h a l f b a k e r y
The phrase 'crumpled heap' comes to mind.

meta:

account: browse anonymously, or get an account and write.

 user: pass:
register,

# Gulper rocket

Gulp.
 (+7, -1) [vote for, against]

[FOR PEOPLE IN A HURRY or diagnosed with less than a week to live. Use a modest-sized pressure vessel between the main tanks and the engine, so you can pressure-pump the fuel in gulps without turbopumps or high-pressure main tanks.]

So, we want to build a rocket. It's going to be a liquid-fuelled rocket (eg, kerosene + liquid oxygen), because these are the most efficient, mass-to-thrust wise.

The first bit is easy. We build a spherical combustion chamber, with inlets for fuel and oxidiser at the top, and big hole at the bottom. The big hole leads to the rocket nozzle, which is where the flames come out.

So far so good.

Now, in order to work efficiently, the combustion chamber has to operate at very high pressures (and temperatures), since this pressure is what provides the thrust. So, we have to bite the bullet and make the combustion chamber out of something very strong and heat-resistant. This is expensive and heavy, but there's no way around it.

Now.

The only remaining problem is that we have to get the fuel and oxidiser into the combustion chamber itself, and to do so at a fairly healthy rate. Here we encounter a problem: if the combustion chamber is at high pressure (typically 100 atmospheres), then we have to be able to push the fuel and oxidiser into it against that sort of pressure.

So.

What we would normally do is to have a couple of pumps (one for fuel, one for oxidiser). These pumps, however, have to deliver fuel and oxidiser at very high pressure and quite high flow rates, which means they are going to be expensive and heavy. We can live with it for something like the space shuttle (where their weight and cost is relatively small compared to the total), but not for a smaller rocket, since they become more expensive and more heavy (relatively speaking) as the rocket gets smaller.

So.

We decide to use a pressure-pumped design instead. All we do is to pressurize the fuel and oxidiser tanks, and connect them directly into the combustion chamber (maybe via an on/off valve). Great!! No complex pumps - we just use gas pressure (eg, helium) to push the liquids into the combustion chamber.

But.

The problem with this is that now, *everything* has to be strong enough to withstand 100bar of pressure. The fuel and oxidiser tanks have to be pressurised to 100bar, to push the stuff into the combustion chamber. A regular Coke can will take, maybe, 10bar. A large fuel tank capable of taking 100bar will be very heavy.

Damn.

The V1 flying bomb got around this problem by using a pulse-jet, which solves a similar problem by pulsing the propellant. Fuel is squirted into the combustion chamber at low pressure (easy), and then ignites; a check valve stops the pressure wave from pushing up the fuel lines. After the ignition cycle, the pressure drops and more fuel is injected, and so on. The engine is then driven by a series of pulses, not so different from a normal internal combustion engine.

But.

This type of pulsed rocket is not very efficient. It's driven by a rapid series of explosions, with wildly fluctuating pressures, so nothing is really ever optimal. Apart from anything else, the combustion is only happening for a fraction of the time.

So.

Enter the Gulper Rocket.

Here's* one standing in front of you, stripped down so you can see its innards.

At the bottom, we have the usual combustion chamber, with a bell- shaped nozzle underneath it.

Above that are two spherical pressure-tanks, side by side. One is for fuel, the other is for oxidiser. Each is about the same size as the combustion chamber, and a pipe leads from the bottom of each pressure tank into the combustion chamber. In each pipe is a simple non-return valve, allowing fuel to flow into the combustion chamber but not back. This valve has a spring bias to it; so, liquid at low pressure is held back; liquid at high pressure pushes open the valve and flows; but a reverse pressure shuts the valve securely. No actuators are needed.

With me?

Now, above the pressure tanks are the main fuel and oxidiser tanks. These are huge (with perhaps a few hundred times the volume of the pressure tanks) and full of liquid fuel and oxidiser. However, they're only strong enough to contain the liquid, and therefore are extremely light (just as on a regular rocket with fuel pumps). Big pipes connect each of these main tanks to the corresponding pressure tank, and a simple one-way valve prevents back-flow.

Now, here's how it works. First, the main tanks are pressurized, but only slightly - a few bar. In fact, the pressure from the boiloff of the liquids can be used to provide this pressure, or it can come from a separate helium tank. This modest pressure quickly pushes fuel and oxidiser into their respective pressure tanks, until these are full, but doesn't have high enough pressure to open the valve between the pressure tanks and the combustion chamber.

Once the pressure tanks are full, they are driven to a very high pressure (say, 100bar); there are various ways to do this and it's not difficult.

Now, the high pressure in the pressure tanks slams shut the non- return valve connecting them to the main tanks. At the same time, it overcomes the springy resistance in the valves connecting the pressure tanks to the combustion chamber, and fuel and oxidiser flood in and are ignited.

