Hi Everyone,
Ok, isn’t it true that military missiles are
fueled by solid propellants? And don’t solids
require external oxygen? (i.e., they can’t bring it with 'em, like liquid-fuelled rockets).
If I’m correct thus far (about solid fuels), then how can something like the Minuteman or MX put a payload into orbit?
I used to make a solid rocket propellant using sugar and potassium nitrate. The saltpeter provided the oxygen.
I hope this clears things up: http://www.sciencenet.org.uk/database/Physics/Rockets/p00845c.html
I’ve tried to find out what the nature of the “oxidizer” is, but I as of yet I haven’t succeeded. Clearly, solid oxidizers (saltpeter is one, as has been mentioned) are not a recent invention; modern gunpowders can work in vaccuum since they contain solid oxygen-containing compounds as well. I don’t know what they are though; I seem to remember the firearm propellants being based on nitro-cellulose but I don’t know if that’s the state of the art.
There are some exceptions to the solid-fueled rule for military rockets. The Titan is (or was) a liquid-fueled rocket, used both for large nuclear warheads and putting spacecraft into orbit (Gemini, if I remember correctly).
Nitrates like Saltpeter are classical oxidants, and are used in “black” gunpowder
(a mixture of charcoal, sulphur and saltpeter) as well as the “caramel candy” experimenter’s rocket fuel mixture discussed above. The basic chemistry is that the oxidizer must contain oxygen in compound form. The chemical reaction releases this oxygen to allow the combustion of the fuel.
As a side note, I seem to recall that we mixed potasium chlorate into the molten sugar instead of potassium nitrate. Must have been more than one formula.
Also considering all the dumb things we did, I wonder how we ended up with all our parts?
The Titans of the last 20 years or so use a liquid-fuel main body with two solid-fuel boosters strapped onto the sides. The upper stages often use liquid fuel with a separate (hopefully!) oxydizer.
Unless things have changed very recently, the Atlases are all wet.
“… how we ended up with all our parts?”
Who said we did ,Diver? A friend of mine lost past the first knuckle of his right ring finger in an Estes rocket launch involving shredded magnesium and home made nitro cellulose.Previous launches had been uneventful,which is why the guncotton entered the picture.We didn’t notice it was gone at first,just " Dayum, that hurt." Never did find it, vaporised or low orbit maybe… Later on he got full scholarships to Rice and wound up a real live nukeyewler Fizzicist with two phd.'s,messin around with them real rockets and stuff. The only time his nonfinger caused any comment was when I was a best man at his wedding. The Photographer wanted to take a cliche photo and told him to look at his watch and hold up his hand " Only 5 min. of freedom." The shutterbug thought he’d committed a fauxpaux
but I just said " We got less time than we thought, hurry it up." Amature rocketry and pyrotechnics is great fun, but when ya got a real live super brain messin around witcha it is a real blast.
“Pardon me while I have a strange interlude.”-Marx
I believe the solid oxidizer in rockets is something like sodium perchlorate; has a lot of extra oxygen atoms available.
The rocket motor with which I am most familiar was a single “granule” of Nitrocellulose, with a very carefully designed cross section. It included its oxidant in the material with its propellant. The main difference between rocket fuel, and bomb material is the shape of the individual granules, and the pattern in which they burn. Both must have the oxidant present with the fuel, in order to function as desired.
Such material is highly unstable, unless it is created with an ignition temperature high enough to preclude spontaneous ignition at expected extremes of environmental temperature. With the rocket booster I dealt with, the ignition temperature was fairly high, which was reached with a charge of explosive fired into the top end of the granule. That material could be stored separately. The propellant would then burn along the inside of the several tubes along the axis of the granule, blowing out the other end.
The burn would continue, all along the length of each tube, which allowed the material to be burned very rapidly, but with a more even distribution of speed of burn vs. time of burn than would be possible with a single tube. (The surface area of a single tube would burn more slowly at first, and more rapidly later, as the surface area of burn increased.)
The shape, location, diameter, and precise axial alignment of the burn chambers (tubes) were very important elements of design. I am sure the much larger solid rocket boosters used now have similar design parameters, although the desired boost rate, and burn time would dictate different specific answers.
It ain’t rocket science, ya know. Oh, wait a minute, yes it is!
Ours is a world of nuclear giants and ethical infants. We know more about war than we know about peace, more about killing than we know about living.
– **General Omar Bradley **
Diver:
You’re not the only one.
I used to make solid fuel rocket engines (not terribly good ones) when I was younger by melting the saltpeter and sugar together, then casting them. I found they burned faster this way.
I did a few safety tests first, putting some on tinfoil and a cookie sheet in the oven with a thermometer, to see when it would spontenously ignite, and it melted, bubbled for a minute or two, and didn’t burn, up to a hundred (F) hotter than melting, so I felt fairly safe. I also lit off a bowl full (as much as I’d be working with) to see what it would do. Both experiments showed it to be fairly safe, and with a fairly low yeild if I was careful.
