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Project Orion: A discussion of Science and Science Fiction


Spacescifi

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Just now, Spacescifi said:

As far as I know (very much feel to correct me if wrong), external pulse propulsion using pure fusion is far simpler to make compared to a pure fusion continous plume rocket, and pulsed pure fusion rocket would be midway difficulty between the two.

It’s not really a question of “simpler” because energy-positive pure fusion isn’t actually something we can do at all, in pulse form or in continuous form. It’s like saying “personal energy shields are simpler than lightsabers” when we have the technology to make neither.

6 minutes ago, Spacescifi said:

I am aware of the problems associated with trying to sustain a fusion reaction. It's hard. It is far easier to just get pulses of fusion and utilize that.

But we can’t do either. If there was a way to get pure fusion pulses then there would presumably also be a way to get continuous fusion energy. In tokamak fusion designs the fusion is continuous by nature; there is no pulse at all.

7 minutes ago, Spacescifi said:

You propose using small fusion pellets at a fast enough pulsed rate to make the ship fly like a continous plume.

Well no, not really. That was tongue-in-cheek, to illustrate that you don’t need pulses at all.

8 minutes ago, Spacescifi said:

But how do intend to do that.... in atmosphere?

Detonating a fusion reaction inside the throat of a nozzle seems like a bad idea,

If it’s happening in a combustion chamber then it’s not in the atmosphere, is it? That’s the point of a combustion chamber: it’s a place where the heat and pressure are concentrated so they can expand out the nozzle to produce thrust. The engine doesn’t particularly care what happens once the exhaust has left the nozzle.

Sure, you’ll need some sort of propellant to absorb the heat of the fusion reaction and expand into exhaust, but whether there is or isn’t an atmosphere present doesn’t really matter. 

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Just now, sevenperforce said:

It’s not really a question of “simpler” because energy-positive pure fusion isn’t actually something we can do at all, in pulse form or in continuous form. It’s like saying “personal energy shields are simpler than lightsabers” when we have the technology to make neither.

But we can’t do either. If there was a way to get pure fusion pulses then there would presumably also be a way to get continuous fusion energy. In tokamak fusion designs the fusion is continuous by nature; there is no pulse at all.

Well no, not really. That was tongue-in-cheek, to illustrate that you don’t need pulses at all.

If it’s happening in a combustion chamber then it’s not in the atmosphere, is it? That’s the point of a combustion chamber: it’s a place where the heat and pressure are concentrated so they can expand out the nozzle to produce thrust. The engine doesn’t particularly care what happens once the exhaust has left the nozzle.

Sure, you’ll need some sort of propellant to absorb the heat of the fusion reaction and expand into exhaust, but whether there is or isn’t an atmosphere present doesn’t really matter. 

 

We could actually do pure fusion explosives according to the latest blog on Toughscifi by matterbeam.

https://toughsf.blogspot.com/2022/03/fusion-without-fissiles-superbombs-and.html?m=1

It would be challenging to make it as powerful as a nuke, as examples he cited were less powerful, but given time and tech advancement there is no reason why such technology could not reach nuke level energies that I can see.

 

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5 hours ago, Spacescifi said:

We could actually do pure fusion explosives according to the latest blog on Toughscifi by matterbeam.

https://toughsf.blogspot.com/2022/03/fusion-without-fissiles-superbombs-and.html?m=1

It would be challenging to make it as powerful as a nuke, as examples he cited were less powerful, but given time and tech advancement there is no reason why such technology could not reach nuke level energies that I can see.

Making it “as powerful as a nuke” really isn’t the goal. As the article points out, if you need higher thrust, you simply fire pulse units faster.

Basically everything proposed in that article is Z-pinch or a variation thereon. The one exception is a continuous fusion accelerator combustor powered by a nuclear reactor.

He cites proposals for HE-initiated pulsed fusion, but none of them are small enough to avoid using a pusher plate approach. And scaling them up increases their efficiency, so bigger is better. But it exceeds the weight-per-pulse of the smallest possible fission-fusion Orion long before it approaches the specific energy of a fission-fusion Orion. So it’s basically just a wimpy Orion. The only reason to use pulsed pure fusion propulsion is if you can’t get your hands on enough fissile material to build a proper Orion. 

