly wouldn't want to eliminate it since the operating cost of the WK2 is a tiny fraction of the cost of any rocket. On the question of suborbital vs orbital, I have seen supposed NASA rocket scientists claim that "Burt Rutan doesn't know that it takes a lot more energy to get into orbit." Rutan graduated third in his class at Cal Tech and I'm sure he knows exactly how much energy it takes to get into orbit, but he also knows how much money it takes, something NASA is not to clear about. Air launch of a liquid-fueled reusable vehicle all the way into orbit "may" be a viable approach, I don't think anyone will really know until it is tried. When it is tried, I hope Rutan, Tim Pickins, SteveCNC and the other smart people behind Virgin Galactic are around to lend their expertise.

 

[SteveCNC]

The fuel for a hybrid is only 2 components , the gas used is just Nitrous Oxide and the solid portion is a type of rubber that from what I've read about it there is some sort of oxidizer in it also although to me it looks like dirty plexiglass and it machines like it too . The fuel plug is in the shape of a cylinder with a hole running lengthwise (the size is roughly 25% of the outer diameter of the fuel plug) . I give a percentage because this engine is scalable and when the project started the very first engine was pretty small , only about 5" diameter by 12" long roughly but it was just proof of concept at that point . The fuel plug is sleeved with a custom fit phenolic piece roughly 1/8" thick for the smaller ones and the length of the fuel plug . Both ends of the motor assembly come off if so desired and I'm not sure but they probably take both ends off for refueling to ensure propper fit of the fuel/sleeve . When I give them the parts they are not put together , that's something they do at their end .

here's a basic description of the motor only , no support hardware: thought some might enjoy this

There's a port fitting (Nitrous injection point ) machined on the back end cap right in the middle plus normally 4 starter containers are welded on at angles that intersect at the end of the fuel plug when installed . It's all machined of course so the mating surfaces and such line up exactly right when welded and then it gets x-rayed for cracks/gaps etc . The geometry of the cap itself isn't all that simple , it's designed with ribs along ellipse' and raised areas in order to maximize strength but be as light(thin) as possible (a little tricky to program using only a 4 axis machine but very much an enjoyable challenge). The starters they use are somewhat like a "D" size model rocket engine , it's a dry chemical motor , fired remotely that heats up the fuel plug enough to lite-up when the nitrous is turned on . Once it's running the ignition self perpetuates as it burns the solid fuel plug from inside out along the lengthwise hole . Of course there is an isolation device between the end of the igniter and the inside of the motor that blows out when the igniter is lit but it's just a small disk of inconel that's scored in 2 places to break away properly . All of the motors I made had either 2 or 4 starters on the end so they could only be started a limited number of times though I'm sure if you went really big you could fit many more starters or come up with maybe a propane or something else you could fire as many times as you want . There's also a couple of small ports in the back end presumably for sensors of some sort (probably temp and a load cell) . The other end of the motor for me is just a fitting where a nozzel is attached . As far as I know none of the hybrids I made were ever gimbaled , they were always a fixed nozzel . The ends of the motor attach to the cylinder using a bunch of rivets , I didn't have to do that part but I am sure they aren't your every day type rivet (probably monel) .

Here's a story about the process of Research and Development of the hybrid you might enjoy :

The second engine I made was about 10" diameter and maybe 20" long and that's the one they kinda killed . It wasn't totally their fault , a software glitch caused windows98 to blue screen during the test fire and well can you say "we need a failsafe shut down !" . Anyway the recording software that was collecting sensor data during the test fire is what glitched and it just so happened that the same computer was running the motor . By the time they got the computer rebooted and able to shut down the motor it had burned all of the fuel plug , went through the phenolic liner and then burned a hole in the side of the motor itself . That hole was within an inch of one of the titanium tanks holding the nitrous oxide , needless to say they got it shut off just in time to not cause a real problem . Once you get titanium lit up it doesn't go out so easy (I've seen a titanium fire inside a machine once , not one I was running but in one at the other end of the shop I was working in at the time ) , plus mixed with free running nitrous , I'm pretty sure the entire assembly would have been nothing but a pile of molten junk . As it was it was not looking very good when I saw it :shock: I guess that's all part of R&D though .


Now as far as getting a hybrid engine vehicle that could get all the way up to orbit and park with a Bigelow space center (my wishful thinking) , it is possible given the fact that it is scalable and you can always have more than one on the vehicle I'm sure .