Whoomf.

Quite a long whoomf, of course (several seconds), because it's using the reservoir of fuel and oxidiser in the pressure tanks.

When the pressure tanks are empty, the fire goes out. At the same time, the flow of pressurizing gas to the pressure tanks is shut off. There's now no high pressure in the combustion chamber or anywhere else, and the cycle starts again, with the modest pressure in the main tanks refilling the pressure vessels ready for another burn.

So, in effect, we have created a pressure-pumped rocket, but one in which only a portion of the fuel/oxidiser needs to pressurized at any one time. This is better than a pulse-rocket, since each "pulse" can last for several seconds (thanks to the reservoir provided by the pressure tanks), lighter and simpler than a pump-fuelled rocket (I hope), and also lighter than a pure pressure-pumped rocket in which the entire fuel and oxidiser tanks must be capable of withstanding combustion pressures.

Maybe.

=========

*you are now 50% of the way through this. Bear with me.

 — MaxwellBuchanan, Jul 16 2010

Can you explain how pumping pressure into the little tank uses less uggg than pumping fuel into the combustion chamber?
 — pocmloc, Jul 16 2010

 It doesn't use any less uggggg.

The point of the pressure-tank is that only it (and not the main tank) needs to able to withstand the pressures which prevail in the combustion chamber itself. You need the same amount of uggggg to get fuel into the chamber, whether it's directly from high-pressure main tanks (which are heavy), via a high-pressure pump (which is costly) from low-pressure tanks, or via an intermediate gas-pressurised pressure tank, as here.
 — MaxwellBuchanan, Jul 16 2010

Depends. Suppose the pressure tanks have 1% the volume of the main tanks, then each "gulp" will be 100th of the total burn, or on the order of 10 seconds, with a "coasting" phase of maybe 1 second while the system recycles.
 — MaxwellBuchanan, Jul 16 2010

 //Would be lucky not to fall apart under the shock loads no ? //

 I'm not sure how you mean. It's basically a regular rocket engine which is fired for ten seconds, off for one, on for ten.. so anything that can take a regular rocket thrust would be fine.

//use some jet engines and feed oxidiser in at high altitudes// Jet engines are way less efficient than rocket engines, and no good for going into spacington space, even if you feed them oxidiser.
 — MaxwellBuchanan, Jul 16 2010

//gas-pressurised// where does the gas come from?
 — pocmloc, Jul 16 2010

[+] like it. Might use external pressure source for liftoff and thereafter use inertia of fuel mass for hydraulic ram effect through check valves to charge pressure vessels during each gulp, saving more weight.
 — cudgel, Jul 16 2010

 // where does the gas come from?//

 There are choices. Some rockets (mainly last-stages) do use gas-pressure-driven fuels, in the usual way, with pressure-tolerant fuel tanks. They normally use helium; a small tank of liquid helium can provide all the gas necessary.

Another option would be to cunningly use the boiloff pressure of the fuel/oxidant itself. This would need some extra plumbing, but should be doable. In the main tanks, you'd bleed most of the boiloff as usual to keep the pressure low; in the pressure tanks, you'd let the boiloff accumulate to much higher pressures to drive the fuel into the combustion chamber. You'd probably want to heat the pressure tanks by having them close to the combustion chamber, to provide rapid and high boiloff pressure.
 — MaxwellBuchanan, Jul 16 2010

 // use inertia of fuel mass for hydraulic ram effect through check valves to charge pressure vessels//

The problem is that the pressure tanks need to be filled whilst the rocket is "coasting" between bursts, so there'll be no inertial force of the type you need. You could have two complete systems in parallel, I guess, so that one gulped while the other one was firing; in that case, your system would work.
 — MaxwellBuchanan, Jul 16 2010

 We like this idea. It may well be unworkable, but the failures during the prototype testing stage will be spectacular enough to more than justify the attempt.

[+]
 — 8th of 7, Jul 16 2010

 So the idea is that you still need the same work done (pressure X Volume) to deliver the fuel to the combustion chamber, but the fact that you have a much smaller high-pressure fuel vessel results in weight savings.

Nice. Can you feed a small ammount of fuel to the oxidiser pressure tank, and a small ammount of oxidiser to the fuel pressure tank - in order to create the high pressure? If metered correctly, this might be very simple. Would work even better with hypergolic fuels.
 — Custardguts, Jul 18 2010

 Yes, that's the idea.

 As for cross-feeding the fuel and oxidiser, I'd be wary of that - flashbacks and all that. On the other hand, it's an interesting suggestion: use very-non-stoichiometric combustion in the pressure tanks to pump the liquids into the combustion chamber. Nice!

