SRB vs solid fuel ramjet

[Idobox]

Solid fuel ramjets are basically a ram intake, a tube full of solid fuel and a nozzle, and they are commonly used for missiles.

What struck me is how similar to SRBs they are, and it got me thinking: why is nobody developing SRB/ ramjet hybrids for space launchers?

The way I see it, you would take a big solid fuel ramjet, add some system to open and close the intake, and add some fuel+oxidizer mix in the middle. At start up, it would work like a SRB, burning through its fuel and gaining speed. As time goes on, the rocket would go faster, and the fuel+oxidizier mix would get consumed, until it is almost completely consumed and the thing flies close to mach 1, when you open the intake and use atmospheric oxygen to burn pure fuel, or a fuel rich mix.

Obviously, such a design would be more expensive than either a SRB or a solid fuel ramjet, with the intake control and and layers of different fuel mixes, but it would combine the thrust at zero speed of SRBs with the much higher ISP of ramjets, while staying relatively simple. The intake control could be something as simple as a one-way valve, opening only when ram pressure becomes larger than chamber pressure.

Another variation on the concept would be to start as a hybrid rocket rather than SRB. That way, you could use a single grain with no solid oxidizer, but you would need a turbopump and a tank of oxidizer, which would probably result in something heavier and more expensive than a SRB/ramjet.

[Thiel]

AFAIK most air breathing proposals falls apart because the rocket doesn't stay in the atmosphere for long enough for it to make sense when you take the added structural weight in to account.

[shynung]

Congratulations, you have just invented the air-augmented SRB!

With a clever arrangement, there is never any need for closeable intakes or valving. Make the intakes direct ambient air into a chamber, inside if which is a rocket nozzle exhaust. The rocket nozzle exhausts hot gas into the chamber, which then goes out through the chamber's nozzle (separate from the rocket's). This effectively turns the chamber into an injector jet, sucking ambient air into it and ejecting them along with the rocket exhaust, increasing effective exhaust velocity similar to a turbofan, and thereby effective I_SP. Even better, this design works with virtually any rocket engine, including solids and NTRs.

AFAIK most air breathing proposals falls apart because the rocket doesn't stay in the atmosphere for long enough for it to make sense when you take the added structural weight in to account.

Since this design requires only an expansion chamber with intakes and a nozzle attached, it could be placed at the business end of the rocket, then staged away when the air gets too thin to get any significant boost.

Edited April 22, 2014 by shynung

[Idobox]

I found the Crow missile that works pretty much like I described, so the concept made sense in the 50s

http://en.wikipedia.org/wiki/Crow_%28missile%29

Congratulations, you have just invented the air-augmented SRB!

With a clever arrangement, there is never any need for closeable intakes or valving. Make the intakes direct ambient air into a chamber, inside if which is a rocket nozzle exhaust. The rocket nozzle exhausts hot gas into the chamber, which then goes out through the chamber's nozzle (separate from the rocket's). This effectively turns the chamber into an injector jet, sucking ambient air into it and ejecting them along with the rocket exhaust, increasing effective exhaust velocity similar to a turbofan, and thereby effective ISP. Even better, this design works with virtually any rocket engine, including solids and NTRs.

I know the air augmented rocket. The main problem is the weight of the air duct, and since SRBs are mostly used for the super high TWR, it's a killer.

My proposition would weigh pretty much the same as a normal SRB, but would burn longer with the same thrust.

And the best arrangement I can think of is to use some SRB fuel as a blocker: it would burn and open the intake after some time.

AFAIK most air breathing proposals falls apart because the rocket doesn't stay in the atmosphere for long enough for it to make sense when you take the added structural weight in to account.

Apparently, the SRB of the shuttle separated after 120s, at 45km, while traveling at 1400m/s. You could imagine a SRB that switches to ramjet after 40s, when reaching the speed of sound, turning earlier to get as much horizontal speed as possible while in ramjet mode, which could last significantly longer (more fuel for the same mass).