I really think it's a much better way to go as opposed to the pressurized beer cans that fly these days (the atlas series is one such rocket type) and cause severe environmental damage in the process . I'm not unfamiliar with the Atlas2 and up rocket series since I've personally made over 4500 different parts involved in them from the valves that keep the beer can pressurized as it uses fuel to the computer housings , the motor framework , stage seperation units , and I was involved in the first Atlas IIAS , I did the SRB attachment hardware used on top (first they wanted it made of 7075 alum. then midway through they said stop ! make it out of 718 Inconel then later went back to 7075 for production after the first launch sensor data confirmed the load values ) . Anyway I know what's involved inside of both those rockets and the hybrid , and a hybrid vehicle can be just an incredibly simpler design . Right now as far as I know Virgin only has a very Low orbit flight vehicle that cannot make it to a space station , but could do insertion to land far distances away if desired or just return to initial point after some weightless time (pass out the barf bags) . But , if they wanted to , I have no doubt a ship could be designed around hybrid propulsion that could do anything they wanted . I can envision the possibility of a first stage that could , under autopilot , return to initial point for use again , because it is such a simple design something like that is very possible .


The Hybrid is also cleaner burning ! The fuel is less hazardous by a vast degree not only to the environment but to handle as well , and far cheaper to boot . What do you suppose it cost to fill up a Nitrous Oxide tank ? I can go buy it myself and add it to my car if I really wanted to (cheap horse power though the motor won't like it for long) . And the solid fuel portion of the fuel is some kind of rubber like compound , though I am sure it's not cheap to create , it can't be that much . As I recall it was far less than $2000 for a section about 6" diameter by 14" long roughly and you could handle it with your bare hands so it's not overly hazardous to be around . But I am sure like most things when you go production the cost goes way down , that was during R&D so cost is at maximum .

 

[Swampcat]

Do you know if the oxidizer was pressure-fed of turbopump-fed? That would affect how heavy the oxidizer tanks were since pressure-fed tanks are thicker and heavier.

I can't speak to the details of this particular design, but I fly amateur hybrid motors and know that nitrous is self-pressurizing. A drawing of SpaceshipOne's motor on Wikipedia supports self-pressurization only.

The Hybrid is also cleaner burning ! The fuel is less hazardous by a vast degree not only to the environment but to handle as well , and far cheaper to boot . What do you suppose it cost to fill up a Nitrous Oxide tank ? I can go buy it myself and add it to my car if I really wanted to (cheap horse power though the motor won't like it for long) . And the solid fuel portion of the fuel is some kind of rubber like compound , though I am sure it's not cheap to create , it can't be that much . As I recall it was far less than $2000 for a section about 6" diameter by 14" long roughly and you could handle it with your bare hands so it's not overly hazardous to be around . But I am sure like most things when you go production the cost goes way down , that was during R&D so cost is at maximum .

The fuel grain for SpaceDev's motor is made of HTPB, which is a synthetic rubber. N2O runs $4/lb at my local automotive speed shop where I buy it, but that's retail.

Hybrid motors for amateurs are available commercially from 22mm diameter up to 152mm. Check out HyperTEK and Contrail Rockets for more information.

 

[exoscientist]

Do you know if the oxidizer was pressure-fed of turbopump-fed? That would affect how heavy the oxidizer tanks were since pressure-fed tanks are thicker and heavier.

I can't speak to the details of this particular design, but I fly amateur hybrid motors and know that nitrous is self-pressurizing. A drawing of SpaceshipOne's motor on Wikipedia supports self-pressurization only.

Thanks for that. I gather that it is pressure-fed too. Since you fly amateur hybrids perhaps you can give me an idea about the weight of hybrid engines. This page gives the empty mass of the SpaceShipOne hybrid as 300 kg (660 lb):

SpaceDev Hybrid.
http://www.astronautix.com/engines/spaybrid.htm

At a thrust given on that page of 16,523 lbf this is a thrust to weight ratio of about 25 to 1. Since hybrids have to have a thick and heavy fuel tank because it doubles as the combustion chamber, it would make sense that the thrust/weight ratio is this low, comparable to that of solid fuel motors.
Astronautix however is not always the most accurate source so I would like another source to get an idea of the weight of the hybrid engines. Another point of uncertainty is whether that Astronautix page is including the weight of the oxidizer tank in calculating that engine empty weight. The oxidizer tank likewise has to be thick and heavy because it is pressure-fed so it would make the T/W ratio even worse if indeed included.
Logically if you're calculating the weight a liquid fueled engine you wouldn't include the weight of the tanks when calculating the empty weight. However, for a solid fueled motor you should include the weight of the (single) tank because it also serves as the combustion chamber. For a hybrid logically it seems to me you should only include the empty weight of the fuel tank because it also serves as the combustion chamber, but not the weight of the oxidizer tank. However, since hybrids have not been commonly used I don't know if this idea would be followed when giving the empty weight of the engine. It may be they also include the empty weight of the oxidizer tank.