Z-pinch can be scaled down very small, small enough to produce pulses which (combined with a magnetic nozzle) can exceed the specific impulse of even ion thrusters. But the specific energies involved are still VASTLY lower than the specific energies you can get from fission-fusion Orion. And even if you multiplied the energy efficiency by 100x with advanced tech you’d still be better off with an ordinary Orion. 

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On 6/18/2022 at 7:59 AM, Terwin said:

When using an actively cooled chamber/nozzle, I do not think that the heat is the greatest problem

A 100 t ship, TWR = 1, ISP*g = 100 km/s.

Jet power = 100 000 * 10 * 100 000 = 1011 W.

Let the waste energy (in form of X-rays) be 50% of total.

1011 W of waste power.

Let the chamber be a sphere 10 m in radius.

Equilibrium temperature ~= (1011 / (5.67*10-8 * 4 * pi * 102) )0.25 ~= 6 000 K

***

A 10 000 t ship, TWR = 1, ISP*g = 100 km/s.

Jet power = 107 * 10 * 100 000 = 1013 W.

Let the waste energy (in form of X-rays) be 50% of total.

1013 W of waste power.

Let the chamber be a sphere 50 m in radius (proportionally).

Equilibrium temperature ~= (1013 / (5.67*10-8 * 4 * pi * 502) )0.25 ~= 8 600 K

***

So, the temperature of the chamber would be several times greater than  a solid material can survive without active cooling.

And the terawatts active cooling of tens-meters nozzle would mass so much that the ship would consist of it totally.
But even worse, it should find a way to quickly emit terawatts into space and, and additionally - not heating its own hull.

On ground you can use massive external coolers and the infinite heat sink of the ocean. But not in space.

So, this limits the encased reaction designs with low waste power, so low thrust.

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13 hours ago, sevenperforce said:

Making it “as powerful as a nuke” really isn’t the goal. As the article points out, if you need higher thrust, you simply fire pulse units faster.

Basically everything proposed in that article is Z-pinch or a variation thereon. The one exception is a continuous fusion accelerator combustor powered by a nuclear reactor.

He cites proposals for HE-initiated pulsed fusion, but none of them are small enough to avoid using a pusher plate approach. And scaling them up increases their efficiency, so bigger is better. But it exceeds the weight-per-pulse of the smallest possible fission-fusion Orion long before it approaches the specific energy of a fission-fusion Orion. So it’s basically just a wimpy Orion. The only reason to use pulsed pure fusion propulsion is if you can’t get your hands on enough fissile material to build a proper Orion. 

Z-pinch can be scaled down very small, small enough to produce pulses which (combined with a magnetic nozzle) can exceed the specific impulse of even ion thrusters. But the specific energies involved are still VASTLY lower than the specific energies you can get from fission-fusion Orion. And even if you multiplied the energy efficiency by 100x with advanced tech you’d still be better off with an ordinary Orion. 

 

11 hours ago, kerbiloid said:

A 100 t ship, TWR = 1, ISP*g = 100 km/s.

Jet power = 100 000 * 10 * 100 000 = 1011 W.

Let the waste energy (in form of X-rays) be 50% of total.

1011 W of waste power.

Let the chamber be a sphere 10 m in radius.

Equilibrium temperature ~= (1011 / (5.67*10-8 * 4 * pi * 102) )0.25 ~= 6 000 K

***

A 10 000 t ship, TWR = 1, ISP*g = 100 km/s.

Jet power = 107 * 10 * 100 000 = 1013 W.

Let the waste energy (in form of X-rays) be 50% of total.

1013 W of waste power.

Let the chamber be a sphere 50 m in radius (proportionally).

Equilibrium temperature ~= (1013 / (5.67*10-8 * 4 * pi * 502) )0.25 ~= 8 600 K

***

So, the temperature of the chamber would be several times greater than  a solid material can survive without active cooling.

And the terawatts active cooling of tens-meters nozzle would mass so much that the ship would consist of it totally.
But even worse, it should find a way to quickly emit terawatts into space and, and additionally - not heating its own hull.

On ground you can use massive external coolers and the infinite heat sink of the ocean. But not in space.

So, this limits the encased reaction designs with low waste power, so low thrust.

 

 

Hahaha!

So you are telling me that ultimately I was right all along!

Project Orion is the most efficient way we can devise to launch heavy spacecraft to orbit.

