Launching lightcraft from the edge of space OR Rockets With No Gravity Turn

[sevenperforce]

I'm sure most of us here have seen, or are familiar, with the Lightcraft -- a design which would use a ground-based laser to accelerate it through the atmosphere and into space. