Sky ramp

[farmerben]

This topic deserves its own thread.  How about rail launching the first stage of a spacecraft.  

 

http://www.g2mil.com/skyramp.htm

This is the most thorough analysis of the topic I have seen.  It appears to be the work of amateurs rather than bureaucracies.  The claims appear reasonable to me.  I wonder if anybody else sees flaws in their analysis.

[YNM]

Why not just go with space pinball as well ?

 

[Bill Phil]

Most of the fuel consumed in getting to 1000 mph is solid fuel. It's also a result of the tyranny of the rocket equation more than air drag.

It could work, though.

[p1t1o]

The crux of it is whether that 40% of fuel (or whatever the figure is in todays best launchers) is more expensive than the cost of developing this project, and the final cost of launches.

Delta-V efficiency is not as relevant as financial efficiency.

[Bill Phil]

3 hours ago, p1t1o said:

The crux of it is whether that 40% of fuel (or whatever the figure is in todays best launchers) is more expensive than the cost of developing this project, and the final cost of launches.

Delta-V efficiency is not as relevant as financial efficiency.

Yes. But financial efficiency is closely tied to the vehicle. Having a smaller percentage of the vehicle as propellant greatly improves the ability of a vehicle to be reusable, and could allow a reduction in the number of required engines as well. 

This kind of thing enables SSTO vehicles with current engine technology. And even if it's not an SSTO, the increase in payload can be so significant that per kilogram costs could be massively reduced.

[wumpus]

4 hours ago, p1t1o said:

The crux of it is whether that 40% of fuel (or whatever the figure is in todays best launchers) is more expensive than the cost of developing this project, and the final cost of launches.

Delta-V efficiency is not as relevant as financial efficiency.

So for a Falcon 9 (so far the cadence champ) that is ~$100k.  There are amusement park rides that do this to ~100kmh and presumably can't be much more expensive than a few years of Falcon 9 fuel.  Scaling it up to 1600kmh is a bigger question.  If the cost of fuel became a limiting factor (presumably a post BFR/New Armstrong ship), I'd recommend scaling this up to ~mach 1, then using a scramjet to get up to as fast as possible (NASA data shows somewhere between mach 6 and mach 10).  Note: acceleration to high speed really should be done at elevation, but you might get away with a low-TWR rocket merely maintaining velocity and by the time it leaves the atmosphere it has burned enough fuel to accelerate.

Using a scramjet also involves a stage separation (not cheap) and presumably recovery.  Reuse of scramjets may also require de-rating that speed quite a bit.  I'd recommend enough SRBs to get the TWR to 1.5 (if only for a little way) for now.

[Gargamel]

On 7/25/2018 at 9:51 AM, farmerben said:

http://www.g2mil.com/skyramp.htm

I couldn't even finish the first couple paragraphs as the page was hurting my eyes.  It's hard to take a scientific proposal seriously if it is written in the layout equivalent of comic sans. 

[kerbiloid]

(It's a usual "Arial,Arial,Helvetica", not Comic Sans.
Just a lot of animation and psychodelic background.)

Though what's bad in a magnetic ramp accelerating a ramjet spaceplane to ~2 M to ignite the ramjet?

[Lisias]

45 minutes ago, Gargamel said:

I couldn't even finish the first couple paragraphs as the page was hurting my eyes.  It's hard to take a scientific proposal seriously if it is written in the layout equivalent of comic sans. 

Old dog, new tricks. I think this guy is from the old ANSI terminals era,.when all we had was 16 colours to do the job. :-)

[farmerben]

One huge advantage is that at launch you can get away with low thrust relative to fuel mass onboard.  TWR = 1 gets you a long way.  With radially mounted external fuel tanks, a single aerospike vehicle is practical.  

External fuel tanks are stages, but quite a contrast with engine stages.  Dropping a tank into the sea isn't as big a deal.  