 

A pure fusion pulsed rocket would be better pf course because you don't have a huge explosion riding behind you every few seconds, but even with that and increasing the amount of fusion pellets that ignited it, you still won't have the thrust an Orion will have.

 

If you are using more pellets to do what Orion can do with less, then you are losing on the fuel efficiency end and may as well just 2-stage it.

 

There is nothing wrong with 2 staging, but I honestly think the the main problem with project orion as an SSTO is not launch but landing the thing.

As you mentioned it would take several first stage large disposable boosters to even get it in the air.

To land such a huge craft would require it to be huge and mostly chemical propellant tank and rocket nozzles anyway with s pusher plate and bomb assembly tacked on.

 

 

So strangely enough now my thinking is... different.

 

Launch small  or lighter weight spacecraft to orbit using advanced more efficient pure fusion bombs with pusher plates.

 

Since the spacecraft weigh less, the fact that the thrust may be less than a nuke may not matter so much. Since with less weight thrust increases anyway.

 

And if worried for crew safety liquid breathing is possible, plus acceleration in pulses is very much survivable.

Edited by Spacescifi
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38 minutes ago, Spacescifi said:

Hahaha!

So you are telling me that ultimately I was right all along!

Project Orion is the most efficient way we can devise to launch heavy spacecraft to orbit.

???

?????????????????????

In which of my words you could even see that???

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30 minutes ago, kerbiloid said:

???

?????????????????????

In which of my words you could even see that???

I'm still trying to figure out what kind of definition of "efficiency" spacescifi is using.

Not energy.  Probably not even mass (the ship itself needs to be massive, unless you are stuffing the pusher plate with cargo as well.  But building an expendable Orion is certainly not efficient).
Cost is unlikely, because the Orion only goes to LEO *once*.  While it might be able to bring unbelievable massive cargoes along, that will come at a huge cost, and will  only be used for that one flight up (you can go as far as you want in space, but you aren't going to land that thing).  I'd be shocked if a few hundred Starship flights wouldn't be cheaper.  Even Falcon can probably beat it, even if it takes the whole fleet launching 1000 flights during the time it takes to construct an Orion.

The only thing that is perfectly clear is that in the 20th century it was the only way to lift an ultra-giga-heavy-spacecraft to orbit (Saturn V was a "heavy lift rocket").  This probably remains the case as I'm not aware of any replacement nuclear rockets being capable  of going from the Earth's surface to LEO (most are great once you get them to LEO (or further depending on exhaust evilness).

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4 hours ago, Spacescifi said:

Hahaha!

So you are telling me that ultimately I was right all along!

Project Orion is the most efficient way we can devise to launch heavy spacecraft to orbit.

Compared to a pure-fusion Orion as advocated by that article, a fission Orion is likely a better choice, as it is much simpler.  On the other hand, the article was advocating for a deep-space fusion Orion because it could refuel from ice or any other hydrogen source.  Should the technical hurdles be over-come, there is the argument to be made that a pure-fusion Orion would be superior to a fission Orion with regards to ISRU.

On the other hand, compared to a more realistic use of fusion for propulsion(mini-mag, fusion torch, micro-pulses, etc.) any form of Orion is the lesser choice. 

 

4 hours ago, Spacescifi said:

If you are using more pellets to do what Orion can do with less, then you are losing on the fuel efficiency end and may as well just 2-stage it.

A 50g fusion pellet will not have the same out-put as a 100lb fission bomb.  Then again, a 100lb fission bomb weighs more than 900 50g pellets, so even if the pellet has two orders of magnitude less impulse, it may still have a higher isp.

 

On 6/18/2022 at 3:54 PM, sevenperforce said:

But we can’t do either. If there was a way to get pure fusion pulses then there would presumably also be a way to get continuous fusion energy. In tokamak fusion designs the fusion is continuous by nature; there is no pulse at all.

To the best of my understanding, we have not yet successfully demonstrated a sustained reaction in a tokamak style reactor, but we have demonstrated fusion pulses(NIF).

While it would be an awful design, we could theoretically put a NIF style ignition, powered by an on-board fission reactor, to super-heat the reaction mass for a deep space(low-thrust, high ISP) vessel.

This would not require anything we cannot do today, so I would argue that a fusion micro-pulse propelled spacecraft is possible with today's technology, but a continuous fusion torch is not yet in our grasp.  

