Prometheus Turbo Rocket concept

[KerikBalm]

Those numbers seem too optimistic. The rocketfan Isp in his images is double what the Sabre is supposed to get, and remains really high for a long time.

That said, I really like the idea of making use of air augmented engines, even in the simpler Gnom sense.

 

@Dragon01 and @Spacescifi

As to the Fusion, not all designs require long magnetic confinement times (ICF fusion or dense plasma focus fusion, for example).

Also, quartz glass is not very heat resistant at all. The idea of the nucelar lightbulb is that quartz is transparent to UV, which can heat doped Hydrogen propellant (doping to increase UV absorption) to head the propellant to higher temperatures than the quartz glass. The problem is that the Uranium gas inside the quartz glass needs to be really really really hot to be emitting enough UV for this to work. If that gas touches the walls of the quartz container, it will melt right through the quartz. So the proposed solution is to have a continuous injection of an inert gas (neon, I think they proposed) and creating a vortex state inside the lightbulb. They also need to suck out gas continuously and pass it through radiators for cooling. They also need at way to separate the uranium gas from the neon gas (in effect, it would also be reprocessing the fuel as it goes), and re-injecting both (note the uranium as it leaves the center of the lightbulb, goes subcritical and stops reacting and releasing significant energy). So they need to have a gas vortex hot at the center, cool enough at the edges that it doesn't melt the quartz, so that enough UV radiation is released, that doped hydrogen heats up to hotter than the quartz is - note the hydrogen flowing past the quartz can't be very hot or it would melt the quartz too, so you can add the doping agent farther away, or vary the flow rate so that the hydrogen gets hottest when its not in direct contact with the quartz (since its heated from the UV emanating from the quartz bulb, not contact with the quartz)

[Spacescifi]

2 hours ago, KerikBalm said:

Those numbers seem too optimistic. The rocketfan Isp in his images is double what the Sabre is supposed to get, and remains really high for a long time.

That said, I really like the idea of making use of air augmented engines, even in the simpler Gnom sense.

 

@Dragon01 and @Spacescifi

As to the Fusion, not all designs require long magnetic confinement times (ICF fusion or dense plasma focus fusion, for example).

Also, quartz glass is not very heat resistant at all. The idea of the nucelar lightbulb is that quartz is transparent to UV, which can heat doped Hydrogen propellant (doping to increase UV absorption) to head the propellant to higher temperatures than the quartz glass. The problem is that the Uranium gas inside the quartz glass needs to be really really really hot to be emitting enough UV for this to work. If that gas touches the walls of the quartz container, it will melt right through the quartz. So the proposed solution is to have a continuous injection of an inert gas (neon, I think they proposed) and creating a vortex state inside the lightbulb. They also need to suck out gas continuously and pass it through radiators for cooling. They also need at way to separate the uranium gas from the neon gas (in effect, it would also be reprocessing the fuel as it goes), and re-injecting both (note the uranium as it leaves the center of the lightbulb, goes subcritical and stops reacting and releasing significant energy). So they need to have a gas vortex hot at the center, cool enough at the edges that it doesn't melt the quartz, so that enough UV radiation is released, that doped hydrogen heats up to hotter than the quartz is - note the hydrogen flowing past the quartz can't be very hot or it would melt the quartz too, so you can add the doping agent farther away, or vary the flow rate so that the hydrogen gets hottest when its not in direct contact with the quartz (since its heated from the UV emanating from the quartz bulb, not contact with the quartz)

 

What is 'long'?

Engineers conisider it a feat to sustain fusion for a few seconds even.

And the magnetic field problem is just there,  causing issuez for dense plasma focus I have read about.

 

From: https://en.m.wikipedia.org/wiki/Inertial_confinement_fusion

Inertial confinement fusion relies on lasers to heat a pellet for fusion to occur. Currently success is eluding them unless the news has forgetten to mention it.

Throughout the 1980s and '90s, many experiments were conducted in order to understand the complex interaction of high-intensity laser light and plasma. These led to the design of newer machines, much larger, that would finally reach ignition energies.

