Pulsejet as a 1st stage rocket?

[SpaceMouse]

So i have been fascinated with the concept of a valveless pulse-jet pretty much since i learned of its existence. Largely due to its simplicity. I've had the thought before that as a inexpensive engine it might work as a 1st stage for a rocket. It's air breathing and as such doesn't need complex oxidizer setups. I made one in KSP with reasonably accurate TWR and a few other things, and mentioned it in my project thread but got very little response (so it probably lacks some accuracy). Question is, what could be done (if anything) to improove its thrust/efficency? I know there was a bit of development in the 60's but it more or less started and ended there. This probably isn't terribly scientific but, any ideas?

[_Augustus_]

I see no reason why not. In fact, why don't rockets use scramjets and SRBs as a first stage?

[magnemoe]

Pulsejet makes sense for something like an cruise missile, you could here use an rocket upper stage to increase speed to hit well defended targets. 
However I don't think performance is good enough for an orbital rocket, its not high supersonic and an cargo plane can lift a lot. 

[SpaceMouse]

Admittedly, even in KSP the usefulness is fairly limited. I maintain its a good 1st stage though, even with horrible fuel consumption, its still better ISP than a rocket (probably). In typical Kerbal fashion, you can strap a whole lot of them under something. with the 1/6th everything on Kerbin, Its reasonably useful for aircraft too.

I actually put it to use as a 'budget' rocket, and used a SRB as a 2nd stage.

[DerekL1963]

8 hours ago, SpaceMouse said:

Largely due to its simplicity. I've had the thought before that as a inexpensive engine it might work as a 1st stage for a rocket. It's air breathing and as such doesn't need complex oxidizer setups.

It's no less stupid than any other airbreathing "1st stage".  Which isn't saying much given how abysmally stupid the idea of airbreathing first stages are in the first place.

8 hours ago, _Augustus_ said:

I see no reason why not. In fact, why don't rockets use scramjets and SRBs as a first stage?

Because scamjets (and no, that's not a misspelling) and other jets are, compared to a rocket, really only useful in a narrow range of speeds and altitudes - and then you need to switch to pure rocket propulsion anyhow.  (Which means in terms of SpaceMouse's question, a complex oxidizer setup.)  Essentially you spend (tens of?) millions of dollars to save a couple hundred thousand dollars worth of LOX.

Edited October 2, 2016 by DerekL1963

[rdem]

A rocket's altitude changes so fast. So, to sustain a high trust with an airbreathing engine you have to follow a shallow path. And that creates much friction and heat.

[DDE]

Pulsejets per se are non-starter since you still get all the trouble of an airbreather, although they are more easily used as an expendable airbreathing stage.

Pulse detonation rockets are a whole 'nother matter.

Spoiler

 

 

[SpaceMouse]

Kinda forgot about the pulse detonation engine.  Might have to do a pulse detonation rocket too.

I still want to see if i can get any improvement out of the crazy 'jet' with no moving parts.

[K^2]

Pulsejets are inherently limited to a low chamber pressure, which means low efficiency. You can't cheat thermodynamics. You can submit and build a proper engine.

Scramjet as a first stage isn't a terrible idea, but then you are forced to use a booster and the actual duration in flight during which it will be useful isn't great. A much better idea is an air-augmented rocket.

[magnemoe]

11 hours ago, DDE said:

Pulsejets per se are non-starter since you still get all the trouble of an airbreather, although they are more easily used as an expendable airbreathing stage.

Pulse detonation rockets are a whole 'nother matter.

  Reveal hidden contents

 

 

You have more information about this, it looks like engine is powerful for bell size
Have heard about them for jets but not rockets, who benefits do they have for an rocket engine? 
Looks like the jet version is hard. 
 

[DDE]

8 hours ago, magnemoe said:

who benefits do they have for an rocket engine? 

Better Isp because the energy is displaced in an honest-to-Marx explosion. This one's a kerolox model.

Edited October 3, 2016 by DDE

[K^2]

To clarify, what we sometimes incorrectly refer to as an explosion in conventional rocket engine is still formally a combustion. The wave front propagates at subsonic speeds, matched by the speed of the fuel and oxidizer inflow. This means that pressure at which combustion happens is entirely determined by the pressure at which the fuel and oxidizer are fed and which combustion chamber can withstand.

Detonation wave travels at supersonic speeds. For conventional rocket engine, detonation is bad news and usually leads to RUD of the engine and the rest of the rocket. On the plus side, however, the pressure is generated entirely by the dynamics of the gas, and can be dramatically higher than anything you can achieve in a conventional rocket, leading to higher overall efficiency. If you can build an engine that is designed to withstand detonation, you can potentially build a lighter engine with better ISP. But this involves extremely hard engineering and mathematical problems.

[magnemoe]

Still it should be easier to do in an rocket engine than an jet engine as you have more control. 
Would also be easier with smaller lower trust engines. 
 

[KerikBalm]

On 10/2/2016 at 5:01 AM, DerekL1963 said:

 Essentially you spend (tens of?) millions of dollars to save a couple hundred thousand dollars worth of LOX.