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21 hours ago, wumpus said:

I'm still trying to figure out what kind of definition of "efficiency" spacescifi is using.

If I've understood it correctly, it's something like this: He wants a patently unrealistic idea to be a realistic idea.

It has repeatedly been explained to him in thorough detail why the idea is unrealistic (I would say "good/bad idea", but everybody keeps stressing that the needs of the plot outweigh everything else, so an unrealistic idea can be a good idea if it makes a good story). But he still insists on applying realism, so we're discussing the real-world merits of the idea rather than its function as a story setpiece. Well, "discussing". It's mostly a many-to-one explanation of where all the hitches are.

At times, some of the explanations of why the idea is unrealistic appear to sink in. We've got plenty of people who are good at explaining. Then somebody says something that can be interpreted as an argument for the patently unrealistic idea, or he gets some sort of inspiration on how to slightly alter a minute detail, and spacescifi immediately jumps back to the initial position of "right, then it's a realistic idea after all!", instantly disregarding everything that was carefully explained to him previously. And then we're back to explaining the drawbacks of antimatter Orion SSTOs from scratch yet another time. 

Try as I might, I do not think I can make a more charitable summary than that. I've seen it play out literally dozens of times by now. It keeps looping back by way of "OK, but if X, then everything you've said doesn't apply after all, right?" There seems to be some fixation on certain ideas and an unwillingness to let them go (or at least, acknowledge their drawbacks) and a continuous re-set to square one.  Questions are asked (oh, how they are asked!) but the answers so rarely taken to heed unless they align with the initial notion.

As such, it is not really about "efficiency". It's just about the merits of a concept whose merits have repeatedly (Repeatedly! Repeatedly!) been discredited. For the hundredth-and-whatnot time, Orion can work however one wants it to in a sci-fi story. But in real life, there are too many drawbacks and literally any attempt to improve it would make it obsolete instead. And yet, it keeps being insisted that we review the idea that the square peg might fit in the round hole, one more time. That is, at least, how I understand what is going on.

Edited by Codraroll
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Getting stuff back down again (a fully loaded Orion) is not happening.

It would take multiple shuttle orange giant boosters to land a huge orion or similar like sevenperforce said.

So at best one could load an orion Spacex with starships as shuttlecraft.

 

I suppose we could gradually disassemble an Orion in orbit to bring it back piece by piece via spacex launch but I digress.

 

Orion is basically a semi-disposable massive cargo freighter to orbit.

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1 hour ago, Spacescifi said:

shuttle orange giant boosters

The shuttle oranges are not boosters.

(And even if they were, it's an oddd idea to use hydrolox for landing).

1 hour ago, Spacescifi said:

we could gradually disassemble an Orion in orbit to bring it back piece by piece via spacex launch

Wut 4???

1 hour ago, Spacescifi said:

Orion is basically a semi-disposable massive cargo freighter to orbit.

Not disposable (even semi), not freighter, not to orbit.
Other parts of the phrase are true.

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  • 2 weeks later...

I was thinking about classic belly lander space plane SSTOs and realized solid fuel similar to the sprint missile would be ideal.

 

Space is at a premium on a manned SSTO so having solid fuel which takes up the least amount of payload space is ideal.

 

Yes I know you cannot throttle solid fuel rocket engines and they will burn all their fuel. That is the plan.

 

Reaching orbit: Take off a runway at high g acceleration (not 100g but definitely 10g) crew can be do liquid breathing to handke g-forces. Burn through the atmosphere until air plasma surrounds leading edge of the hull. Activate magnetohydrodynamic fields to deflect and use the air plasma as a source of thrust to cruise out of the atmosphere and into space.

 

In space: Any SSTO would have planet use solid fuel sprint-like engines and space only engines that use chemical propellants.

 

Conclusion: Who knows if it would work, but in scifi defense screens and shield fields can often deflect uber plasma and particle beams moving at a significant fraction of lightspeed.

 

If they can deflect that... atmospheric frictional plasma can also be.

 

 

Edited by Spacescifi
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8 hours ago, Spacescifi said:

realized solid fuel similar to the sprint missile would be ideal.

What made you think that? Why Sprint specifically?

8 hours ago, Spacescifi said:

Space is at a premium on a manned SSTO

It is? I always thought it was mass.

