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Hypersonic flight: A new fuel for jets and missiles is on the cards

68 points3 yearseconomist.com
jjk1663 years ago

There are a lot of issues that led to these fuels being abandoned the first time around, most of which are unavoidable.

1) Boron Carbide and Boron Nitride particles form in the exhaust. These are extremely abrasive, destroying downstream engine components. They also make the exhaust plume highly detectable over long distances - think of the particles like soot from a coal burning ship.

2) The fuels are extremely corrosive, and thus you can't leave a tank full with them, and even when you do fuel up, you can't run on pure boranes, you need to mix them with other fuels. This means you can't keep a boron fueled missile on standby, so its speed advantage is rendered moot except for surprise first strikes. Further, in diluting the fuel the performance gains are reduced.

3) Boron based fuels suffer from incomplete combustion, dramatically reducing real performance below theoretical values.

4) Manufacturing and storing these chemicals is incredibly dangerous and expensive. One of the major reasons for the initial program's abandonment was that a production plant exploded. These safety concerns were damning in the age of atmospheric nuclear tests, one can imagine the difficulty of production in a modern regulatory environment.

In short, boron based fuels offer only questionable performance gains at very high economic, technical, and human costs. I won't say there is no future at all for them, but the idea that they are the fuel of the future is laughable.

pfdietz3 years ago

There is an interesting boron fuel combination that would be useful for an air turborocket. (See https://en.wikipedia.org/wiki/Air_turborocket for what that is.)

The combination is diborane (B2H6) and hydrazine (N2H4). These react to form boron nitride (BN) and hydrogen. This reaction is exothermic and by itself would give an Isp of maybe 250 s, IIRC. The gas is hydrogen rich, and would be great for feeding into the air turborocket, or into a scramjet, for combustion with air.

ricardobeat3 years ago

The original research was suspended in part due to how dangerous these fuels were, hydrazine doesn't sound like a very promising option being extremely toxic and flammable.

bernulli3 years ago

Surely the Isp, i.e. thrust per propellant weight flow rate, would have to be much higher for that air turborocket? 250s is not that great already for a rocket that has to carry all of the propellant; the big advantage of an airbreathing engine is that only a fraction of the accelerated massflow is propellant.

msandford3 years ago

I believe that's the ISP of the initial reaction that makes a hydrogen rich gas that you feed into your air breathing thing. That's how it reads to me anyhow. You'll likely get extra ISP from the hydrogen/air combustion too.

pfdietz3 years ago

That's correct; it was the Isp of that combination if just used in an ordinary rocket engine, not an airbreathing engine.

knolan3 years ago

For anyone interested in the development of rocket propellant and related fuel I’d highly recommend ‘Ignition!: An Informal History of Liquid Rocket Propellants’ by John D. Clark.

tim3333 years ago

Start of the chapter on it:

>Fifteen years ago people used to ask me "What is an exotic fuel any- way?" and I would answer "It's expensive, it's got boron in it, and it probably doesn't work." I had intended, originally, to entitle this chap- ter "The Billion Buck Boron Booboo," but decided against it on two grounds. The first was that such a title might conceivably be consid- ered tactless by some of the people who authorized the programs con- cerned. The second reason is that it would not be completely accurate. Actually, the boron programs did not cost a billion dollars. It just seemed that way at the time.

nyankosensei3 years ago

I also recommend ‘The Green Flame’ by Andrew Dequasie. It’s the fascinating personal story of a chemist embarking on a new career and ending up working with borane fuels.

david-gpu3 years ago

Non-paywalled mirror: https://archive.is/lT5dT

sudosysgen3 years ago

Russia and the USSR have already been working on such fuels, which are due to be used on the Zircon hypersonic missiles.

In order to achieve the same goals while avoiding the safety issues of boron fuels, the Decilin-M fuel for the Zircon missile is thought to use engineered hydrocarbons (the exact composition is unknown) with polyethylene to thicken the solution with aluminium nanoparticle added.

david-gpu3 years ago

Wikipedia has an article about this, for those interested.

https://en.wikipedia.org/wiki/3M22_Zircon

jcun41283 years ago

Curious, says range is at most 400mi if used ballistically. I wonder even at Mach 9 can a laser stop this thing or would ships just use railgun/lasers against each other? Maybe it's surface to ship.

wongarsu3 years ago

The problem with ship-to-ship laser combat is that line-of-sight only a couple kilometers. Two objects mounted in 20m height can only see each other for 36km.

Railguns are a lot more likely, but I think we still don't have "barrels" that survive firing more than a couple shots before replacement.

sudosysgen3 years ago

If your lasers can be reflected easily by mirrors, then the missile will simply be painted with a mirror coating.

mc323 years ago

Can they bounce the beam off of a sat or flying object?

wongarsu3 years ago

Maybe a good use for quadrocopters or other low-cost drones.

If you use a satellite, you might as well put a larger concave mirror on it to focus sunlight. A nearly 100 year old weapon concept that luckily never happened [1]

1: https://en.wikipedia.org/wiki/Sun_gun

chrisseaton3 years ago

You need ballistic weapons to get around the fact that the surface of the earth is curved.

bernulli3 years ago

The key point of hypersonic weapons is that they are maneuverable.

chrisseaton3 years ago

Right - ballistic or guided - I really meant kinetic as opposed to energy weapons like lasers.

jhgb3 years ago

If it's heading at you at Mach 9, just dump a load of BB pellets against it.

jcun41283 years ago

Maybe it would be more like a missile intercepting this incoming missile then... I've seen some of those missiles that can turn 90 degrees and accelerate to some insane speed. Don't know if the sprint missile does the 90deg turn but I think another Russian missile does the 90deg turn after launching.