[Cunjo Carl]

Rocket sleds are an awesome concept, and as a total layman I prefer them quite a bit to hyperloops, but they do have a number of interesting issues.

The main things that stick out in my mind are that the ground effect (uneven aerodynamic drag/lift because the ground is so close) is going to get really crazy when the rocket goes transonic! Also, and there'll be notable mechanical stresses on the rocket just from being supported and propelled from the side. Plus, after release the launch vehicle would require much more aerodynamic control than your typical 'flying cylinder' vertical launcher. All this sounds like a lot of extra structural requirements on the rocket, which'll raise drymass considerably. The question is, are these increased structural drymasses somehow less than the drymass of the extra fuel tank that would have been needed to hold the 'wasted' fuel of a normal launch.... despite being less awesome somehow I think the normal launch would win out.

There's a lot of stuff on this particular site that doesn't quite sound right to me, though I'm admittedly no expert on the topic. One example as case and point, they posit that the first stage speeding up along the ground is actually a good thing for the crew's safety, and to be honest, that's bizarre! With a proper LES, your chance of escaping a rocket catastrophe is really quite good. If you were traveling mach 2 only a few dozen meters off the ground, and the launch vehicle or its rocket-sled breaks or explodes or otherwise falls from track, what could you possibly do to save the crew? The webpage says the crew could "... eject from an escape capsule, similar to aircraft ejection seats". At mach 2 just off the ground? With the upwards inflecting track? Really?

Also, I found these statements particularly funny:

" There are other bogus space launch concepts that are proven failures, yet continue to be advocated and funded, like maglev launch and reusable boosters. "
" Shooting a spacecraft upward off a rocket-powered sled a better alternative. "
" Building a ramp up a mountainside may seem complex, but it's simple compared to the difficulties of building massive rollercoaster tracks. "
" Some people have expressed concern about air pressure build-up if a tunnel is used.  One solution is to make the tunnel wide enough so its not a problem. "

Thanks for sharing, @farmerben! I do really like the concept of rocket sleds by the way. It's mostly that they have a lot of technical difficulties that would need to be addressed (like anything), and this particular page tends to call them "minor problems" and not really deeply discuss them, while also ascribing them many benefits which I don't think they really have. I do like it as an introduction to the topic though!

 

Edited July 29, 2018 by Cunjo Carl

[farmerben]

I thought the tone of the page was amateurish, but those points are somewhat defensible.  If there is a more serious look at the topic, I haven't found it yet.

Escape capsule is more than just a seat, that system might take more dry-mass than others.

A straight rail is simpler than a curvy roller coaster.  More precise perhaps, but simpler.  

Inside a tube at supersonic speeds air pressure builds up on the nose of the vessel.  On the surface this would be no more of a problem than in the air.  

[Cunjo Carl]

On 7/29/2018 at 1:46 PM, farmerben said:

I thought the tone of the page was amateurish, but those points are somewhat defensible.  If there is a more serious look at the topic, I haven't found it yet.

Escape capsule is more than just a seat, that system might take more dry-mass than others.

A straight rail is simpler than a curvy roller coaster.  More precise perhaps, but simpler.  

Inside a tube at supersonic speeds air pressure builds up on the nose of the vessel.  On the surface this would be no more of a problem than in the air.  

 

Again, I'm a fan of the rocket sled idea, and I do like the approachable aspect of the page, but I'll point out the following challenges:

Launch Escape System

Spoiler

 

Well, let's actually look at the situation. In order to make the curve they're talking about (it's a family of curves, but for one that looks reasonably like the one in the picture,) you'd need ~3.5-5G of acceleration upwards, including to our normal gravity. This force would normally be supplied by the track as it follows the shape of the mountain. When the escape system fires, it'll need to provide the force instead in addition to the force to actually escape. So, our escape system would need to have the ability of our current escape systems with the following extras:  Let me know if you'd like the derivation!