The largest operational ICF experiment is the National Ignition Facility (NIF) in the US, designed using the decades-long experience of earlier experiments. Like those earlier experiments, however, NIF has failed to reach ignition and is, as of 2015, generating about ¹⁄₃ of the required energy levels.^[1]

 

 

 

 

 

[Guest]

Actually, quartz glass is very heat resistant. Not quite the record-holder, but it's up there. It kind of has to be, given the environment it works in. Uranium gas in the lightbulb will melt anything, including amorphous carbon and the like. Quartz is just good enough and transparent enough (and not only in UV spectrum, it's prime lens material for anything short of far IR), that it won't evaporate from radiative heating alone. Then it just boils down to keeping convective heating at bay and cooling the thing to prevent the buildup.

15 hours ago, Spacescifi said:

Only with scifi heat resistant materials could a gas core ever lift something like this with 40 crew and cargo to orbit:

Not true. Making your engine hotter increases your specific impulse, not thrust. For more thrust, you need to make your engine bigger. An Enterprise-sized payload can be easily lifted with GCNRs, in fact the Liberty Ship was conceptualized to lift 1000T to LEO, and still be able to fly back and land. It used an array of nuclear lightbulbs. 

15 hours ago, Spacescifi said:

Sure magnetic fields don't melt, but they also leak plasma. Which prevents a sustained fusion reaction from taking place.

Not true, if you design them right. Tokamaks are capable of steady state operation, and several research programs are underway to do just that. 100 seconds have already been achieved, and there's a reactor in India which is supposed to have a 1000s discharge time (dunno if they did that yet). ITER is the most ambitious project (aimed at achieving both steady state operation and breakeven), but far from the only one.

[Spacescifi]

5 minutes ago, Dragon01 said:

Actually, quartz glass is very heat resistant. Not quite the record-holder, but it's up there. It kind of has to be, given the environment it works in. Uranium gas in the lightbulb will melt anything, including amorphous carbon and the like. Quartz is just good enough and transparent enough (and not only in UV spectrum, it's prime lens material for anything short of far IR), that it won't evaporate from radiative heating alone. Then it just boils down to keeping convective heating at bay and cooling the thing to prevent the buildup.

Not true. Making your engine hotter increases your specific impulse, not thrust. For more thrust, you need to make your engine bigger. An Enterprise-sized payload can be easily lifted with GCNRs, in fact the Liberty Ship was conceptualized to lift 1000T to LEO, and still be able to fly back and land. It used an array of nuclear lightbulbs. 

Not true, if you design them right. Tokamaks are capable of steady state operation, and several research programs are underway to do just that. 100 seconds have already been achieved, and there's a reactor in India which is supposed to have a 1000s discharge time (dunno if they did that yet). ITER is the most ambitious project (aimed at achieving both steady state operation and breakeven), but far from the only one.

 

Kay I will concede that I was likely wring about the thrust thing. As far as I know you just need to increase the mass flow rate, likely via air intake along with propellant... which might drive up the heat load further on the quartz. Since oncoming air at high speed will be hot, not cold.

Do not get me wrong. I want it to work, but the design challenges are formidable.

As for fusion, i have my doubts, since how do you do it on a spacrship when it takes an entire massive facility to achieve it on Earth? All that TWR ya know?

Unless our materials science. Advances and we can build future truck engine size power plants as as good the facilty size ones we use for fusion nowadays.

 

[Guest]

7 minutes ago, Spacescifi said:

Kay I will concede that I was likely wring about the thrust thing. As far as I know you just need to increase the mass flow rate, likely via air intake along with propellant... which might drive up the heat load further on the quartz. Since oncoming air at high speed will be hot, not cold.

It will drive it down. Air at high speed is going to be hot, but there's hot, and there's inside of an GCNR.  Even if you have hot air coming in, it will be cooler than the reactor. As a matter of fact, increasing the mass flow rate, by itself, will never result in more heat. It will reduce it, and thus Isp. Remember what I already told you about thrust power some time ago. Thermal constraints in this kind of rockets limit Isp, not thrust.

Fusion is hard, but it's not impossible. We just need to know more about how exactly it behaves. Nuclear reactors were, at one point, limited to building-sized facilities, as well. We just need to learn to walk before we can run. 

