Antimatter... How To Handle It For Rocket Staging To Orbit?

[Spacescifi]

My Guess?

Given how potent AM is I think you only need a single main engine nozzle but it had better be thick and sturdy (to stuff it with cooling apparatus tech).

AM is volatile, so I think at best all one could do is send pellets containing AM into the reaction chamber... injecting more for more thrust, less for less.

 

Still, to hold several grams worth of AM per pellet SAFELY would indicate a high level of AM safety manufacturing... likely AM neutral materials because otherwise you need uber magnetic fields and you can't have those reliably with small pellets.

 

Suddenly the idea of a pulsed AM rocket stage sounds easier and safer... mainly because the flow of AM is not constant, you are only feeding discrete packages/pods of AM at a time.

 

 

Am I right or wrong?

 

Scott Manley said he thinks AM is more likely than fusion to replace chemical rocket staging to orbit.

 

Fusion creates a lot of deadly neutrons which requires a lot of heavy shielding which makes spacecraft heavier than they should be to get off the ground.

 

AM makes a lot of gamma rays which are easier to shield against with shielding not so heavy as would be required for excess neutrons.

 

Even the ambient air would absorb gamma rays!

[kerbiloid]

A challenge: AM-propelled SPTO rocket.

Spoiler

SPTO = Single Piece To Orbit,
opposite to
MPTO = Many/Multiple Pieces To Orbit,
SCTO = Single Cloud To Orbit,

 

4 hours ago, Spacescifi said:

engine nozzle but it had better be thick

Thicker nozzle - quicker overheat. Because of the heat conductivity and temperature gradient.

V-2 appeared exactly when they replaced thick nozzles with thin ones and pipes.

4 hours ago, Spacescifi said:

AM is volatile

Oh, yeah!!11...

It's a very exact term - "volatile".

Together with surrounding matter.

4 hours ago, Spacescifi said:

AM <...> SAFELY

Oxymoron.

1 g AM (+ 1 g M) = 40 kt

4 hours ago, Spacescifi said:

AM neutral materials because otherwise you need uber magnetic fields

Another AM.
Antihydrogen in antialuminium tanks.

The problem is with anticoncrete touching the ground, and the air blowing at the rocket.

Either the magnetc field, or a cooled super-cold cryomatrix with rare inclusions of AM atoms, slowly decaying and warming the tank.

4 hours ago, Spacescifi said:

Scott Manley said he thinks AM is more likely than fusion to replace chemical rocket staging to orbit.

A nonsense blessed by Scott Manley stops being nonsense.

Let's just hope he is too busy with Kerbal videos to become a NASA engine director.

4 hours ago, Spacescifi said:

Fusion creates a lot of deadly neutrons

Aneutronic fusion doesn't.

And anyway it's easier to have a bulky fusion powerplant on ground to produce hydrolox from water and/or to power the launch vehicle with microwave beam.

4 hours ago, Spacescifi said:

AM makes a lot of gamma rays which are easier to shield against with shielding not so heavy as would be required for excess neutrons.

Until the shielding gets melted by the gamma rays.

4 hours ago, Spacescifi said:

Even the ambient air would absorb gamma rays!

It absorbs neutrons even better.

170 m of exponential distance for nuke blast neutrons, 250...400 m for gammas.

P.S.
(Starts counting posts  till AM Orion proposal).

P.P.S.
(For SSTO.)

Edited June 23, 2022 by kerbiloid

[KSK]

In no particular order:

Your scaling is way off.  Annihilating 1g of antimatter releases 1.8x10^14 J of energy (from E=MC^2).  A 1 kiloton explosion releases approximately 4.2x10^12 J. So annihilating 1g of antimatter in one go is roughly equivalent to a 43 megaton nuclear explosion. I suppose you could propel a spacecraft with Tsar Bombas but it seems unwise.

Edit. My mistake - 43 kiloton explosion. Ignore the Tsar Bomba comment, although 43 kilotons is still a significant explosion to contain in a rocket engine.

Outside of Star Trek, there are no such things as AM neutral materials.

Therefore storing antimatter pellets is a non starter. Any antimatter storage is going to depend on storing charged antiparticles in some kind of magnetic trap.

You’re correct that a lot of the energy from antimatter annihilation is released as gamma rays. Which are essentially useless as rocket exhaust for a ground to orbit vessel (unmatched ISP,  negligible thrust). You might want to think about ways of harnessing gamma rays to heat propellant. Atomic Rockets has various designs if you’re interested.