It would need an additional 2.5-4G worth of force, which is not too nasty on its own because LES in reusable systems are ~7G applied already. However, it would need to do this in a supersonic cross wind (meaning it will need to be aerodynamically stable in the 'forward' direction while the escape rocket is firing out the bottom.) It then needs to be able to bleed off off the supersonic speeds without falling into the curved mountain approaching at 3.5-5G, meaning it will need to boost up a considerable distance so it has enough time to (somehow) deploy drogues, then chutes and finally land starting in those high-G conditions. When it lands, we'll be on a steep (45 degree) grade so we somehow need to keep it from rolling off! Now all the while we've been slowing down our astronauts, the wreckage has been splattering and slowing down as well, so somehow we need to avoid this. I have no concept of how this could be done without a glider, which would be far too complex, but would admittedly look awesome in a sci-fi film.

Now because this all comes sideways out of the rocket and fully encapsulates the crew, the LES can't just be dumped in the upper atmosphere like normal, so we would need to drag whatever monstrosity can do all of the above all the way to orbit and back! There's no easy way to cut it. This is an engineering nightmare. Can it be done? Maybe, but there will be a ton of concessions on the craft design because of it. Exactly what those are and how we could solve them would be a fun thing to imagine! It's far from cut and dry though, as it's presented.

 

Building with roller coaster technology

Spoiler

 

The rocket sled track will need to be much more impressive than a typical roller coaster. There are some truly amazing roller coasters that are feats of engineering (and large budgets) , but when we compare their plans to a typical roller coaster, we see they're hugely different in scale. The rocket sled rail would be as far removed from a roller coaster as a roller coaster is from a thick plank of wood. It's fine to not agree with the exact estimates I've made, the point still remains that they're extremely different magnitudes of project.

Their example craft is the little suborbital X-33, which weighs in at 130tonnes fully fueled. By the time we factor in the sled and actually getting to orbit with a payload we'd be talking several times the launch mass, which I'm rounding roughly to 500tonnes. The curved track will increase the experienced gravity to ~3.5-5G. So this would need to do ~100 times the normal force, 20 times the speed, and be dramatically longer and smoother than any roller coaster. From a conceptual standpoint they may be similar, but when you sit down to start pondering the brass-tacks of actually building it, this would be a very different beast. Maybe more akin to building the Shinkansen rail.

Properly designed, I certainly don't think any of this is impossible, and I think there's a lot of good things to be learned by trying to figure out options for all these little troubles, but it's definitely not the slam-dunk easy that the webpage presents it as!

~3-5G, 500tonnes, 5km, 680m/s. Curved, orbital rocket sled rail
~3-5G, 5tonnes, 1km, 35m/s. Typical 'easy' Roller coaster ... Then making an equivalent step down we have:
~3-5G, 50kg, 200m, 2m/s. Someone jumping along a rail of wood planks (like 4x4s)

 

 

 

Launching an orbital rocket through a tunnel

Spoiler

For tunnels, their example craft the X-33 is about 20m tall/broad, and we might assume the sled that pushes it would be similarly sized. How large would the tunnel need to be for the transonic shockwave of a passing rocket to not have damaging or destabilizing effect on itself and the rocket? Even small planes can shatter windows across a good distance (~100m) of open sky, what about a massive orbital rocket ramming through a closed tube? I think a lowball would be 3 times the diameter of the rocket+sled, and even this would result an insanely large tunnel the likes of which has never been seen! It would rival the cost of any space program to build! Oh well, I guess Elon is covered in any case .

 

It's fun to imagine how we might approach these challenges, but the one thing we can say for sure is that they're not cut-and-dry as presented. Some researchers would get to make their careers by them! Nothing wrong with that. Why am I a fan of rocket sleds though? I used one on an SSTO in KSP once, and it worked amazingly! So I hatched a plan... and as we know, if it works in KSP it'll work in real life too, right?