[Spacescifi]

13 minutes ago, Dragon01 said:

It will drive it down. Air at high speed is going to be hot, but there's hot, and there's inside of an GCNR.  Even if you have hot air coming in, it will be cooler than the reactor. As a matter of fact, increasing the mass flow rate, by itself, will never result in more heat. It will reduce it, and thus Isp. Remember what I already told you about thrust power some time ago. Thermal constraints in this kind of rockets limit Isp, not thrust.

Fusion is hard, but it's not impossible. We just need to know more about how exactly it behaves. Nuclear reactors were, at one point, limited to building-sized facilities, as well. We just need to learn to walk before we can run. 

 

We know how fusion works. What we have trouble with is magnetic field containment and not breaking even for the amount of energy expended vs what we are producing via fusion.

 

You have faith that we will develop a process to do what star's do... without the star's worth of mass, relying on wispy plasma and uber magnetic field control instead.

I hope you're right, but material limits and physics will have it's say in the end either way.

If it allows for it great.

I have found that in the universe the answer to whether or not something is possible is often more like, "It depends on meeting certain criteria." Rather than a hard "No, you cannot do that!".

Including things that have occurred that seem to defy known undersyanding of physics.

Edited September 25, 2019 by Spacescifi

[Guest]

5 minutes ago, Spacescifi said:

I have found that in the universe the answer to whether or not something is possible is often more like, "It depends on meeting certain criteria." Rather than a hard "No, you cannot do that!".

This is why science is great. It doesn't really limit you, only makes you jump through the hoops sometimes, and the result is usually rather impressive. Those who think science is about hard limits usually don't understand the matter correctly.

[Spacescifi]

1 hour ago, Dragon01 said:

This is why science is great. It doesn't really limit you, only makes you jump through the hoops sometimes, and the result is usually rather impressive. Those who think science is about hard limits usually don't understand the matter correctly.

 

There are a few areas where I really do think the universe says, "LOL you cannot do that!"

But for the most part doing new stuff with science is like applying for a job. If you have the required experience (knowledge) and the skill (available resources), and you can impress the interviewer (the experiment is a practical success), then the universe is like, "Have at it, have fun, knock yourself out."

Edited September 25, 2019 by Spacescifi

[wumpus]

2 hours ago, Dragon01 said:

This is why science is great. It doesn't really limit you, only makes you jump through the hoops sometimes, and the result is usually rather impressive. Those who think science is about hard limits usually don't understand the matter correctly.

There are a few.  You'd be surprised how close to the limit (Carnot efficiency) modern power plants really are, similarly for data transfer if you can afford the latency (I'm sure anything bouncing back and forth to geosync does this) you can hit as close to the Shannon limit as you want.  The one that surprised me was where I worked out that cooling panels for space warefare had some very hard limits (the fins on the ISS might be 1% of this limit, but the ISS hardly has the power needed to blast another spacestation) and they would make ideal targets for overheating (you can angle them to avoid two enemies, but not three coming from different directions).  This was almost entirely due to Carnot limits and the melting points of potential black bodies/thermal transfer lines.

Typically all that means is that you've reached the top of this "tech tree", go climb another if you want to get any higher.  And come up with some clever solutions to get around the hard limits (it might be possible to make a methane-based fuel cell that works more efficiently than burning methane and running it through a Carnot engine to power a generator.  Or better yet, make sufficiently cheap solar cells and possibly a battery system).

[Guest]

1 hour ago, wumpus said:

There are a few.  You'd be surprised how close to the limit (Carnot efficiency) modern power plants really are, similarly for data transfer if you can afford the latency (I'm sure anything bouncing back and forth to geosync does this) you can hit as close to the Shannon limit as you want.  The one that surprised me was where I worked out that cooling panels for space warefare had some very hard limits (the fins on the ISS might be 1% of this limit, but the ISS hardly has the power needed to blast another spacestation) and they would make ideal targets for overheating (you can angle them to avoid two enemies, but not three coming from different directions).  This was almost entirely due to Carnot limits and the melting points of potential black bodies/thermal transfer lines.