Why are you fixated on pulsed rockets?

As far as I recall, you have your shielding requirements the wrong way around. Neutron radiation can be shielded against using low atomic number materials. Hydrogen is excellent. Materials containing a lot of hydrogen such as polyethylene are acceptable and more convenient. Water is good (one reason why nuclear waste is stored underwater. These are relatively low density materials so your shielding is relatively light.

Gamma radiation shielding on the other hand tends to rely on dense materials.

Without doing way more research than I care to do, I see no reason at all why neutron shielding should be lighter than gamma ray shielding. It might be, but I would want to see a link to a relevant source rather than a bland assertion.

”because otherwise you need uber magnetic fields and you can't have those reliably with small pellets.”

I’m not sure what you’re getting at here because at face value this is gibberish. Real life counterexample - the Tevatron particle accelerator at Fermilab. It collided protons and antiprotons. (Well technically it collided small bunches of protons and antiprotons at a time to increase the collision probability) Both were kept circulating in a beam pipe where they were steered and accelerated by magnetic fields.

The system was very reliable and manipulated the tiniest  pellets (if you can call an individual antiproton a ‘pellet’) of antimatter with  magnetic fields.

Edited June 23, 2022 by KSK

[kerbiloid]

2 hours ago, KSK said:

Gamma radiation shielding on the other hand tends to rely on dense materials.

With high atomic number (iirc, ~Z⁵), which are usually also dense because their electronic clouds are compact.

[KSK]

58 minutes ago, kerbiloid said:

With high atomic number (iirc, ~Z⁵), which are usually also dense because their electronic clouds are compact.

You put it better than I did - thanks!

My understanding is that the high atomic number is the key part for gamma radiation shielding.

[JoeSchmuckatelli]

9 hours ago, kerbiloid said:

A nonsense blessed by Scott Manley stops being nonsense

/meme 

[magnemoe]

10 hours ago, JoeSchmuckatelli said:

/meme 

This, now antimatter has some safety issues not found in other very dangerous stuff like nuclear bombs or spend fuel rods. 
Like it will blow up if you get an air leak or if containment fails because power out, structural damage or software issues. 
Its interesting as an starship fuel, but think its use as an weapon is questionable because how dangerous it is and you will take damage in combat. 
 

[sevenperforce]

16 hours ago, KSK said:

Why are you fixated on pulsed rockets?

Something something heat something pusher plate something melt?

Antimatter is a PARTICULARLY bad way to build a pulsed rocket, because the only way to pulse antimatter is to have antimatter pellets in little containment vessels that can be “switched off” and go boom. Even the most advanced conceivable antimatter containment vessels mass orders of magnitude more than the mass of antimatter they can contain. As with anything else impacted by the square-cube law, you’re going to do better with a larger vessel than a small one. So you’re just torpedoing your specific energy right out of the gate by using pulse units instead of having a large containment vessel that slowly releases the antimatter into the combustion chamber.

Another thing @Spacescifi seems to consistently ignore: neither fission explosions nor fusion explosions nor antimatter explosions are a source of propulsion. They are all a source of energy. Energy alone does not produce thrust; you need to put that energy into a reaction mass (commonly termed propellant) or you don’t move.

Every rocket has a source of energy and a source of reaction mass. Sometimes they are the same; sometimes they are not.

Even in classic Project Orion, exploding the thermonuclear warhead doesn’t do anything to the pressure plate. It’s just an extremely bright flash of light, nothing more. The only reason you get any effect on the pressure plate is because the warhead has a bunch of tungsten on one side that is vaporized by the flash of light and flung at the pressure plate at ridiculous speeds; the tungsten vapor bouncing off the plate is what provides the impulse. Project Orion is just a very inefficient nuclear thermal rocket which uses tungsten vapor as its propellant and thermonuclear explosions as its energy source. In this sense it is no different from a NERVA which uses liquid hydrogen as its propellant and a nuclear reactor as its energy source, or an ion thruster which uses xenon as its propellant and solar panels as its energy source.

Even the RS-25 SSME engine doesn’t technically use the same energy source and propellant. The RS-25 burns liquid oxygen and liquid hydrogen together, but it injects additional liquid hydrogen as additional propellant along with the steam produced by the hydrolox reaction. By adding extra liquid hydrogen, the molecular weight of the exhaust goes down and so the specific impulse goes up.