 What I'm hoping to come up with would be a system which, once started, would be entirely self-driving (ie, no need for externally-controlled valves etc), and therefore as simple and light as possible.

Hypergolic fuels would, as you point out, be a nice way to go, especially as there'd be no need to re-ignite the combustion chamber on each cycle. However, as far as I know, the hypergolics are all pretty evil chemicals. I presume reignition can come from a spark- or glow-plug type system? Or perhaps there's a catalyst which will ignite kerosene/LOX mixes (just as platinum will ignite hydrogen/LOX, I think)?
 — MaxwellBuchanan, Jul 18 2010

 Conventional designs do not to my knowledge require fuel tanks capable to combustion chamber pressures. The fuel pumps draw large volumes and low pressure in and eject them at high pressure into the chamber.

 why not use an automatic machine gun type approach and store pressure from each firing sequence in a storage tank to pressurize the feed tanks after they receive their fuel charge.

if you used some sort of spring loaded piston arrangement you could in essence create an internal combustion rocket motor where the exhaust is what provides the motive force.
 — jhomrighaus, Jul 18 2010

 //Conventional designs do not to my knowledge require fuel tanks capable to combustion chamber pressures. The fuel pumps draw large volumes and low pressure in and eject them at high pressure into the chamber//

 I discussed, in the idea (above, left) the fact that most rockets use turbopumps to pump fuel into the combustion chamber. Some other rockets (see above, left) use high- pressure fuel tanks instead. Both solutions involve complexity, cost and/or weight.

 //if you used some sort of spring loaded piston arrangement you could in essence create an internal combustion rocket motor where the exhaust is what provides the motive force.//

I also discussed the use of "pulse jet" (or pulse rocket) techniques, and explained why they're not very efficient (see above, left).
 — MaxwellBuchanan, Jul 18 2010

 // store pressure from each firing sequence in a storage tank to pressurize the feed tanks after they receive their fuel charge.//

 I've been trying to think of a way to implement that, but without success. Basically, you have to pressurize the pressure tanks to a pressure higher than that of the combustion chamber. This could be done with some sort of hydraulic jack type of system, but then you're talking about pistons which will be in contact with liquid oxygen or with fuel, and with combustion gases at some ridiculous temperature. I think that's a tall order.

 However, using liquid helium to provide the pressure isn't so bad - a small amount of liquid helium will be enough to pump (ie pressurize) many times its own volume.

 Also, I wonder if the heat of the combustion chamber could be used to enhance boiloff in the pressure tanks, and avoid the need for external pressurization?

 I'm moving towards a system which is totally self-driving. Low-pressure boiloff pushes liquids from the main tanks into the pressure tanks, which are closed when full by a non-return valve. Heating of the pressure tanks causes boiloff and raises their pressure sufficiently to open the escape valve letting liquid into the combustion chamber. When the pressure tanks are empty, the whole cycle begins again.

So, I think this can be done with an entirely passive system and no need for additional pressurizing gas.
 — MaxwellBuchanan, Jul 19 2010

 //fuel gets cycled around the outside of the combustion chamber?//

 Yes, that would be the sort of thing. I'm imagining some sort of glorified steam engine, in which the heat of combustion drives the pumping of the fuel. However, I want to avoid all pistons (and anything bar the simplest valve), since these will be tricky under harsh conditions.

I could do this if I were sober.
 — MaxwellBuchanan, Jul 21 2010

 No, you couldn't.

You said that about the genetically modified hedgehogs, and look how that ended up ...
 — 8th of 7, Jul 21 2010

Hey, no, I said genetically modified HEDGES. Just because they turned out to be able to walk doesn't mean they're insectivores.
 — MaxwellBuchanan, Jul 21 2010

 We did warn you. "Triffid" and "Privet" are NOT interchangeable. Admittedly, you improved the security of your properties, but terms like "collateral damage" and "acceptable casualties" are not - as we understand it - normal in the context of herbacious borders in your society.

Correct us if we're wrong (Like that could ever happen).
 — 8th of 7, Jul 21 2010

 //"acceptable casualties"//

That is a pleonasm. By definition, "casualties" are "casual", and therefore acceptable.
 — MaxwellBuchanan, Jul 21 2010

We defer to your superior legal representation.
 — 8th of 7, Jul 21 2010

 So in essence, it's a staged-pressure V1.

How much time in between pulses would it take to refill the pressure tanks under a low-pressure fill system? And what is the rocket doing during this time period? Coasting off course? Falling?
 — RayfordSteele, Jul 21 2010

How about a pair of gulper-sets? One fires while ullage reloads the other; then they switch. Feed alternates between the two, and combustion is continuous.
No relight, no back-pressure recovery time, no fry/freeze temp oscillations of your combustion chamber, no whiplash for the astronauts, no (ok, less) pogo-stress for the payload...
 — lurch, Jul 21 2010

 //So in essence, it's a staged-pressure V1. //

 Yes, pretty much.