Bob Clark

 

[exoscientist]

... By the time they got the computer rebooted and able to shut down the motor it had burned all of the fuel plug , went through the phenolic liner and then burned a hole in the side of the motor itself . That hole was within an inch of one of the titanium tanks holding the nitrous oxide , needless to say they got it shut off just in time to not cause a real problem . Once you get titanium lit up it doesn't go out so easy (I've seen a titanium fire inside a machine once , not one I was running but in one at the other end of the shop I was working in at the time ) , plus mixed with free running nitrous , I'm pretty sure the entire assembly would have been nothing but a pile of molten junk . As it was it was not looking very good when I saw it :shock: I guess that's all part of R&D though ...

Do you think this might have something to do with how the Scaled Composites nitrous accident happened?
BTW, that accident and your explanation here of a near accident suggests the claims by Scaled Composites and Virgin Galactic that hybrids are completely safe should not be expressed by them in such absolute terms.


Bob Clark

 

[SteveCNC]

Bob ;

The thing about that accident is , it was a series of events that led up to it , first off they only used a small piece of rubber for that test figuring they would only burn less than 2 minutes so no need for a full size plug . The other thing that allowed it to happen was the fact that they completely filled the nitrous tanks . So now you have 2 minutes worth of solid fuel and 20 minutes worth of nitrous running without a way to shut it off for about 3 minutes . That's why I mentioned the fact there was no failsafe shut off on that test fire when in retrospect there should have been . That would have prevented any problem at all , though I am sure failsafes are in place on the motors that actually get airborn . The nitrous is the most dangerous part of the entire system and it's not all that dangerous when compared to typical rocket fuel . Plus I also have no doubt that the actual flight design does not have the nitrous tanks surrounding the motor casing at least not on SS2 .

 

[SteveCNC]

As for the nitrous tanks being heavy , I really don't think they are because there's no need for them to be so . While I didn't personally make either the SS1 or SS2 nitrous tanks I did make all of the tanks through R&D plus a few satelite motors and they were all titanium and very light .

One thing that we do where I am working right now is wrap our tanks in carbon fiber . Of course the tanks I'm talking about are aluminum wrapped in carbon fiber and they contain Nitrogen used for helicopter emergency water landing devices . But it would make any tank far stronger to burst strength without a lot of weight , and since scaled is all about carbon fiber I am certain their tanks are wrapped in the stuff even though none of the tanks I made over the years were but then again none of them carried people either .

The pictures of SS1's motor/tank assembly are interesting , they show the nitrous tank as a sphere attached to the end of the motor which might be the case for SS1 I really am not sure how they configured it , I just know how the hybrid motor components are designed , the nitrous can go anywhere , just need a little plumbing . The 660lbs of weight must include that tank though , I don't see any way to get to that much weight without it included along with mount structures . Probably includes the entire propulsion module assembly , though it makes sense to make the entire motor/tank/control system all one unit mounted into an airframe so it can also be removed in one unit . Makes it easy to work on when you can get to all sides easily .

 

[Swampcat]

First of all, I want to apologize for giving a wrong url. Loki Research has absolutely nothing to do with hybrids. I meant to give Hypertek which is a well known manufacturer of commercial amateur hybrid motors. I will fix the link above.

Thanks for that. I gather that it is pressure-fed too. Since you fly amateur hybrids perhaps you can give me an idea about the weight of hybrid engines.

I doubt much can be extrapolated from amateur hybrids. Most are monotube, floating injector designs. For what it's worth, though, you can find specifications for all motors at the manufacturer's websites. Check here for Hypertek's M series which are not monotubes.

BTW, that accident and your explanation here of a near accident suggests the claims by Scaled Composites and Virgin Galactic that hybrids are completely safe should not be expressed by them in such absolute terms.