8 hours ago, Spacescifi said:

so having solid fuel which takes up the least amount of payload space is ideal.

I would think high ISP would be better.

8 hours ago, Spacescifi said:

Reaching orbit: Take off a runway at high g acceleration (not 100g but definitely 10g)

That sounds unpleasant.

8 hours ago, Spacescifi said:

crew can be do liquid breathing to handke g-forces.

Even more unpleasant.

8 hours ago, Spacescifi said:

In space: Any SSTO would have planet use solid fuel sprint-like engines and space only engines that use chemical propellants.

Sprint uses chemical propellant too.

8 hours ago, Spacescifi said:

Conclusion: Who knows if it would work,

I could name a few individuals...

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There are many problems with this idea. Picking a couple of the most obvious ones:

At 10g constant acceleration, you’re above the Karman line in 45 seconds and travelling at 4.5 km/s (so a considerable way off orbital speed) Actual acceleration will increase as you burn propellant and your vehicle mass drops, meaning that you get above the Karman line even faster. 

Either way, your MHD fields have an extremely small time window to work in before you’re above enough of the atmosphere that you no longer have much plasma to work with.

Solid rockets are a horrible choice for a reusable vehicle - which I’m assuming is the point of an SSTO. They’re heavy, inefficient, and a complete pain to refuel.

Solid rocket motors burn from the inside out, all the way along the rocket. Therefore their entire casing is effectively one big combustion chamber and has to be built accordingly.  The Shuttle SRBs, for example, were built from 2cm thick steel and weighed 91 tons apiece.

Incidentally, you should read up on the Shuttle SRB igniters at some point. They were impressive.

The two advantages of  solid rockets are storability and metric shed-loads of thrust. This made them an excellent choice for a last ditch ABM.  For a crewed spacecraft, they are a poor choice.

 

Edited by KSK
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A Sprint-like booster together with ion thruster.

Sprint starts and reaches the Karman line at 4.5 km/s vertical speed, then enables the ion thruster horizontally to circularize the orbit before the re-entry after the overmoon jump.

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10 hours ago, Shpaget said:

It is? I always thought it was mass.

Shuttle SRBs (also 1970s design) had an Isp of 275s.  But if launched from air, you can probably start with vacuum numbers.  Vega stage 3 (presumably close to vacuum) gets 295s.  Figure your crew, passengers, cargo, and non-fuel bits of the spacecraft and plug it into the rocket equation to see how big a monster it will be.  Don't forget that the entire length of the rocket surrounding the solid rocket fuel will need to withstand the pressure of the combustion chamber and be fairly massive even if carbon fiber.

6 hours ago, KSK said:

Solid rockets are a horrible choice for a reusable vehicle - which I’m assuming is the point of an SSTO. They’re heavy, inefficient, and a complete pain to refuel.

Refuel?  If you somehow manage to get a single stage to orbit with a Isp of 300ish it isn't going to be reused.  There won't be any cargo space to speak of, let alone heatshields and landing gear.

Anyone bringing up the SSTO fantasy (with current rocket engines, or worse airospikes) merely shows that they can't handle simple equations like Δv=Isp*g*ln(mo/mf).  Equation one of rocket science.

Δv=9400m/s

g=9.8m/s2

Isp=300s (generous)

mo=total mass of rocket (fueled)

mf=final mass of rocket (crew, passengers, cargo, and non-fuel bits of the spacecraft)

Plug away and try to come up with some viable rocket that even gets to orbit and I'll be impressed.  Don't even think of safely landing it.

 

 

 

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6 hours ago, kerbiloid said:

A Sprint-like booster together with ion thruster.

Sprint starts and reaches the Karman line at 4.5 km/s vertical speed, then enables the ion thruster horizontally to circularize the orbit before the re-entry after the overmoon jump.

Have to be a much more powerful than typical ion engine.  SMART-1 took 4 months to get to the Moon (~4000m/s delta-v?  Don't know where to find requirements for circular orbits).  You'll need more than twice that  in 4-5 days.  But at least it isn't an SSTO.