Typically all that means is that you've reached the top of this "tech tree", go climb another if you want to get any higher.  And come up with some clever solutions to get around the hard limits (it might be possible to make a methane-based fuel cell that works more efficiently than burning methane and running it through a Carnot engine to power a generator.  Or better yet, make sufficiently cheap solar cells and possibly a battery system).

Carnot limits, in particular, can be worked around in one way: increase temperature delta. Run the reactor hotter, the heat sink cooler, or both. Once again, it's not a hard limit, but rather an obstacle that can be engineered around. Or you can use MHD generators, which bypass Carnot limits entirely, due to them not being thermal engines in first place. Modern power plants have reached the limit of one particular way of doing things, this doesn't mean that there isn't any room for improvement. 

Cooling panels in space warfare aren't as much of a weakness as it might seem, because it will likely be an one-dimensional affair, no matter the number ships involved. Children of a Dead Earth is an excellent simulation that shows that. It has its limitations, but either way, radiators are hardly the weak point on a properly designed ship. Indeed, if it was allowed by the simulation, some of my warships could have had their radiators placed flat against the hull (which is made of the same material, because carbon FTW). The price is, of course, them running extremely hot, and reactor efficiency tanking as a result, but this turns out to be the way to go.

You're generally not trying to pump up a single number. You're trying to achieve a specific result, within physical constraints. In most cases, whatever you're trying to do, you won't be prevented from doing so, but there will be a price to pay, in space context that is usually in waste heat or propellant volume.

[sevenperforce]

22 hours ago, Spacescifi said:

All you really need is unusually heat resistant materials that we do not currently have.

With those, you could fly a gas core NTR up to orbit and back. Since thrust per amount of propellant burned is higher the hotter it is.

In real life, thrust would be lower due to not having such heat resistant material available that would dramatically allow higher thermal energies in rocket thrust.

 

Fusion is a dead end right now in my opinion.

NTR is much more scifi and easier to achieve too in scifi. No fussing about with magnetic fields, just make a scifi heat resistant material.

In real life it is easier than fusion but lower thrust unless augmented with air or certain propellants/mixes.

You don't need more heat-resistant materials; you can just use active cooling.

21 hours ago, Spacescifi said:

Quartz may work, but I do not see it being able to survive the thermal energy needed to lift 500 ton SSTO'S (average jet liner weight). I am thinking it would have to be lightweight solution at best. Orion pusher plate thrust it is not.

Only with scifi heat resistant materials could a gas core ever lift something like this with 40 crew and cargo to orbit..

There is no relationship between gross vehicle weight and "thermal energy".

[sevenperforce]

6 hours ago, Spacescifi said:

As far as I know you just need to increase the mass flow rate, likely via air intake along with propellant... which might drive up the heat load further on the quartz. Since oncoming air at high speed will be hot, not cold.

Nope, doesn't make a difference. For an airbreathing or air-augmented engine, you want the maximum mass flow regardless of how large or small your vehicle is, because high mass flow will decrease thrust-specific fuel consumption, meaning you use less propellant. If you want a bigger SSTO, you simply need more engines.

Edited September 25, 2019 by sevenperforce

[Spacescifi]

23 minutes ago, sevenperforce said:

You don't need more heat-resistant materials; you can just use active cooling.

There is no relationship between gross vehicle weight and "thermal energy".

 

Active cooling only takes you say far and has limits. What are you doing? Using cryogenic propellant as cooling? Or just tanks of coolant?

 

To lift a 1000 ton SSTO into orbit without staging,  I am thinking that a lot of your mass will likely be coolant and propellant, limiting crew and cargo capacity dramatically more than any airliner jet.

Ironically, this nuclear rocketry scheme would likely better be used as a set of staging rockets to launch a 1000 ton SSTO. 

The 1000 ton SSTO could go to the moon, and spend a day per ton of water processing from moon ice. Later it could process it into just enough LH to take off from the moon, while extracting tons of LOX too. The whole fuel processing task may take a month or more. Meanwhile crew conserves what food they have and grow fruits and veggies onboard tge ship's hydroponic garden.

From there it could fly to mars, send a shuttle down with a nuclear rocket, send it back up, and go home.