All that to say, the fundamental concept of a pulsed antimatter engine is TERRIBLE. Antimatter is the perfect energy source. It requires no ignition. It reacts with literally everything. It can be most easily manipulated and controlled in small quantities. So if you have a way to contain and release antimatter, then fire that stuff into whatever propellant you choose and enjoy arbitrarily high specific impulse.

[Spacescifi]

43 minutes ago, sevenperforce said:

Something something heat something pusher plate something melt?

Antimatter is a PARTICULARLY bad way to build a pulsed rocket, because the only way to pulse antimatter is to have antimatter pellets in little containment vessels that can be “switched off” and go boom. Even the most advanced conceivable antimatter containment vessels mass orders of magnitude more than the mass of antimatter they can contain. As with anything else impacted by the square-cube law, you’re going to do better with a larger vessel than a small one. So you’re just torpedoing your specific energy right out of the gate by using pulse units instead of having a large containment vessel that slowly releases the antimatter into the combustion chamber.

Another thing @Spacescifi seems to consistently ignore: neither fission explosions nor fusion explosions nor antimatter explosions are a source of propulsion. They are all a source of energy. Energy alone does not produce thrust; you need to put that energy into a reaction mass (commonly termed propellant) or you don’t move.

Every rocket has a source of energy and a source of reaction mass. Sometimes they are the same; sometimes they are not.

Even in classic Project Orion, exploding the thermonuclear warhead doesn’t do anything to the pressure plate. It’s just an extremely bright flash of light, nothing more. The only reason you get any effect on the pressure plate is because the warhead has a bunch of tungsten on one side that is vaporized by the flash of light and flung at the pressure plate at ridiculous speeds; the tungsten vapor bouncing off the plate is what provides the impulse. Project Orion is just a very inefficient nuclear thermal rocket which uses tungsten vapor as its propellant and thermonuclear explosions as its energy source. In this sense it is no different from a NERVA which uses liquid hydrogen as its propellant and a nuclear reactor as its energy source, or an ion thruster which uses xenon as its propellant and solar panels as its energy source.

Even the RS-25 SSME engine doesn’t technically use the same energy source and propellant. The RS-25 burns liquid oxygen and liquid hydrogen together, but it injects additional liquid hydrogen as additional propellant along with the steam produced by the hydrolox reaction. By adding extra liquid hydrogen, the molecular weight of the exhaust goes down and so the specific impulse goes up.

All that to say, the fundamental concept of a pulsed antimatter engine is TERRIBLE. Antimatter is the perfect energy source. It requires no ignition. It reacts with literally everything. It can be most easily manipulated and controlled in small quantities. So if you have a way to contain and release antimatter, then fire that stuff into whatever propellant you choose and enjoy arbitrarily high specific impulse.

 

 

Obviously a way to contain AM is needed, but how one transfers it to the reaction chamber without reacting with anything else along the way is the challenge.

 

Like the only way I can think of is a vacuum chamber that rapidly opens to release AM that is shot into the reaction chamber where propellant is going.

 

Which is honestly the main reason I am thinking pulse methods of propulsion.

 

I mean if you could suspend AM particles magnetically in a pipe and funnel how more or less for thrust into a chemical reaction chamber I suppose that could work.. but again you would some type of vacuum otherwise the gases from the reaction chamber would enter the pipe and blow it it and the ship to smithereens.

 

In space detonating fusion pellets work great with magnetic nozzles due to vacuum.

 

But we do not have that luxury on a first stage rocket im atmosphere.

Edited June 24, 2022 by Spacescifi

[kerbiloid]

1. Any AM storage would mass many times greater than the stored AM, while the AM releases just several hundred times more energy than the fusion.

2. As the AM releases the energy in form of gamma photons, they will fly trough the thin layer of the fusion material without significant interaction, and almost all energy will just irradiate the rocket itself.
So, this makes only homeopathic amounts of AM useful, and their input in energy negligible.

3. AM has no passive safety. Once the active protection fails on blackout, everything explodes. It can't just be lying and sleeping, like a nuke.

All of this makes AM absolutely useless as a propellanr itself, only as ignition sparkle.
But as the fusion pulse rocket anyway needs strong magnetic fields to confine and reflect the fusion plasma, it's much easier to light the fusion by magnetic striction or by beams of electrons or Xrays.

So, there is no place to use the AM, except some very special applications.

Edited June 24, 2022 by kerbiloid

[kerbiloid]

The Orion/Casaba projectile is just a former classic two-stage thermonuke where the fusion core is thrown out and only the fission primer is left.

Spoiler

The fusion core (the alarm clock on the picture) is absent, and its peanut end is shaped like a classic de Laval nozzle.