Gravityloss3 years ago

That's just after launch at basically no speed. Of course you must exit the launch tube along the direction of the tube (usually with launch charge) and then turn.

rbanffy3 years ago

Unless you dump a significant amount of BB pellets, you’ll still have high-speed missile parts flying in your direction.

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jhgb3 years ago
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pie4203 years ago
carapace3 years ago

It's not Plichta's silane fuel.

Instead of hydrocarbon use hydrosilicon. Thought to be unstable (like "big bada boom" unstable) Peter Plichta discovered that "Silanes with chains of seven or more atoms of silicon per molecule are stable and can be pumped and stored very much like gasoline and other carbon-based liquid fuels."

http://www.rexresearch.com/plichtasilane/plichta.html

They burn nitrogen and are said to make lightning instead of flames.

> The combustion process of hydrosilicons is fundamentally different from the exclusively oxygen based combustion we know from burning hydrocarbons. In a sufficiently hot reaction chamber, silanes separate into atoms of hydrogen and silicon, which immediately mix with the oxygen and nitrogen of the air. The hydrogen from the silanes and the air's oxygen now burn completely leaving only water vapor, bringing the temperature of the gases close to 2000 degrees C.

> Since there is no more oxygen, no silicon oxide can be formed in the following phase. What happens instead is an extremely energetic reaction of the 80% nitrogen in the air with the silicon atoms present, that forms a fine powder called silicon nitride (Si3N4).

SiempreViernes3 years ago

It's funny how articles about hypersonic weapons always talk about their speed when the defining property is manoeuvrability; Obviously you can't turn very well if you go super fast, so the solution is to go slower: hypersonic missiles aren't actually the fastest missiles around.

rbanffy3 years ago

If you know where your target will be, you don’t need to manoeuvre too much. If you are getting there on Mach 9, the target will very much be wherever it was when you launched.

SiempreViernes3 years ago

A car going 30 km/h clears the 30m blast radius of a 900kg bomb in about three seconds, and the 340m fragmentation radius in about 40 seconds.

This means that to hit a moving target at Mach 9 (or about 3 km/s) you need to launch your 11m long rocket within either 9 or 120 km depending on the desired probability for a kill.

As a reference, an aircraft carrier can go over 50 km/h, the Chinese hypersonic weapons go about 1.7 km/s and have a range of at least 1000 km. How does that work out you wonder? Simple, they put nukes on their hypersonic missiles.

mLuby3 years ago

As impressive as an aircraft carrier with solar escape velocity would be, I'm betting that's a typo.

A 50km/s vessel carrying hypersonic missiles would be like bringing turtles on a transatlantic flight!

elric3 years ago

> an aircraft carrier can go over 50 km/s

I'm assuming you mean km/h there? I'd love to see a boat going that fast.

SiempreViernes3 years ago

Yeah, thanks for pointing out the typo. Probably wouldn't have spotted it in time to edit it otherwise.

bernulli3 years ago

But you also don't want your target to know that it is being attacked to prevent countermeasures.

rsfern3 years ago

The current sort of arms race around hypersonics is precisely because there are no good currently existing countermeasures for a missile traveling at like Mach 10. I’m sure it’s being worked on, but there’s a lot of effort going into the mutually assured destruction short of strategy.

sudosysgen3 years ago

Missile maneuvrability is measured in lateral acceleration.

Hypersonic missiles pull just as many Gs as any other missile, they are not any less maneuverable.

jhgb3 years ago

If they only pull as many Gs as any other missile, then their turning radius will be larger proportional to the square of the velocity ratios. For example a Mach 10 missile pulling the same Gs as a Mach 2 missile will turn in a circle 25 times larger.

Or, to put it differently, if the missile is approaching its target and X seconds before impact decides to change the impact point, or perhaps evade a countermeasure, a Mach 10 missile will be only able to make 4% of the correction a Mach 2 missile with the same transverse acceleration would be able to make from the same distance.

sudosysgen3 years ago

Turning radius doesn't matter though. What matters is being able to outmaneuver interceptors and cause them to lose energy. What matters for that is only Gs.

The turning radius is completely irrelevant if you want to evade a countermeasure. All that matters is how far you are from where the countermeasure is going. That's measured in Gs.

Similarly, there's not going to be any issue changing impact points, the only issue is changing targets, but I don't see where that would be useful.

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jhgb3 years ago
SiempreViernes3 years ago

Hypersonic get their claim for manoeuvrability by comparison with their faster cousins the ballistic missiles, not short range (less than 60 km) air-to-air missiles that have sort bursts of Mach 4.

sudosysgen3 years ago

This is wrong. Hypersonic vehicles are thought to have maneuvering accelerations over 15Gs. That's more than most cruise missiles.

Of course they will not be as maneuverable as A2A missiles. But what's the point? If you want to evade A2A missiles already going slower than you you're using energy to your advantage so 10Gs is more than enough.

As it stands hypersonic missiles are more maneuverable than all other missiles including cruise missiles short of specialized A2A missiles. So for the purpose of evading interceptors and penetrating defence systems it's more than enough.

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jhgb3 years ago
seiferteric3 years ago
adeelk933 years ago

That is indeed what the article says

mikewarot3 years ago

Have they solved the erosion problems with it that lead to it being dropped in the 1960s?

grouphugs3 years ago

it's a really bad time for new weapons