Hybrid motors are considered "relatively" safe when compared to other types of rocket propulsion. You're still dealing with pressure vessels and a liquid oxidizer, but N2O is much safer to handle than O2, while as SteveCNC said, the fuel is about as safe as rocket fuel gets.

The other thing that allowed it to happen was the fact that they completely filled the nitrous tanks . So now you have 2 minutes worth of solid fuel and 20 minutes worth of nitrous running without a way to shut it off for about 3 minutes .

I'm certainly no expert on hybrid motors, but my understanding is that once the fuel is consumed, chamber pressure drops and the remaining N2O would simply flow through the chamber until tank pressure equalized to ambient. There would have to be some kind of residual ignition source left in the chamber after the fuel is consumed. It is true that "blowback," where combustion chamber pressure exceeds N2O tank pressure allowing hot gases to flow in reverse through the injector into the tank, is the most common failure scenario in hybrids, but that generally requires a nozzle blockage or failure of injector bulkhead o-rings/seals.

 

[SteveCNC]

While I did take chemistry in college I'm by no means an expert on chemical reation chains , however one thing I do know is that at extremely high temperatures chemical reactions can occur more easily , so a substance that would normally not burn, can , which is the case here . With the high temperature the phenolic was able to react with the nitrous oxide and once we started seeing chunks fly out the bottom ( I watched the video) the motor case was exposed and I guess at that temperature even inconel can burn if there's an oxidizer handy . One thing you gotta remember , they won't be using up all the fuel during normal operations . That was merely a fluke of research not wanting to waste more solid fuel than needed during testing coupled with no failsafe shut off and a glitched control computer . All 3 things had to occur to cause that chain of events and I am sure that at least 2 of those are not an issue on the real ship .

 

[vulture4]

SteveCNC - since you've worked on other vehicles, do you see any simpler approach to fabricating the nozzle of a liquid fueled engine with its cooling passages in the nozzle walls? Usually these are welded or brazed together out of hundreds of pieces of tubing, but this is very expensive. In the Shuttle engine there are literally hundreds of welds, some of tubing about the size of spagheti. Could there be a more practical approach to forming the cooling passages using machining? The other part that might be an interesting challenge is the injector plate where the fuel enters the combustion chamber.

 

[SteveCNC]

I personally have never worked on that particular part of the rocket , all the way down to it but never done a nozzle . I was looking at some pictures of the shuttle nozzle and it looks like it's made from sheet and bar run through a ring maker . I used to work for a company that made large rings of that sort cept they were the races for really big jet engines made of either titainium or 625 inconel . But the reason things that have to be made in that manner is because a casting is too brittle by nature where sheet and bar is rolled material , much stronger in many ways to a casting . While it might be possible to make a nozzle that could work using castings with journals built in and everything but your probably gonna pay a pretty high weight/mass cost to do so just to get the same strength . And while you could make it completely from billet it wouldn't be as strong as made from sheet . Sheets properties are hardened skin with a soft inside basically almost case hardened because of how it's made with rollers , that makes it actually stronger than billet because your in virgin material with billet and even if you heat treat it , it will never be the same as sheet because it hasn't been rolled to compress the skin . There is a process to do something similar but it dosen't penetrate as deep and it's far more work than it's worth on something that size . I'm afraid they already have pretty much the best solution for what they need .

The only other solution to the plumbing and welding would be to create nozzle that didn't need to be cooled . The materials would have to be ceramic and perhaps a hasteloy wire mesh inside and done as a single casting , might be lighter or at least close to the same and would eliminate the water system entirely . But castings really aren't my thing , I don't normally like working with them as a machinest and I've only made an aluminum bust once in high school over 35 years ago (got an A ) . Other methods wouldn't be able to take the heat but I believe hastoly can handle roughly the heat needed without going limp . That's another metal I don't like to run on a mill is hasteloy X it's not too bad if your running it on a lathe or edm but I prefer not to have to mill it , but it can handle a lot of heat .

 

[MeteorWayne]

nozzel? plumming?
Scary!

 

[kk434]

Launching rockets from planes has been tried before (X-15 planes, pegasus rocket)and sounds like a wery good idea but somehow it is not widely used now. Is there a problem with this design?

 

[scottb50]

Launching rockets from planes has been tried before (X-15 planes, pegasus rocket)and sounds like a wery good idea but somehow it is not widely used now. Is there a problem with this design?

Well let's see under a thousand pounds to orbit and it needs an L-1011 to get it airborn, that might be one of them.