It looks100% clear that the classic picture is incorrect at least twice,

Spoiler

and the other pictures where the tungsten is replaced with plastic "propellant disk" are wrong as well.

***

The classic peanut nuke (see the first picture) is encased in a high-atomic-number shell made of uranium or even tungsten, no matter.
The shell gets heated by the primerX-rays, becomes millions K hot and immediately starts shininng itself, mostly in X-ray range due to its temperature.
So, while it's not actually an X-ray mirror, it emits X-rays on being illuminated by X-rays, so acts like a mirror bottle of a thermos.

This secondary X-ray compresses (by the radiation pressure) the fission core many times greater than a chemical explosive could.

At the same time it heats upp the medium (the filler) inside the shell.
The medium (the filler) is a styrofoam, whose cells are filled with a hydrogen-rich but non gaseous hydrocarbon (like pentane). 
The filler evaporates, gets ionized, and turns into the cloud of hydrogen (and some carbon) plasma, i.e. a proton gas.
The proton gas gets heated up inside the X-ray thermos, and, thanks to its low atomic mass, its pressure is very high.

So the X-ray emission of the superhot shell and the protonic gas together compress the fusion core making it many times denser, and that's how the fusion boom works.

The filer is not beryllium, of course. The beryllium is just a "lense" or a "diaphragm" between the fission and fusion parts of the  peanut, to reflect back the primer neutrons while the case is getting warmed by the X-rays.

So, on the classic scheme of the orion/casaba nuke we must have the green "filler" be not "beryllium (oxide)", but "hydrocarbon-impregnated styrofoam".
We should place a small beryllium (oxide) disk int the throat between the green and the purple.

The tingsten membrane stays where it is.
No "plastic propellant disk" like on some heretic pictures could be here, because this membrane is a part of the X-ray mirror bottle.

The nozzle becomes tungsten because it's the former uranium-or-tungsten peanut case, just reshaped from a sphere into a nozzle.

The nozzle and the membrane form the tungsten X-ray mirror bottle, which gets heated up by the primer X-rays, uniformly emits X-rays itself, turns the green (styrofoam, not "beryllium") filler into proton gass, whose pressure kicks out the membrane, turning the X-ray energy into kinetic energy of the ionized tungsten jet.

A funny fact.
This means that actually the classic nuke Orion is a classic, nozzled, hot hydrogen rocket, using tungsten as kinetic mediator.
The main difference is that the reaction zone is placed outside of the rocket, letting the structure be lightweight.

***

The primer (the purple) is depicted also strangely.
It look spherical, which means that it probably uses a 32 (or more) point initiation system, like Fat Man or Daby Crockett.
This means 32 electric detonators per nuke, and this look weird for the thousand-like amount of the nukes onboard. To complicated, too many failure sources.

The design is American, and at the same time since 1956 the Americans used the 2-point initiation Swan design (and its derivatives) to make the nukes thinner and simpler.

Spoiler

It needs just two detonators instead of 32, and (ta-dam!) it's shaped like a cantaloupe.

Also Casaba is "howitzer", so its caliber was limited.

This probably means that one can be sure that the Orion/Casaba primer (the purple) should be not round but elliptic.

***

A brief version:
green is "styrofoam", not "beryllium";
a beryllium patch in the nozzle throat;
nozzle is made of tungsten, too;
purple primer is elliptic (and with two fuse tubes).

***

This make the Orion generic design absolutely natural, when you are a thermonuke scientist taking a  thermonuke from the shelf, throwing out the fusion core, and using a saw and a hammer to shape the secondary sphere into a de Laval nozzle.

***

Obviously it's designed not for the propulsion, but as a nuclear shrapnel-like direct-shot projectile , that's why Casaba is "howitzer".

Just then tactical howitzer projectile was applied for the propulsion.

Edited June 24, 2022 by kerbiloid

[Spacescifi]

 

 

So perhaps Scott Manley was wrong (impossible lol)?

He's only human.... I guess popularity and being right most of the time kinda makes it easy for people believe anything you say is right... when in reality it is better to be willing to never assume someone is right without verification... but people are often too lazy for that, especially when it is not a life or death matter.

So the way things are shaping up on this thread, I am not sure there will be ever be a replacement for chemical and solid boosters as a first stage.

Pulsed fusion is more doable than sustained fusion reactions, but given the complexity involved, it does not seem to make much sense to bother with a fusion anything type of rocket for a first stage.