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8 hours ago, wumpus said:

Shuttle SRBs (also 1970s design) had an Isp of 275s.  But if launched from air, you can probably start with vacuum numbers.  Vega stage 3 (presumably close to vacuum) gets 295s.  Figure your crew, passengers, cargo, and non-fuel bits of the spacecraft and plug it into the rocket equation to see how big a monster it will be.  Don't forget that the entire length of the rocket surrounding the solid rocket fuel will need to withstand the pressure of the combustion chamber and be fairly massive even if carbon fiber.

Refuel?  If you somehow manage to get a single stage to orbit with a Isp of 300ish it isn't going to be reused.  There won't be any cargo space to speak of, let alone heatshields and landing gear.

Anyone bringing up the SSTO fantasy (with current rocket engines, or worse airospikes) merely shows that they can't handle simple equations like Δv=Isp*g*ln(mo/mf).  Equation one of rocket science.

Δv=9400m/s

g=9.8m/s2

Isp=300s (generous)

mo=total mass of rocket (fueled)

mf=final mass of rocket (crew, passengers, cargo, and non-fuel bits of the spacecraft)

Plug away and try to come up with some viable rocket that even gets to orbit and I'll be impressed.  Don't even think of safely landing it.

 

 

 

 

Ok... I believe you.

 

What if you tried a hybrid rocket SSTO utilizing the same technology of sprint mixed with hybrid rocket technology?

 

https://en.m.wikipedia.org/wiki/Hybrid-propellant_rocket

 

You may add airbreathing rocketry or even pulse detonation rocketry if it helps it SSTO any better... anything but project orion since that has been well discussed.

 

I am throwing known tech against the metaphorical wall to see what may stick.

Edited by Spacescifi
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9 minutes ago, Spacescifi said:

I am throwing known tech against the metaphorical wall to see what my stick.

I think that if there were known technologies that could make a useful SSTO, we would have one by now.

Generally speaking you will not find a plausible engine that can give you the dv to get to orbit with a useful cargo fraction in one stage and still have enough thrust to get off the ground. 

Staging is a way to 'cheat' the rocket equation posted above, by discarding dry-mass that is no longer needed, letting you have multiple stages that each have a high fuel fraction.

Each stage you add, just adds another chunk of dv to the existing rocket when you add it to the bottom.

The trade-off is that each additional stage counts all later stages as part of it's dry-mass, but a SSTO would need to be much larger than any multi-stage rocket for the same task, so even with huge early stages, staging still makes an over-all smaller rocket.

 

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4 hours ago, Terwin said:

I think that if there were known technologies that could make a useful SSTO, we would have one by now.

Generally speaking you will not find a plausible engine that can give you the dv to get to orbit with a useful cargo fraction in one stage and still have enough thrust to get off the ground. 

Staging is a way to 'cheat' the rocket equation posted above, by discarding dry-mass that is no longer needed, letting you have multiple stages that each have a high fuel fraction.

Each stage you add, just adds another chunk of dv to the existing rocket when you add it to the bottom.

The trade-off is that each additional stage counts all later stages as part of it's dry-mass, but a SSTO would need to be much larger than any multi-stage rocket for the same task, so even with huge early stages, staging still makes an over-all smaller rocket.

 

 

Well at least I learned hybrids are thrustier than chemical even though the delta V is less. They can also he throttled or cut off unlike solid fuel boosters.

 

So it seems like the only SSTO that could be made right now with a decent cargo load (40 tons) would be an SSTO with hybrid rockets for lift off and orion nuclear pulse propulsion.

 

It is unlikely it could land itself safely, but it could at least get up to orbit as an SSTO.

 

It would have to be lighter weight than traditional orion and crew would likely need liquid breathing for the acceleration but it could in theory be done from a thrust POV.

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For my money, Skylon is probably your best bet. Horizontal takeoff, horizontal landing spaceplane with an airbreathing rocket engine that switches over to an internal LOX supply once it's out of atmosphere. On paper it works. In practice, a lot of things have to go right, and the vehicle mass needs to stay within budget if it's to take any significant payload to orbit.

It's capable of getting  to low-Earth orbit only (although I suppose on-orbit refueling would let it go elsewhere) and only has a modest cargo capacity. Plus it has to glide home, Shuttle-style. But it is an SSTO.

More generally, if you look at the equation that @wumpus posted:   Δv=Isp*g*ln(Mo/Mf), there are only a few ways of getting more delta-V out of a rocket.