Its not just LOX, but fuel. The effective Isp is much much higher because of the use of air as propellant. When I say air as propellant, Im not talking the ~20% O2 for an oxidizer, I mean 100% of the air as working mass.

For the same energy it takes to expell 1 kg of mass at 4000 m/s, you can expell 4kg of mass at 2000 m/s for double the thrust. 16 kg of mass at 1000 m/s for 4x the thrust... High effective Isp means a much lighter mass of fuel needed, which means a much smaller first stage, and thus lower required thrust, etc.

The problem is that it only works if your first stage can get to a significant fraction of orbital velocity. A pulsejet IIRC is pretty limited in terms of Mach, and its not going to get you anywhere close.... you'll still need your rocket stage to be a nearly full size rocket.

This is why air launch from modified airliners/subsonic bombers makes very little sense: the gains are so small, and you can't really build an airbreathing craft big enough to handle a large rocket payload.

An F-15 going mach 2 in a zoom climb starts to look a little more appealing as it can eliminate some of the gravity losses and you get a bit more tangential/horizontal velocity... but it still doesn't make much sense.

Scramjets could in theory get to a significant fraction of orbital velocity. In thoery a scramjet going at mach 10 on hydrogen fuel could get over 5,000 Isp, or 1,200 Isp on hydrocarbon fuels. Of course in reality we're only seeing our engines get about half their HC fuel maximum, so lets say 2,000 Isp for H2, or 600 Isp for HC fuel.

So far the fastest scramjet that I can find information about reached Mach 6 or 2042 m/s... which isn't bad, and would be super effective in KSP but still leaves ~6km/sec to go for the next stage. The first stage of the Saturn V delivered 3,550 m/s (not sure what the final speed was... ie how large the drag losses were), and the first stage was about 5/6th the mass of the entire rocket. Going from 260 seconds of specific impulse to over 2000 would decrease the mass needed by over 10 fold, and thus the thrust needed. Still, the airbreather would need to lift a payload of 680 metric tons for a payload to the moon... 8x what a C-5 cargoplane can lift... lets look at something smaller... the Saturn 1B second stage... 114 metric tons... still would be heavier than anything an aircraft has ever lifted... and that S1B only got 21 metric tons to orbit.

So, keeping in mind the limits of aircraft design, we'd really not be looking at anything much better than a 10 ton to orbit design, using a scramjet propelled aircraft as a first stage, as far as I can tell... looking at these stats, I really don't see how Skylon would get 15 tons to orbit as a single stage.

Getting scramjets to mach 8/9/10 would enourmously improve their usability and payload to orbit.

But... that mach 6 one that flew... it only fired for 5 seconds, so most of that dV was delivered by a rocket booster anyway...

We'd need an airbreathing craft more like the SR-71 that can takeoff under its own power and accelerate to scramjet speeds without rocket boosters, or else the airbreathing stage is just a few second gimmick between two rocket stages

[Matuchkin]

On 01/10/2016 at 5:13 PM, _Augustus_ said:

why don't rockets use SRBs as a first stage?

[magnemoe]

1 hour ago, KerikBalm said:

Its not just LOX, but fuel. The effective Isp is much much higher because of the use of air as propellant. When I say air as propellant, Im not talking the ~20% O2 for an oxidizer, I mean 100% of the air as working mass.

For the same energy it takes to expell 1 kg of mass at 4000 m/s, you can expell 4kg of mass at 2000 m/s for double the thrust. 16 kg of mass at 1000 m/s for 4x the thrust... High effective Isp means a much lighter mass of fuel needed, which means a much smaller first stage, and thus lower required thrust, etc.

The problem is that it only works if your first stage can get to a significant fraction of orbital velocity. A pulsejet IIRC is pretty limited in terms of Mach, and its not going to get you anywhere close.... you'll still need your rocket stage to be a nearly full size rocket.

This is why air launch from modified airliners/subsonic bombers makes very little sense: the gains are so small, and you can't really build an airbreathing craft big enough to handle a large rocket payload.

An F-15 going mach 2 in a zoom climb starts to look a little more appealing as it can eliminate some of the gravity losses and you get a bit more tangential/horizontal velocity... but it still doesn't make much sense.

Scramjets could in theory get to a significant fraction of orbital velocity. In thoery a scramjet going at mach 10 on hydrogen fuel could get over 5,000 Isp, or 1,200 Isp on hydrocarbon fuels. Of course in reality we're only seeing our engines get about half their HC fuel maximum, so lets say 2,000 Isp for H2, or 600 Isp for HC fuel.