The only way the ship is landing on Earth is to send up booster rockets to link up with it and slow it for landing.

As I doubt after the moon adventure will have enough propellant left to effect a safe landing.

7 minutes ago, sevenperforce said:

Nope, doesn't make a difference. For an airbreathing or air-augmented engine, you want the maximum mass flow regardless of how large or small your vehicle is, because high mass flow will decrease thrust-specific fuel consumption, meaning you use less propellant. If you want a bigger SSTO, you simply need more engines.

 

I see. So a proper star trek shaped vessel weighing 1000 tons launched into orbit would need a whole row of rocket nozzles along the rear wall to increase the thrust.

Not like this:

Edited September 25, 2019 by Spacescifi

[sevenperforce]

18 minutes ago, Spacescifi said:

Active cooling only takes you say far and has limits. What are you doing? Using cryogenic propellant as cooling? Or just tanks of coolant?

Correct, you use your propellant as your coolant. Pump it through the nozzle and throat walls to cool them, tap off what you need for autogenous pressurization, and then pump the rest into the thrust chamber. You lose nothing at all by preheating your propellant.

18 minutes ago, Spacescifi said:

To lift a 1000 ton SSTO into orbit without staging,  I am thinking that a lot of your mass will likely be coolant and propellant...

Trying to orbit anything without staging will require that most of your mass is propellant. Though the percentages get better as you get bigger. If a stretched Falcon 1 first stage was launched with a single Block 5 Merlin 1D, it could just barely make orbit without payload. A Falcon 9 Block 5 first stage could make orbit readily with about 1-2% payload fraction. SuperHeavy could make orbit with 3-5% payload fraction. For any given engine type, the square-cube law means that bigger is always more efficient.

18 minutes ago, Spacescifi said:

Ironically, this nuclear rocketry scheme would likely better be used as a set of staging rockets to launch a 1000 ton SSTO.

If it is using staging, it is not an SSTO.

The great SSTO paradox is that any engine good enough to use on an SSTO is even better to use on the first stage of a TSTO.

[wumpus]

19 hours ago, Dragon01 said:

Carnot limits, in particular, can be worked around in one way: increase temperature delta. Run the reactor hotter, the heat sink cooler, or both. Once again, it's not a hard limit, but rather an obstacle that can be engineered around.

Except that while in space you can have an arbitrarily hot "hot side" (I used 4000K as that seemed the limit for anything turning into plasma), you can't actually use the 3K of space in vacuum as your "cold side" (unless you accept infinitely large radiators).  Using closed-loop cooling (open loop is asking to run out of coolant) and minimizing surface area lead me to heatsinks running at 2600K (they need to be hot to emit a lot of heat via blackbody radiation).  I was surprised how large such radiators simply *had* to be to sink a lot of power.  Perhaps either such vessels will either have the surface area (perhaps with fractal surfaces that emit in 180 degrees, perhaps in a single direction and contributing toward thrust) or perhaps they will simply not need such power and avoid the issue altogether.

But I should point out that this was so odd because it was the "exception that proves the rule".  Normally there are enough variables such that any one parameter can be reduced to what you need, in this case the minimum size for a heat sink (in vacuum) was simply a shocking size.

[Guest]

4 hours ago, wumpus said:

(unless you accept infinitely large radiators)

Not infinitely. Just really darn large, like megastructure-level large. If your fuel is very scarce and you need to get every bit of energy you can out of it, it can be worthwhile. Or if you can use a magnetic field to set up some sort of gas cooling contraption the size of a planet (easier in deep space, since otherwise solar wind will interfere). Or if your fuel is really scarce (say, you're mining positrons off a radiation belt), but the coolant is common (like hydrogen in a gas giant's atmosphere), you may be able to afford an open cycle. Alternatively, a plasma core reactor can run the hot end hotter than 4000K, though admittedly, you probably want to use MHDs at that point, anyway. If thermodynamic efficiency is crucial, there are ways of getting at it. It's just that it isn't most of the time, it matters for a coal burner who wants to shave off a few dollars from the coal bill, and a commercial nuke plant which wants more power out of a smaller reactor (and a lower uranium bill, as well). If fuel is not a significant part of operating costs, thermodynamic efficiency doesn't matter.

