Start and stop an SRB?

[Themohawkninja]

While I know that you can\'t lower the thrust of a SRB, because the solid fuels\' burn rate can\'t be varied, I was wondering about what we could add to SRBs, and it occurred to me... can we stop an SRB mid-burn, and then re-light it at a later point?

[Vostok]

I think even IRL once an SRB is lit, it\'s a one way trip. Since the fuel contains its own oxidiser, even if you pump the engine full of carbon dioxide/extinguisher substance, it\'ll keep burning.

So, essentially, this would be unrealistic. It\'d also remove the fun from the SRBs, the fun being, if something goes wrong, you\'re essentially sitting atop a large firework, with little hope of rectifying the situation.

[leopardenthusiast]

I suppose that if you had a reversed booster that lasted for however long you wanted to halt the thrust, you could do that. The SRB would still burn as normal though, you\'d just have thrust cancelling it.

Pausing an SRB is impossible.

[rokketz]

I think even IRL once an SRB is lit, it\'s a one way trip. Since the fuel contains its own oxidiser, even if you pump the engine full of carbon dioxide/extinguisher substance, it\'ll keep burning.

So, essentially, this would be unrealistic. It\'d also remove the fun from the SRBs, the fun being, if something goes wrong, you\'re essentially sitting atop a large firework, with little hope of rectifying the situation.

This is why we can\'t have nice things.

[Themohawkninja]

I think even IRL once an SRB is lit, it\'s a one way trip. Since the fuel contains its own oxidiser, even if you pump the engine full of carbon dioxide/extinguisher substance, it\'ll keep burning.

So, essentially, this would be unrealistic. It\'d also remove the fun from the SRBs, the fun being, if something goes wrong, you\'re essentially sitting atop a large firework, with little hope of rectifying the situation.

I guess it would ruin some of the... 'Kerbalness' of the SRBs, but I was curious if it was even possible, because as far as I know, the SRBs are a single stage homogeneous mix of chemicals (judging by the constant appearance of the fireball it makes), so the composition of the motor is the same at any given time, thereby if you can light it once, you can extinguish it, and light it again.

[Rich]

Variable thrust SRB\'s? No problem!

Edited September 1, 2012 by Rich

[Themohawkninja]

Variable thrust SRB\'s? No problem!

How are the engines getting fuel? Can you place fuel lines directly on LFEs?

[Vanamonde]

I believe a solid rocket booster is a hollow cylinder which is ignited from the inside surface. As the surface burns away and exposes the propellant underneath, that fuel ignites and keeps going. It\'s already got its own fuel and oxidizer, so there\'s no way to extinguish the combusion once started. (And since more area is exposed and combusting the longer it burns, they actually get stronger as they go.) And really, since the only advantage of SRBs is that they burn quick and hard, if you\'re going to use it for any other purpose, you\'d be better off replacing it with a LFE that\'s more controllable and will give you a better cumulative change in momentum.

[Rich]

How are the engines getting fuel? Can you place fuel lines directly on LFEs?

Yeah, I ran fuel lines from the central stack directly to the LFE\'s. The lines do not attach cleanly, but the fuel would appear to flow.

[Ziff]

While I know that you can\'t lower the thrust of a SRB, because the solid fuels\' burn rate can\'t be varied, I was wondering about what we could add to SRBs, and it occurred to me... can we stop an SRB mid-burn, and then re-light it at a later point?

This would be an advanced SRB, there are apparently some that can somewhat control thrust using vent ports but I can\'t find a good example and I don\'t trust wikipedia. However, I do know there are pulse rocket motors which are basically several sections of solid fuel separated from each other so they can be fired in segments when needed. Kinda cool.

-Ziff

[Themohawkninja]

This would be an advanced SRB, there are apparently some that can somewhat control thrust using vent ports but I can\'t find a good example and I don\'t trust wikipedia. However, I do know there are pulse rocket motors which are basically several sections of solid fuel separated from each other so they can be fired in segments when needed. Kinda cool.

-Ziff

Pulse rockets are solid? I though they used liquid fuel that is flash-ignited to create a small explosion of thrust, and it does this hundreds (if not thousands) of times per second.

[Capt'n Skunky]

The whole design point of the SRB is that it\'s light once and let \'er rip. Your assertion that the thrust can\'t be varied is incorrect however. By changing the shape of the the way the fuel is packed into the casing they can vary the amount of thrust and I think the shuttles SRBs had this.

Cheers!

Capt\'n Skunky

KSP Community Manager

[Themohawkninja]

The whole design point of the SRB is that it\'s light once and let \'er rip. Your assertion that the thrust can\'t be varied is incorrect however. By changing the shape of the the way the fuel is packed into the casing they can vary the amount of thrust and I think the shuttles SRBs had this.