So far the fastest scramjet that I can find information about reached Mach 6 or 2042 m/s... which isn't bad, and would be super effective in KSP but still leaves ~6km/sec to go for the next stage. The first stage of the Saturn V delivered 3,550 m/s (not sure what the final speed was... ie how large the drag losses were), and the first stage was about 5/6th the mass of the entire rocket. Going from 260 seconds of specific impulse to over 2000 would decrease the mass needed by over 10 fold, and thus the thrust needed. Still, the airbreather would need to lift a payload of 680 metric tons for a payload to the moon... 8x what a C-5 cargoplane can lift... lets look at something smaller... the Saturn 1B second stage... 114 metric tons... still would be heavier than anything an aircraft has ever lifted... and that S1B only got 21 metric tons to orbit.

So, keeping in mind the limits of aircraft design, we'd really not be looking at anything much better than a 10 ton to orbit design, using a scramjet propelled aircraft as a first stage, as far as I can tell... looking at these stats, I really don't see how Skylon would get 15 tons to orbit as a single stage.

Getting scramjets to mach 8/9/10 would enourmously improve their usability and payload to orbit.

But... that mach 6 one that flew... it only fired for 5 seconds, so most of that dV was delivered by a rocket booster anyway...

We'd need an airbreathing craft more like the SR-71 that can takeoff under its own power and accelerate to scramjet speeds without rocket boosters, or else the airbreathing stage is just a few second gimmick between two rocket stages

Agree here, note airplane as first stage makes some sense for very small rockets as you get away with lots of the air resistance and can use an more vacuum optimized nozzle for first stage. 
However getting up to 6km/s you are moving as fast as a falcon9 first stage, this is also the max speed for skylon in air breathing mode. 
At this speeds it makes sense, however hypersonic separation is hard Skylon and some other design solves this by going into space for separation. 
large hypersonic planes are also expensive to develop and build. rockets are far cheaper here. 
 

[KerikBalm]

I think you meant mach 6, not 6 km/sec

There is no way skylong is getting to 6 km/sec while airbreathing

[wumpus]

2 hours ago, KerikBalm said:

Getting scramjets to mach 8/9/10 would enourmously improve their usability and payload to orbit.

But... that mach 6 one that flew... it only fired for 5 seconds, so most of that dV was delivered by a rocket booster anyway...

NASA's X-43A managed mach 9.6 (~3000m/s).  http://www.nasa.gov/missions/research/x43-main.html

Gotcha's: It just barely made positive acceleration vs. drag, it had zero payload and was not reusable. https://hapb-www.larc.nasa.gov/Public/Documents/AIAA-2006-1-317.pdf

Why you would want it?  Basically the thing has a (fuel) Isp of over 2000 (around mach 6) and still over 1000 at mach 9 (I've linked to one that had it, unfortunately this forum lacks a real search function).

https://info.aiaa.org/Regions/Western/Orange_County/ASAT Conference 2013 Presentations/The Specific Impulse Potential of Hydrogen Scramjet.pdf

Basically, a technology that is only slightly less "blue sky" (in that prototypes have at least fired) than most of the blue sky work that is discussed here.  It has a long way to go, and considering that NASA basically handed the research over to the USAF (which seems to be ignoring it), it looks like one of those "the technology of tomorrow, and always will be".

As far as pulsejets go, they sound like more of an expert model rocket maker's chance to move to more exotic launch techniques.  On the other hand, I suspect that the only advantage would be altitude as the things just can't produce delta-v.  Using a weather balloon as a "first stage" would make a lot more sense to get through the atmosphere.

[KerikBalm]

Ah yes, I knew I must have been forgetting one... anyway... the benefits of an engine getting you to over 1/3 orbital velocity with specific impulses in the quadruple digits, usable high in the atmosphere... those are of course very interesting.

A low thrust, low mach, relativiely low altitude pulse jet just doesnt make much sense. The phase of flight in the right speed and altitude range is soo brief that its never going to be even close to worth it.

[HalcyonSon]

Pulsejets in action... https://en.wikipedia.org/wiki/V-1_flying_bomb

[Bill Phil]

Issue with pulsejets... They pulse. This means that they cause vibrations, which can be a real game killer if you're not careful.

[magnemoe]

16 hours ago, KerikBalm said:

I think you meant mach 6, not 6 km/sec

There is no way skylong is getting to 6 km/sec while airbreathing

Yes, obvious misake

[SpaceMouse]

Hmmm. Sorry for the rather late response. The rotating detonation rocket looks like a concept worth bringing to ksp, looks like it might be reasonably easy for it to have a air-breathing stage, since many of the concepts already use a fairly conventional jet turbine. Ive read how they're supposed to be substantially more thermodynamicly efficient, but numbers seem lacking. Any idea what ISP gains might be made from such a concept?

Also, on the subject of ISP, the value i have for the pulsejets ive already made are very crude estimates. Any idea what value i should use for a very simple pulsejet?

[cantab]

As far as airbreathing engines go, I'd say a pulsejet offers one advantage - it's dirt cheap. The question is still whether it can provide enough performance to be worth it as a first stage or a booster. I don't know that much about how they perform, in particular how their thrust and efficiency vary with speed.

[SpaceMouse]

Does anyone have some input as to what ISP increase i could get out of a Pulse detonation rocket/ Continuous detonation wave rocket? I so far can't find much more than 'significantly' better.