 Probably true. However, the peak G will be the same as in a continuous rocket, so it should be survivable, but perhaps vomitiferous.

 //How much time in between pulses//...//what is the rocket doing during this time//

I'm guessing the refil could be done in 1/10th the burn time, so 10 seconds on, 1 off... since the filling pipes can be as wide as reasonable. During the 1 second coast, the rocket is coasting and losing speed, but not much. Toss a rock in the air and it's coasting from the moment it leaves your hand. In fact, staged rockets such as the old Apollo system often have "coasts" of a second or longer between stages - it's not a big problem.
 — MaxwellBuchanan, Jul 21 2010

Why not just use a graviton polarity generator powered from a forced quantum singularity ?
 — 8th of 7, Jul 21 2010

Because it doesn't really exist, actually.
 — MaxwellBuchanan, Jul 21 2010

Like you know ...
 — 8th of 7, Jul 21 2010

I've bought one on eBay - never arrived. Once bitten.
 — MaxwellBuchanan, Jul 21 2010

Fair enough. We blame FedEx, they lose stuff all the time. We are still waiting for our Ronco "Record Vacuum" ... we did the Western Union cash transfer thing just like they said, how long does shipment take from Nigeria to Low Earth Orbit ?
 — 8th of 7, Jul 21 2010

So you had no luck getting a vacuum sent to space? Oddly enough, my brother in Newcastle is still waiting for his shipment of coals.
 — MaxwellBuchanan, Jul 21 2010

We blame the french.
 — 8th of 7, Jul 21 2010

I knew we'd agree on something.
 — MaxwellBuchanan, Jul 21 2010

In this case, it's so easy ...
 — 8th of 7, Jul 21 2010

Wow! Thanks, [bigsleep]. An animation is clearly worth 1,093 words. That's exactly the sort of thing I had in mind.
 — MaxwellBuchanan, Jul 23 2010

 //once the hot stuff gets out of the combustion chamber there's a lot of heat energy there going to waste//

Ideally not. The ideal rocket nozzle allows the combustion gases to expand until their pressure as they leave the end of the nozzle just matches that of the outside atmosphere (which gets thinner, of course, as you go up; hence no one nozzle is ideal at all altitudes) . This expansion equates to cooling, so there's less "wasted" heat. However, the energy needed to pump the fuel into the combustion chamber is a tiny percentage of the energy output of combustion, so siphoning off a tiny amount of heat to pump the fuel will reduce the net thrust negligibly.
 — MaxwellBuchanan, Jul 25 2010

and..."negligibly"?
 — methinksnot, Jul 30 2010

 What if the fuel reservoir were stored entirely inside the combustion chamber. The reservoir would not be especially robust or pressure worthy. The combustion chamber is. The chamber is gradually pressurized perhaps by incrementally adding fuel, or maybe cold and at the same time the reservoir is gradually pressurized.

 The reservoir experiences the same pressure on the inside and outside so does not need to be robust as there is not a pressure difference across the wall. Pumps enclosed within the reservoir operate at low pressure to emit fuel and sustain combustion / pressure outside in the combustion chamber.

A problem is that this would be a huge combustion chamber to enclose the reservoirs. I wonder if the reservoirs were surrounded with some fluid that would transmit the pressure up from the combustion chamber if that would work.
 — bungston, Jun 06 2012

//What if the fuel reservoir were stored entirely inside the combustion chamber.// Interesting idea, but in the end I don't think you gain. Ultimately, you still have to enclose the same volume (ie, the combustion volume plus the reservoir volume) against high pressures, whether the latter is inside the former or not.
 — MaxwellBuchanan, Jun 09 2012

 That won't help, because the pressure created when they're mixed (and again when they react with the fuel) will still propagate back up into the tanks.

 As far as I can see, if you have continuous flow from the tanks into the combustion chamber, and if that flow is pressure driven (rather than pump driven), then the pressure in the tanks has to be higher than that in the combustion chamber. But an intermittent flow uncouples the tanks from the combustion chamber.

I guess that a "gulper rocket" approximates to a machine gun, where a bullet is fed in under low pressure, and then fired, creating high pressure, in alternation.
 — MaxwellBuchanan, Jun 09 2012

I like this idea. If it is difficult to inject fuel into a high pressure vessel from a lower pressure storage chamber, just wait till the pressure drops. Eminently sensible. [+]
 — AusCan531, Jun 10 2012

 [annotate]

back: main index