Now this isn't the *Best*Plan*Ever* (tm), it's just my rough sketch idea of a cool thing to be done with a rocket sled. Rather than being based on the X-33, it's based on Skylon, the fully reusable SSTO space plane coming out *soon* (tm).

The rocket sled (Methalox or turbojet powered) is supported on a ~15km long, perfectly flat and level high gauge rail with one rail electrified. The rocket sled cradles the fuselage of a space plane that is exactly the hopes and dreams of what Skylon could be. A few launch clamps hold the two together. Skylon spools up its engines while the rocket sled fires, pushing forward at 2G! 5km down the rail (22 seconds later) they're traveling mach 1.3, then the launch clamps unlock and Skylon lifts away by aerodynamic forces. The rocket sled engages brakes (reactive brakes maybe) and slows down at 2-3G over another few km, dumping most of the power into the rail, where it's used for something cool back at base camp... lasers maybe? Then, after stopping, the rocket sled uses electric motors to drive back home. Meanwhile, Skylon does what Skylon does (why mess anymore with an already awesome plan). It picks up altitude and velocity up to mach 5 with its miraculous jet engines, then it switches to closed combustion and rockets into orbit. Once there it drops off a payload (or acts as mothership to a returning GTO second stage) , and then flies home. Because it's nice and light weight by now, it'll have an easier time of this than the shuttle. Finally, it uses powered flight to land back at the starting blocks. A crane or something drops it off on the rocket sled again, they're refueled and flown again!
It'd have no launch escape system, because it wouldn't have passengers! Just cargo and a computer at the helm. In the case of an emergency, it would try to save the spaceplane by having it lift away, or having the rocket sled put on the breaks, but it definitely wouldn't be human rated.

What advantages does this have over Skylon as it's currently envisioned? This would let the designers optimize the aerodynamics and engine profile for super sonic flight without worrying about takeoff, subsonic and transonic conditions. It would need to be able to fly in subsonic conditions on return, but not efficiently! As much as the free deltaV, removing the subsonic design constraints is what would really add value to the system. Also, the landing gear (= dead weight) could be made much smaller because they would only need to support the spaceplane _after_ it lands without the fuel/cargo. Additionally, there wouldn't need to be a large launch-abort braking system in the landing gear, because it would be on the rocket sled instead. All of this translates to a huge boost in deliverable payload fraction, which translates to money money money.

Advantages over contemporary flyback boosters: Fully reusable, no lost upper stages. Less assembly required per flight.
Disadvantages: Plenty.

I really like the idea though!

 

Edited August 1, 2018 by Cunjo Carl
ttypo

[Xd the great]

For the disadvantages, the rocket sled on a rail creates a lotta drag. The skylon still have to deal with transonic drag and sonic boom. The whole design requires a ton of extremely flat land...

Why dont you use a large jet instead of all this trouble. Plane launched skylon.

[farmerben]

I'm not a huge fan of putting super heavy and expensive turbo machinery on a spacecraft if you don't need it, and its really only super useful for the first 500 m/s.   Some kind of flying stovepipe has got to be a better deal if it will actually work.  Carrier planes compare to directly rocket sleds, and are obviously better for some applications... The higher the mass of the final stage the more favorable a launcher looks.   

 

Edited August 1, 2018 by farmerben
deleted a mistake

[Cunjo Carl]

39 minutes ago, farmerben said:

I'm not a huge fan of putting super heavy and expensive turbo machinery on a spacecraft if you don't need it, and its really only super useful for the first 500 m/s.   Some kind of flying stovepipe has got to be a better deal if it will actually work.  Carrier planes compare to directly rocket sleds, and are obviously better for some applications... The higher the mass of the final stage the more favorable a launcher looks.  

I'd argue in this case that we need it. The verticality of the sled is one of the hugest technical hurdles. It would ideally be at like 80 degrees! Putting it even at 45 is adding in a lot of technical constraints on the launch vehicle, and that's still an enormous challenge on the sled end as well! A nice flat rail and a HOTOL SSTO (horizontal takeoff and landing) are peanut butter and chocolate. It's the high ISP of the air breathing mode that allows for the slow, low angle of attack climb.