1)   You can increase the specific impulse of the engines (ISP). That tops out at around 400 for chemical rockets and to get that high they need to be burning hydrogen. Hydrogen isn't the easiest propellant to handle and it's low density, which means a hydrogen fueled rocket needs a large tank volume. For an ISP above 400 (roughly) , you're looking at some form of nuclear propulsion (or, I suppose, antimatter propulsion), which still has a respectable thrust to weight ratio. 

2)  You can reduce the dry mass of your vehicle. That lowers Mf in the above equation, meaning that the ln (Mo/Mf) term is higher. Intuitively, that makes sense - a lighter vehicle will go faster for the same amount of propellant expended.

3)  You can add more propellant. That increases Mo in the above equation, meaning that Mo/Mf term is higher again. Unfortunately, more propellant means more, or bigger, engines to push a heavier vehicle, and generally means a higher vehicle dry mass in the form of extra fuel tanks and everything else required to attach those tanks to the vehicle.

The conventional way to get more delta-V out of a rocket is a bit of 1 (use the best engines you can), a bit of 2 (make the unfueled rocket as light as possible) and a bit of 3 (use staging to get rid of excess dry mass as soon as you can).

Airbreathing rockets make use of an external store of propellant (atmospheric oxygen) that they don't need to carry.  That bumps up Mo quite considerably in the above equation whilst (ideally) having no effect on Mf.  The trick is designing an airbreathing system that isn't so heavy (increased Mf) that it effectively cancels out the gains made from that increased Mo.

Edit:  It sounds obvious but Skylon also makes use of aerodynamic lift (it being a winged vehicle and all). I *think* (any actual aerospace engineers feel free to chip in here) that its engines can get away with a lower thrust to weight ratio since they're not brute-forcing the entire mass of the vehicle against gravity, in the way that a vertical take-off rocket needs to. That's helpful since hydrogen burning engines don't tend to have great thrust to weight - see the Shuttle for example, which needed solid rocket boosting to get off the ground.

 

Edited by KSK
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1 hour ago, Spacescifi said:

SSTO with hybrid rockets for lift off and orion nuclear pulse propulsion

What is wrong with the way we explain things?

Time and time again, various members have made it clear that SSTOs are pretty hopeless endeavours and cannot be combined with Orion in any way, shape or form. Yet barely two days pass, and then your conclusion reverts to "I guess an Orion SSTO is the best answer" again, as if nothing had been said to the opposite.

Why doesn't it sink in? Is anything unclear about the explanations?

To reiterate:

1) An SSTO needs to be as light as possible for the rocket equation to allow the endeavour.

2) An Orion drive is a monstrously heavy thing. It does not play nice with the rocket equation.

3) The Orion drive has various insane drawbacks that make it a stupidly bad idea overall, and any attempts to improve it will make it obsolete instead.

Is it 1), 2), or 3) you have a problem with?

Edited by Codraroll
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9 hours ago, Spacescifi said:

 

Well at least I learned hybrids are thrustier than chemical even though the delta V is less. They can also he throttled or cut off unlike solid fuel boosters.

 

So it seems like the only SSTO that could be made right now with a decent cargo load (40 tons) would be an SSTO with hybrid rockets for lift off and orion nuclear pulse propulsion.

 

It is unlikely it could land itself safely, but it could at least get up to orbit as an SSTO.

 

It would have to be lighter weight than traditional orion and crew would likely need liquid breathing for the acceleration but it could in theory be done from a thrust POV.

Look up the Isp (pretty sure its less), and plug those numbers into the rocket equation and see how far you get.  Make sure include all the mass for your nuclear pulse propulsion.

 

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22 hours ago, wumpus said:

Have to be a much more powerful than typical ion engine.  SMART-1 took 4 months to get to the Moon (~4000m/s delta-v?  Don't know where to find requirements for circular orbits).  You'll need more than twice that  in 4-5 days.  But at least it isn't an SSTO.

An overmoon jump gravity maneuver to change the veloicity vector.

Requires a proper launch date to line up the bodies.

10 hours ago, Spacescifi said:

orion nuclear pulse propulsion.

Uranium is flammable, it can be used in a hybrid rocket together with air as oxidizer. See the aluminium rockets.

Beryllium, too.

And the orionuke has a nozzle.

So, Orion can use nukes as SRB.

Edited by kerbiloid
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