A ram intake on top of a SRB weighs practically nothing

[shynung]

Problem is, regular SRB have both fuel and oxidizer in one mix, enabling very fast chemical reactions to take place, thereby generating high-TWR thrust. Using ambient air as oxidizer is problematic, since air constitutes only 21% O₂, making fast chemical reactions problematic. Using a fuel-rich solid-fuel mix gives you somewhere between air-augmented rockets and solid-fuel ramjets, which will become inefficient at lower atmospheric pressures.

Also, no matter how simple, intakes will probably weigh more than simple nosecones of the same material and size, as they are generally more complicated in shape. In the end, you have a vehicle slightly heavier than an unmodified SRB, and burns longer with less thrust.

Edited April 22, 2014 by shynung

[R0cketC0der]

The main reason why we use SRBs is that they are a lot cheaper than liquid fueled rockets. However, if you make SRBs that are a lot more complex, they will cost a lot more, and therefore reduce the advantage over liquid rockets.

[shynung]

The main reason why we use SRBs is that they are a lot cheaper than liquid fueled rockets. However, if you make SRBs that are a lot more complex, they will cost a lot more, and therefore reduce the advantage over liquid rockets.

It'd be somewhere between SRBs and liquid-fuel rockets, yes.

[Idobox]

Problem is, regular SRB have both fuel and oxidizer in one mix, enabling very fast chemical reactions to take place, thereby generating high-TWR thrust. Using ambient air as oxidizer is problematic, since air constitutes only 21% O₂, making fast chemical reactions problematic. Using a fuel-rich solid-fuel mix gives you somewhere between air-augmented rockets and solid-fuel ramjets, which will become inefficient at lower atmospheric pressures.

Also, no matter how simple, intakes will probably weigh more than simple nosecones of the same material and size, as they are generally more complicated in shape. In the end, you have a vehicle slightly heavier than an unmodified SRB, and burns longer with less thrust.

That's true, but you also need less and less thrust as your rocket burns its fuel and gets lighter, and you have to compare the weight of the intake not just to the weight of a simpler nose, but also to that of saved oxidizer.

I'm not saying it's the best solution to every rocket need, but it seems like a slightly heavier booster that can burn longer with the same thrust (just put enough oxidizer to get the same thrust in ramjet and in SRB mode) for longer would be a good thing. Alternatively, you could make a lighter booster that burns with the same thrust for the same time.

The main reason why we use SRBs is that they are a lot cheaper than liquid fueled rockets. However, if you make SRBs that are a lot more complex, they will cost a lot more, and therefore reduce the advantage over liquid rockets.

check this picture from wikipedia

http://en.wikipedia.org/wiki/File:RARE_cutaway.jpg

It is not a lot more complex than a solid-fuel ramjet, which is a very popular second stage for missiles because it is so cheap and reliable.

[shynung]

That's true, but you also need less and less thrust as your rocket burns its fuel and gets lighter, and you have to compare the weight of the intake not just to the weight of a simpler nose, but also to that of saved oxidizer.

Saving oxidizer could work, but only in altitudes the fuel is designed to burn in, which depends on fuel-oxidizer mixture. At other altitudes, the combustion is either going to be oxidizer-rich (too much air), or worse, fuel rich(too little air).

Also, I have to point out that the most common modern solid rocket propellant has ammonium perchlorate (NH₄ClO₄) as the oxidizer, which has double the amount of oxygen atoms in one molecule than regular oxygen (O₂). Completely replacing the former oxidizer with the latter at 21% concentrations will ruin your TWR for sure. Partially replacing it will force you to choose an optimal atmospheric pressure, as I explained above.

In theory, solid-fuel ramjet boosters could work, but there's a lot of things that has to be solved for now.

EDIT: Here's a good read in the subject.

Edited April 22, 2014 by shynung