39 minutes ago, farmerben said:

Take several large helium balloons each carrying a pulley and a cable.  ... At the center is a rocket which can detach its cables at any time.  On the distant ends of the cables are giant winches. 

Zeplins launching rockets? Now we're talking cool

 

11 hours ago, Xd the great said:

For the disadvantages, the rocket sled on a rail creates a lotta drag. The skylon still have to deal with transonic drag and sonic boom. The whole design requires a ton of extremely flat land...

Why dont you use a large jet instead of all this trouble. Plane launched skylon.

While the rocket sled idea has numerous technical hurdles, I'd contend that these particular ones aren't the biggest of deals.

The space plane would need to be able to structurally survive transonic conditions, but given it's designed for surviving the considerably greater aerodynamic pressures of MaxQ (up at mach 3-5.5 somewhere) the transonic region wouldn't place any new design constraints. It's a freebe! That is, unless ground effect causes massive amounts of excess lift that somehow aren't experienced in other regimes (like reentry). Next, the rocket sled will certainly experience drag, but even a (very) highball estimate would be .5G worth when at top speed in addition to the of the 2G acceleration. On the other hand normal rockets generally work up to many G of acceleration in addition to both gravity and aero drag. By comparison this sled's gonna be toodling! Fuel-wise, since it's only responsible for ~450m/s deltaV, the aerodrag won't kill us through the exponentiality of the rocket equation either. Just bring along some more fuel.

On the flip side,  plane-based launchers would need to deal with both its own drag (parasitic), as well as the lift induced drag to keep everything floating. The current state of the art, VMS Eve, isn't able to break the sound barrier-- It provides the height without a ton of speed. Adding in the further constraint that it would be carrying an enormous Hydrogen-fuelled vehicle makes it a tricky sort to imagine. It's already the biggest composite air frame ever built, but it would need to get much larger! Spruce Goose eat your heart out, style thing. It would definitely provide the additional advantage of dropping off the launch craft at a higher altitude (= lower pressure) so perhaps the added difficulties are worth it?

Let's actually put some numbers on these engines and the fuel mass of a rocket sled. Now given this is fantasy, of course I want them to be raptor engines! Sadly, I think those will be massively overkill. Let's check.

Skylon wet mass: 325T  (wiki)
Raptor SL Thrust: 1700kN (wiki)
Raptor SL Isp: 330s (wiki)
Sled Fuel Mass (approx): ~80T = (95+325)*(exp(450*1.25/(330*9.8))-1)  -- Includes 1.25 factor of drag
Sled Total Mass (Guess): ~175T = 80T Fuel + 95T Everything else
Sled+Skylon Wet mass (iterate): ~500T = 175T + 325T
Sled numEngines: ~7 = 500*(2*9.8*1.25)/1700                           -- Includes 1.25 factor of drag
Sled numEngines including a 15 degree outwards cant: ~7.5 = 500*(2*9.8*1.25)/(1700*cos(15deg))     Round it to 8

Well! Lucky me, it looks like we'll get to use raptors after all . These figures include a healthy margin for aerodynamic drag, but do assume fairly high grade components for the sled. On the other hand, a -very- heavy sled might end up being more like 350T fully fueled (240T dry + 110T fuel), and require 10 raptors to push it including the outwards cant. I'm assuming that most of the mass will be attributed to (in order) the frame, wheels, tanks, plumbing, control surfaces, motors+suspension+breaks, rocket engines, and lastly the seats for crazy tourists who might be named Jeb.

Now while I think the above aspects are circumventable by available technology, there's quite a few that aren't.

 Q: How do we keep this sled stable on the track without lifting off or slamming down at any point?
 A: Uh... Canards+Spoilers?
 