 

As far as I can tell, if you want to build a really powerful thrust fusion rocket you had better make it big, since the bigger you make it the more likely the reaction chamber won't explode when you ignite a pulsed fusion reaction. On top that you have radiation concerns.

 

Simply stacking extra chemical rocket boosters that detach and land sounds a lot more sensible for a first stage and safer too.

[Codraroll]

23 hours ago, Spacescifi said:

So perhaps Scott Manley was wrong (impossible lol)?

I think you shouldn't rule out the possibility that you misunderstood what he said. Given your reaction (or lack thereof) to the meticulous explanations by sevenperforce and others on these topics, it seems that there are some concepts you should reconsider your conceptions and conclusions about. 

 

23 hours ago, Spacescifi said:

Simply stacking extra chemical rocket boosters that detach and land sounds a lot more sensible for a first stage and safer too.

Indeed. I have seen others try to say the same for quite a while, but it's good to hear it from you too.

Edited June 26, 2022 by Codraroll

[Spacescifi]

On 6/25/2022 at 2:56 PM, Codraroll said:

I think you shouldn't rule out the possibility that you misunderstood what he said. Given your reaction (or lack thereof) to the meticulous explanations by sevenperforce and others on these topics, it seems that there are some concepts you should reconsider your conceptions and conclusions about. 

 

Indeed. I have seen others try to say the same for quite a while, but it's good to hear it from you too.

 

Reusable chemical first stage boosters COULD be used to lift the mass equivalent of a WWII battleship (like the Bismark) into orbit, but by that point I really think they reach the point of diminishing returns based on TWR.

 

Chemical thrust is limited and once the weight becomes ridiculous I reckon you would see first stage boosters literally using up their boost propellant and using reserves to land mere feet from off the ground.

 

You would probably see a massive disc of reusable rockets attached to one another with the massive battleship load heavy spacecraft in the center of it all.

 

What am saying is there is a breakeven point where using something more dramatic (*cough* external pulse propulsion) is more effective at getting high mass into orbit quickly and probably using less mass resources over all.

 

I reckon to launch the mass of ANY project orion concept using only reusuable boosters would weigh much heavier than the orion project, which result in a very inefficient climb to orbit as rocjets drop off constantly even near the ground lol.

Edited June 26, 2022 by Spacescifi

[Codraroll]

10 minutes ago, Spacescifi said:

Chemical thrust is limited and once the weight becomes ridiculous I reckon you would see first stage boosters literally using up their boost propellant and using reserves to land mere feet from off the ground.

You would probably see a massive disc of reusable rockets attached to one another with the massive battleship load heavy spacecraft in the center of it all.

Which is why you would never launch something of such ridiculous weight in one piece. What mission types could possibly require that?

[Spacescifi]

46 minutes ago, Codraroll said:

Which is why you would never launch something of such ridiculous weight in one piece. What mission types could possibly require that?

 

Why not just send up a bunch of cylinder shaped spacecraft as second stages while omitting the pointy noses?

I am saying sending all of a spacecraft up at once in separate pieces via multiple stage booster launches simultanously while the second stage cylinders would connect in orbit to form a larger spacecraft.

Makes docking with each other easier to for orbital construction?

I understand it will add to drag on the way up, but if all your boosters are reusable then the propellant savings lost should not matter as propellant (methalox) is cheap anyway.

 

Come to think of it, piece by piece orbital construction of an Orion would be ideal.

 

Just send up in one go via multiple separate booster launches with separate pieces, then have all the pieces connect in orbit.

Edited June 26, 2022 by Spacescifi

[Kerbart]

Any exotic energy source that is so volatile and spreads lots of radiation might not be suitable for in-atmosphere use.

 

Larry Niven solved that problem by using ultra-compressed (to nearly solid form) Helium as propellant and save the nasty stuff for the vacuum of space.

[Spacescifi]

47 minutes ago, Kerbart said:

Any exotic energy source that is so volatile and spreads lots of radiation might not be suitable for in-atmosphere use.

 

Larry Niven solved that problem by using ultra-compressed (to nearly solid form) Helium as propellant and save the nasty stuff for the vacuum of space.

 

Can that even be done to helium by man and tech or does it require a gas giant lol?

EDIT: Interesting....

 

https://arstechnica.com/science/2018/12/researchers-find-super-solid-by-looking-at-a-normal-solid/

 

Edited June 27, 2022 by Spacescifi

[kerbiloid]

4 hours ago, Kerbart said:

ultra-compressed (to nearly solid form) Helium

Almost as viable idea as the memetic "liquid vacuum" (stored somewhere).