Cheers!

Capt\'n Skunky

KSP Community Manager

The thrust can\'t be varied en-route though, can it?

[Ziff]

Pulse rockets are solid? I though they used liquid fuel that is flash-ignited to create a small explosion of thrust, and it does this hundreds (if not thousands) of times per second.

That\'s a pulse jet engine. I\'m talking about a pulse rocket motor. Rocket motor\'s use solid fuel, not liquid. ATK makes some seriously advanced ones used in missiles, but I see no reason why they couldn\'t be made into SRB\'s. It works just how I described it. Several segments of solid fuel separated by a barrier. Fuel can be ignited in each segment as needed, but again, once a segment is lit you would have to ride it out until it is finished burning. Imagine instead of a 40 second burn you could have 2, 20 second segmented burns. Here\'s a legit article by the company. (Warning: PDF file ahead.) -> http://www.atk.com/Products/documents4-1/MP%20-%20TSRM%20Propulsion.pdf

-Ziff

[Eldestish]

Just get a damned robotics hinge and mount the SRB on that,Then you can change its angle for different amounts of thrust 8)

[softweir]

In Real Life there have been experiments to abort SRB thrust early by using an explosive to break open a panel at the top of the SRB. This does 2 things: 1) The gasses venting through the panel counter the thrust from the main nozzle; and 2) Venting the gasses causes the pressure and temperature inside the SRB casing to drop to below that necessary for sustained thrust.

There\'s no way to re-ignite one of these once the panel has been blown off!

There has also been some research done into hybrid boosters. These have a solid propellant but liquid oxidiser which is injected into the booster casing, and if the oxidiser flow is changed then some degree of thrust control can be performed, or they can even be shut off. They probably can\'t be reduced in power too far without combustion failing, but it may be possible to re-ignite one if additional pyrotechnics are fitted to do so.

However, so far as I am aware neither system has been used in a launch.

[Themohawkninja]

In Real Life there have been experiments to abort SRB thrust early by using an explosive to break open a panel at the top of the SRB. This does 2 things: 1) The gasses venting through the panel counter the thrust from the main nozzle; and 2) Venting the gasses causes the pressure and temperature inside the SRB casing to drop to below that necessary for sustained thrust.

There\'s no way to re-ignite one of these once the panel has been blown off!

There has also been some research done into hybrid boosters. These have a solid propellant but liquid oxidiser which is injected into the booster casing, and if the oxidiser flow is changed then some degree of thrust control can be performed, or they can even be shut off. They probably can\'t be reduced in power too far without combustion failing, but it may be possible to re-ignite one if additional pyrotechnics are fitted to do so.

However, so far as I am aware neither system has been used in a launch.

That first part sounds more like a fail-safe than anything else. The second example you gave was much more intriguing, and I wonder why the combustion would fail after lowering the liquid part to much.

[Vanamonde]

There has also been some research done into hybrid boosters. These have a solid propellant but liquid oxidiser which is injected into the booster casing, and if the oxidiser flow is changed then some degree of thrust control can be performed, or they can even be shut off. They probably can\'t be reduced in power too far without combustion failing, but it may be possible to re-ignite one if additional pyrotechnics are fitted to do so.

So it might be done, but to what purpose? Does it yield a harder shove at a critical moment or something like that? Because otherwise it just seems like a clunky way to reproduce what an LFE already does.

[vexx32]

That first part sounds more like a fail-safe than anything else. The second example you gave was much more intriguing, and I wonder why the combustion would fail after lowering the liquid part to much.

Simple. Combustion is a checmical reaction, requiring certain ratios of fuel to oxidiser to properly burn. If there is too little oxidiser, it will not burn at all.

[Themohawkninja]

Simple. Combustion is a checmical reaction, requiring certain ratios of fuel to oxidiser to properly burn. If there is too little oxidiser, it will not burn at all.

So I take it that one was only the oxidizer, and the other was only fuel?

[softweir]

Yes. Liquid oxidiser, solid fuel. The solid fuel can\'t burn without oxidiser. If there is too little oxidiser then pressure and temperature in the booster casing drop below those needed to sustain combustion, the surface of the fuel cools down and stops burning. It may still smoulder, but it won\'t provide thrust.

And you\'re right the first option is a failsafe.

[Themohawkninja]

Yes. Liquid oxidiser, solid fuel. The solid fuel can\'t burn without oxidiser. If there is too little oxidiser then pressure and temperature in the booster casing drop below those needed to sustain combustion, the surface of the fuel cools down and stops burning. It may still smoulder, but it won\'t provide thrust.

And you\'re right the first option is a failsafe.

That\'s an interesting concept. Can I have a link to a wiki page or some article that further explains how this liquid-solid motor works?