In space, mass efficiency of the whole system is more important. My CoADE power systems use a 3000K hot end and a 2500K cold end. This gives less than 16% efficiency, but radiators are really compact, for the sheer amount of heat they reject. Reactor mass is insignificant in comparison. They do guzzle weapons-grade uranium as a side effect, but there are always tradeoffs. 

Even the speed of light is not a true "hard limit" thanks to time dilation. You can travel any distance you want in as short a time as you want (given enough energy, of course). Sending someone or something, while staying behind, is what's problematic, but that's much less of a barrier to exploration and making an interstellar society. It does dictate some very unusual traits of such a society, but it doesn't prohibit it.

Edited September 26, 2019 by Guest

[farmerben]

On 9/24/2019 at 1:59 PM, Dragon01 said:

Actually, it's not that hard. Lithium-6 is a very good neutron absorbent, with Boron-10 being up there, too. Both of those are very lightweight, and in fact has been proposed as radiation protection for spaceships (they're good against cosmic ray neutrons, too).

 

There is barely any neutron radiation in space.  Neutrons decay with a half life of about 10 minutes into a proton, an electron, and an anti-neutrino.  There are lots of free neutrons in stars, but they mostly do not escape the star, or if they do they decay before getting to us.  

A meter of water, concrete, or plastic is a good neutron shield because the hydrogen slows neutrons down and they decay before getting all the way through.  I don't think neutron shielding is a difficult problem for spacecraft.    

Lithium-6 plus a neutron becomes tritium and helium plus about 5 MeV.  That means the products are moving at about 3% of light speed.  That is hella good for exhaust velocity.  We don't need to exchange heat with the reactor, we just need lots of neutrons, in order to use lithium as propellant.

[tater]

https://forum.nasaspaceflight.com/index.php?topic=43344.msg2101645#msg2101645

He added an update on NSF with this image:

[Guest]

It does look like a nice design, with all the caveats of comparing paper rockets with metal ones. Air augmentation can really help you with getting more bang out of your buck, but on the other hand, it's a relatively new tech. Even Starship is mostly an incremental design running on methane being ridiculously cheap, stainless steel hull and square/cube law (and I still don't think it'll go as well as everyone hopes). Even FFSC dates back to a 60s Russian engine (the RD-270). Besides Gnom and SR-71 (both somewhat tangentially related), Prometheus doesn't have as much to build on as the others, so these numbers, especially costs, are likely optimistic.

[tater]

^^ Yeah, still interesting. His initial idea was for it to be nuclear. That's phase 2 (gets reused in space, like lunar SS).

[magnemoe]

16 minutes ago, Dragon01 said:

It does look like a nice design, with all the caveats of comparing paper rockets with metal ones. Air augmentation can really help you with getting more bang out of your buck, but on the other hand, it's a relatively new tech. Even Starship is mostly an incremental design running on methane being ridiculously cheap, stainless steel hull and square/cube law (and I still don't think it'll go as well as everyone hopes). Even FFSC dates back to a 60s Russian engine (the RD-270). Besides Gnom and SR-71 (both somewhat tangentially related), Prometheus doesn't have as much to build on as the others, so these numbers, especially costs, are likely optimistic.

Agree, now air augmented is perfect for missiles with an pretty predictable flight envelope in the atmosphere. However even here you might want to drop it at 12 Km or 120 m above ground because enemy. 
SSTO, please stop it does not work, note that you have to survive reentry, land and needing no refurbishing outside that is daily maintenance complex equipment and be as safe as current rockets. 
Now it makes way more sense for an first stage.  Yes you need an reusable upper stage too but that is much easier without the heavy first stage hardware. 

Now SSTO has serious benefits, I don't see Starship P2P to be workable with stacking with superheavy, but it might work with something like this. 

 

[Guest]

Surviving reentry is not a problem if you can survive the flight profile needed to get the most out of a turborocket. It very much does work with a system like that. Reusable upper stage actually makes less sense, because two reusable stages mean double the recovery hardware, so with an SSTO, you'll start to see big savings in that area. This goes double if you go nuclear, on-orbit reuse needs orbital infrastructure that fleet of nuclear space tugs could serve, while a flyback LV would be able to take regular commsat launches and such.