 Q: How do we keep the sled's exhaust plumes from scorching the track?
 A: Maybe put the sled's rocket engines on pylons with a slight outwards cant?
 
 Q: How do we account for most of the mass being mounted on top?
 A: Having the rocket pylons up on level with the mutual center of mass would probably be the way.
 
 Q: So Skylon just flies off this thing without trouble?
 A: For sure!
 
 Q: Where can you build 15km of track without bothering nearby populations with all the sonic booming?
 A: There's a few places, but they're admittedly almost as rare as a nice mountain people wouldn't mind you launching rockets up the side of.
 
 Q: What if a kangaroo or an emu gets on the tracks?
 A: Just build the fences around it high... like reeeeaaly high.
 

 

Edited August 1, 2018 by Cunjo Carl
typo

[kerbiloid]

Spoiler

9 hours ago, Cunjo Carl said:

Q: What if a kangaroo or an emu gets on the tracks?

A panspermia.

 

9 hours ago, Cunjo Carl said:

Q: How do we keep this sled stable on the track without lifting off or slamming down at any point?

How does a magnetic suspension keeps a train stable?

[Cunjo Carl]

2 hours ago, kerbiloid said:

  Reveal hidden contents

A panspermia.

 

How does a magnetic suspension keeps a train stable?

I like the idea! Preface, this is how I remember it working, but it may be a bit off...

There's two varieties, and both can provide magnetic levitation. The first (and the kind I'm much less familiar with) is a 'linear motor' type, where there's permanent or electromagnets in the track and electromagnets in the train. The other rarer kind is called the linear induction motor where there's electromagnets on the train and a conductive but non magnetized track (like plain old aluminum). As the electromagnets on the train turn on, the electrons in the nearby conductive track experience the sudden magnetism, and (by the way of things) rush in circles (called eddy currents) to create an opposing magnetic force. This causes momentary levitation, but it's not a stable situation. Quickly afterwards though, the electromagnets on the train start to turn off, and just in front of them another set turns on. The new set also opposes those eddy currents , and because it's a little in front of them, pushes forward off of them (moving the train forward), while simultaneously creating a new set of eddies very slightly further forward. The process repeats, and the magnets keep pushing off old eddies while creating new ones.  The rate these eddies move within the track is called 'slip' and is a defining factor in how the motor functions. In real situations, the motor will use electromagnets turning on in alternating directions (poles) rather than simply on/off. Also, the above is from a stationary perspective. Because the eddies move much less quickly than the train, the electromagnets on the train need to sweep backwards to keep up with the track. 

This is all what I remember, and it should be mostly correct but may be off in a little detail. I had to study LIMs for a project a good while back, and wound up prototyping one in my garage. What stuck with me most was how to choose wire gagues and wrappings rather than how the blasted things worked! Sometimes life is like that. An LIM to support 500Tonnes of rocket would be truly gargantuan and require enormous amounts of power. There are, however, many things in this world that can supply and handle enormous amounts of power so it's certainly a possibility! It would be interesting to calculate just how much electric power would be involved, but I'm expecting from a paper I just glanced at ~1-2kW/ton of levitation assuming a typical slip (optimized for pushing the train forward by magnets). So, probably 1/2 to 1 megaWatt for my proposed 500T rocket sled ... As a reference, your house probably pulls ~1kW on average, enough to maybe levitate a very small car.

Edit: The 1-2kW/ton value sounds a little low to me. Maybe the ones I made were really low efficiency though! That would certainly sound likely.

I think... I could be wrong, but I seem to remember you could get better levitation efficiencies if your track was ferromagnetic (like carbon steel). The downside was magnetic saturation could be an issue, and you couldn't use high slips so forward motion got a little funky. Still, I think the first thing I'd look into would be this.

For fun, back at home, It is possible to levitate an object in place using a single electromagnet which simply turns on/off quickly by this same effect. It's a common physics demo, called the jumping ring.