Spoiler

Caption: "Vacuum"

 

[Terwin]

15 hours ago, Spacescifi said:

What am saying is there is a breakeven point where using something more dramatic (*cough* external pulse propulsion) is more effective at getting high mass into orbit quickly and probably using less mass resources over all.

 

I reckon to launch the mass of ANY project orion concept using only reusuable boosters would weigh much heavier than the orion project, which result in a very inefficient climb to orbit as rocjets drop off constantly even near the ground lol.

For a chemical rocket, the larger the better.

The fuel mas increases based on volume, and tank mass only increases based on area, giving a higher fuel fraction the larger you go.

While available thrust is also limited based on area, you can always go with a wider diameter to mitigate this. 

Side-mounted boosters are also a good way to increase thrust-area, even if they may not have as much of a benefit as increasing the diameter.  (Falcon heavy shows a good example of this)

 

I am pretty sure that the only reason to have a first stage the burns out within  a few hundred feet of the launchpad is if you are limited to pre-existing rocket parts assembled in a lego fashion(like KSP) and your vessel is on a much larger scale than the parts.

 

While w have not yet figured out how to make engines larger than a specific size(thus limiting per-engine output), SpaceX is a good example of using lots of reliable engines in tandem for huge thrust.

There may come a point where engine reliability limits your ability to increase the TWR of the engine section of a given stage, but increasing engine reliability can push that point out to  ridiculous extremes.

 

On 6/25/2022 at 4:13 PM, Spacescifi said:

As far as I can tell, if you want to build a really powerful thrust fusion rocket you had better make it big, since the bigger you make it the more likely the reaction chamber won't explode when you ignite a pulsed fusion reaction. On top that you have radiation concerns.

No, if you want a pulsed fusion powered engine, you want to make every pulse relatively small(as in grams of TNT equivalent per pulse per engine), so that you can have chamber walls of a reasonable thickness and thus a reasonable twr.

[wumpus]

2 hours ago, Terwin said:

No, if you want a pulsed fusion powered engine, you want to make every pulse relatively small(as in grams of TNT equivalent per pulse per engine), so that you can have chamber walls of a reasonable thickness and thus a reasonable twr.

Except that you can get better performance the smaller and quicker the pulses get until you have a steady stream of antimatter.  Pulses are kludges for when you can't get power any other way.  The V-1 "flying bomb" used a pulse-jet for propulsion, because it gave them the ability to make an early jet and also make it cheap enough to be expendable.  I don't think anyone has made a production pulse-jet since.  There have been (I think it rose and fell in the 1990s) some experiments with pulsed detonation rockets.  The idea is to build an air-breathing rocket that detonates the fuel instead of combusting it.  Mostly I think the goal was to get an air breathing rocket however they could, but also to raise Isp by having supersonic exhaust velocity (detonation implies the reaction moves supersonic and produces a shock wave, not sure if the exhaust gases move any faster).

Pulses are a "last choice" to use a fuel.  Not something to find a fuel that will provide a pulse.

[sevenperforce]

1 hour ago, wumpus said:

Pulses are a "last choice" to use a fuel.  Not something to find a fuel that will provide a pulse.

Yep. Pulses are required when you have an energy source you can't control. Pulsed propulsion is not a solution to things like heating, weight, efficiency, thrust, etc.

1 hour ago, wumpus said:

I don't think anyone has made a production pulse-jet since.  There have been (I think it rose and fell in the 1990s) some experiments with pulsed detonation rockets.  The idea is to build an air-breathing rocket that detonates the fuel instead of combusting it.  Mostly I think the goal was to get an air breathing rocket however they could, but also to raise Isp by having supersonic exhaust velocity (detonation implies the reaction moves supersonic and produces a shock wave, not sure if the exhaust gases move any faster).

In a detonation wave, the supersonic combustion process itself serves to compress the fuel, meaning that (a) you don't need a heavy compressor and (b) it can operate at any speed from a standstill up to Mach 5 or beyond. And a notional PDE would have up to 1000 pulses per second, making it effectively continuous from an engineering standpoint.

I've always thought Iron Man's repulsors looked like some sort of pulsed detonation gadget. 

[kerbiloid]

33 minutes ago, sevenperforce said:

Pulsed propulsion is not a solution to things like heating

If it can direct the energy in preferred direction, it allows to keep the reaction zone far outside of he ship, so it is, as it was shown by the thermonuke developers who had developed Orion.