As far as I understand, Starship P2P won't go to orbit at all, and therefore shouldn't need a first stage at all. Of course, that's assuming it works at all.

[magnemoe]

On 6/27/2020 at 10:44 PM, Dragon01 said:

Surviving reentry is not a problem if you can survive the flight profile needed to get the most out of a turborocket. It very much does work with a system like that. Reusable upper stage actually makes less sense, because two reusable stages mean double the recovery hardware, so with an SSTO, you'll start to see big savings in that area. This goes double if you go nuclear, on-orbit reuse needs orbital infrastructure that fleet of nuclear space tugs could serve, while a flyback LV would be able to take regular commsat launches and such.

As far as I understand, Starship P2P won't go to orbit at all, and therefore shouldn't need a first stage at all. Of course, that's assuming it works at all.

An SSTO has the problem having to 1) carry all of the first stage hardware into orbit 2) have all this survive reentry.  First stage only get into low hyper-sonic speed who is just 1/3-1/4 of orbital speed so it only need rudimentary heat protection like Falcon 9 or Electron first stage. 
An rocket with air intakes will have an far more complex engine structure who is heavy, fragile and pointless at 50 Km. 
Downside of two stages is that you need to integrate the stages who add cost. However to make the SSTO work you need to min-max a lot to get it to work so its an good chance you get an hangar queen. 

Now for starship p2p, first the logistic of it don't adds up for me. Current trend for premium air travel is smaller planes flying directly between places as changing planes take a lot of time. This was Concord main problem and with the security theater  after 9/11 it was faster and cheaper and less hassle to buy an business class flight from Berlin to Chicago leaving the concord with the people wanting to go fast between London and NY. 
Starship share this issue but you also need get an boat or an helicopter out to an carrier to board an rocket aiming for another carrier you have to get off. Now its benefit is that you can service the 15K km / 20 hour flights who is something you want to avoid. 
But its not many of them with high traffic. 

However if it works out I don't see stacking working well with passenger flights, its works but its make stuff more complex and the margins will not be great here.
As I understand to get an +15K km suborbital trajectory you are almost at orbital speed, it makes sense then you think about circulating in KSP.

[sevenperforce]

On 6/27/2020 at 4:44 PM, Dragon01 said:

Surviving reentry is not a problem if you can survive the flight profile needed to get the most out of a turborocket. It very much does work with a system like that. Reusable upper stage actually makes less sense, because two reusable stages mean double the recovery hardware, so with an SSTO, you'll start to see big savings in that area. This goes double if you go nuclear, on-orbit reuse needs orbital infrastructure that fleet of nuclear space tugs could serve, while a flyback LV would be able to take regular commsat launches and such.

It survives the flight profile while the engine is active and providing cooling, etc.; re-entry is much more challenging.

Actually being able to perform controlled re-entry of an orbital vehicle is a huge challenge.

[Spacescifi]

6 hours ago, sevenperforce said:

It survives the flight profile while the engine is active and providing cooling, etc.; re-entry is much more challenging.

Actually being able to perform controlled re-entry of an orbital vehicle is a huge challenge.

 

Our technology is not mature enough to do any legitimately decent payload SSTO stuff with powered landings.... on other worlds... on a regular basis.

Yeah... Elon's Starship is like... can do some of it, but it's mission envelope is quite limited.

As so far I can tell.... an any mission kind of space vessel would be acomplished with antimatter catalyzed fission or fusion bomb pushed pusher plates Both of which could be as small as a baseball... even smaller if you want.

 

If we had that, you would not need to worry about reentry.

Since your bomb fuel weighs so little (less than a gram of uranium/plutonium reacting with a small amount of antimatter), you have thrust to spare.

You can literally pulse stop instead of orbiting and fall straight down... then engage some water propellant enhanced with AM on the way down to land... hopefully near a body of water for refueling.

And no matter what your ship needs to be big.

Size matters, as more space means more propellant, payload etc.

Edited June 29, 2020 by Spacescifi