So I mixed the SP and Sugar, shook them together in a plastic jar, and then poured them into a metal pot and slowly increased the temperature, stirring with an old wooden spoon (after the chopstick incident, which was funny, in retrospect) until molten, then quickly poured them into molds.
I seem to remember using a double boiler to remove the potential for hotspots… can’t remember if that worked, or if I simply used a thick-bottom pot.
Anyways, if worked fairly well. I’d rig a funnel out of a magazine subscription card, pour the mix into a cardboard tube and put a safety fuse (the stuff guaranteed to burn at a set speed) in the mix so it hardened around the fuse.
One problem with this is that the mixture wants to boil just above the melting point, and it’s nearly impossible to get rid of the bubbles. Needless to say, the foam isn’t as effective as solid fuel is.
Fun experiment, and I think, fairly safe. I was always anal about personal safety, trying something only after trying to make it blow up in testing, so see where the limits were.
In further answer to the OP, military missiles use solid fuel so they can be stored (or even transported) fully fueled and therefore ready to go at a moment’s notice.
The earliest ICBMs (Atlas and Titan, etc.) were liquid fueled and required a tremendous amount of maintenance, both in personnel and supplies, to be kept ready. (LOX boils away pretty rapidly.)
Liquid fuels are preferred because they have a significantly higher specific impulse, i.e. thrust to weight ratio. Since the amount of payload reaching orbit is paramount in most rocket launches, liquid fuels are used in most civilian rockets. It’s only the military, for reasons cited, and booster rockets, which are supposed to add extra thrust without adding too much extra headache, that are built around solid fuels.
That’ll do, pig. That’ll do.
Thanks, everyone, for all your answers.
Regarding the post above this one, don’t ICBMs put a “bus” into orbit, which then launches the MIRVs?
Only if you define “orbit” very loosely.
The MIRV bus certainly goes into space, but it is on an elliptical trajectory headed towards its target(s). When Alan Shepherd did the same thing (on top of an ICBM, incidentally) it was considered a sub-orbital flight.
That’ll do, pig. That’ll do.
Hmmph! That’s Alan Shepard.
Sorry Alan.
One other thing. Theater ballistic missiles such as the Scud of Desert Storm fame are liquid fueled. That’s because their technology lags the good old US of A. That also makes them more vulnerable to attack on the ground since they have to uncover long enough to fuel before they can launch. They also need a fuel tanker to accompany the transporter-erector-launcher (TEL) and are therefore more visible. And, in general, the TEL is designed to travel over “unspecified terrain” while fuel trucks tend to be limited to roadways. You can see that liquid fuel is quite a liability in the field. But for now it’s the only way they can get enough oomph to use their toys.
I thought solid fuel rockets were easier and cheaper to produce than liquid fuel rockets. Why are newer missiles solid fuel and older ones liquid? Are newer warheads smaller and lighter? Or were there technological problems that prevented larger solid rocket motors till recently?
By the way, in case we’ve forgotten, the US Space Shuttles still use solid rocket boosters. Come to think of it, isn’t the Shuttle the only manned launch vehicle that uses/used solid rocket motors?
Yep.
SRB’s have burn characteristics that make them undesirable for most manned vehicles. Although the shuttle uses them, it it only in the first few moments of lift, and they are needed there because this big bus really takes a lot of energy to get moving.
While a solid fuel motor is burning, the shape, and size of its burning surface changes. Much is done to control the natural geometry, which I mentioned in my other post, but the hard facts of geometry and physics make the thrust increase for the entire burn, until it is done. (the surface area of the burn increases at the fuel burns, and so, more of it burns per second as time passes.)
Liquid rockets are also variable in thrust, but it is varied at will, by controlling the rate of fuel flow. Since the weight of the rocket goes down as it uses fuel, solid rockets produce more acceleration as time passes. That is not necessarily a good thing, especially if you have human passengers. In the case of the shuttle, you have both solid and liquid fuel engines running at the same time, and can use the variable thrust to compensate for any undesired increase in acceleration.
I must study politics and war that my sons may have liberty to study mathematics and philosophy. My sons ought to study mathematics and philosophy, geography, natural history, naval architecture, navigation, commerce and agriculture in order to give their children a right to study painting, poetry, music, architecture, statuary, tapestry, and porcelain.
– John Adams, (1735 - 1826)
It’s not hard to build a rocket, but it’s hard to build one that will fly as far as you want it to. Range is the holy grail of missile technology and the way you increase range is to increase the thrust-to-weight ratio. Liquid fuels have a significant advantage in thrust-to-weight ratio, which is why they’re so popular.
When you’re just starting out it’s hard to build a missile structure that’s light enough to use solid fuels and still deliver a significant payload at a significant range. The U.S. has had sufficient experience to build highly efficient solid-fuel rockets. The primary enabling technologies are lightweight materials, lightweight structures, lightweight guidance packages and lightweight payloads. If other countries had these technologies they would use solid fuels for their tactical and strategic missiles like we do.
Another difficult technical problem is getting the solid fuel molded to the proper shape without any flaws. As tris explained, the shape of the charge is extremely important. A void or crack in the fuel charge means a sudden increase in surface area, i.e. an explosion.