Edited August 2, 2018 by Cunjo Carl

[kerbiloid]

http://www.dailymail.co.uk/sciencetech/article-4839534/China-s-supersonic-flying-train-travel-2-500mph.html

Of course Daily Mail, but about a high-speed Chinese project.

Edited August 2, 2018 by kerbiloid

[YNM]

5 hours ago, kerbiloid said:

Daily Mail ... high-speed Chinese project.

Banal, and... Bogus !

 

That article aside, I guess the hardest problem with such systems is just the sheer forces we're trying to deal with. Not to undermine any extent of human ingenuity but I kind of doubt it it'd be viable in any near future.

There could be a few solutions to offer :

1. A looped system with an exit. Like circular particle accelerators. The challenge will be to handle the centripetal forces and the switch mechanism.

2. A very high-G short-rail system. The problem will be the high initial G-loading.

3. A very long straight (plan-wise) rail. The problem will be the sheer size of it and the slight centrifugal forces, from Earth's curvature and slope change.

[dreadanaught]

On 7/26/2018 at 8:02 AM, wumpus said:

There are amusement park rides that do this to ~100kmh and presumably can't be much more expensive than a few years of Falcon 9 fuel.  Scaling it up to 1600kmh is a bigger question. 

1

Those amusement park rides sometimes can't go faster due to the acceleration loads it would place on the people riding the thing.

[wumpus]

1 hour ago, dreadanaught said:

Those amusement park rides sometimes can't go faster due to the acceleration loads it would place on the people riding the thing.

The US Navy is trying to replace steam driven catapults with a system called EMALS for electric acceleration.  It will get to 150mph, but is so far unreliable (10% failure rate) and is priced at US carrier volumes but is unlikely to hurt anyone who can meet military pilot health checks (might be a problem with the general public but not astronaut crews for some time).

[Rakaydos]

My question is, with all the power needed for an electromagnetic track and magnetic acceleration, how much rocket fuel could you literally create out of thin air (and water) with Electrolysys + Sabatier reaction?

[wumpus]

56 minutes ago, Rakaydos said:

My question is, with all the power needed for an electromagnetic track and magnetic acceleration, how much rocket fuel could you literally create out of thin air (and water) with Electrolysys + Sabatier reaction?

Electrolysis is ghastly inefficient and the Sabatier reaction reaction would only ruin your Isp to get better thrust at a significant cost of electricity.  Then there's the whole issue of thermodynamic efficiency of the thermal rocket cycle (which might be saved thanks to extreme hot/cold delta, I don't know).  I think even a railgun is more efficient than heat engines, imagine the efficiency of an electric car if it didn't have to carry batteries around.  The real problem is that now you are fighting the rocket equation.  For low thrust rockets (which make even more sense on skyramps), you can burn half your mass in fuel getting to mach 3 and half your mass again to mach 6.  Simply using electricity to accelerate a much lighter spacecraft has efficiencies all of its own.

[Rakaydos]

6 minutes ago, wumpus said:

Electrolysis is ghastly inefficient and the Sabatier reaction reaction would only ruin your Isp to get better thrust at a significant cost of electricity.  Then there's the whole issue of thermodynamic efficiency of the thermal rocket cycle (which might be saved thanks to extreme hot/cold delta, I don't know).  I think even a railgun is more efficient than heat engines, imagine the efficiency of an electric car if it didn't have to carry batteries around.  The real problem is that now you are fighting the rocket equation.  For low thrust rockets (which make even more sense on skyramps), you can burn half your mass in fuel getting to mach 3 and half your mass again to mach 6.  Simply using electricity to accelerate a much lighter spacecraft has efficiencies all of its own.

...I'm not suggesting some kind of absurd Sabatier-thermal rocket. Just a Methane fuel plant for fueling a conventional reusable rocket to carry the same payload instead of the ramp.

Edited August 11, 2018 by Rakaydos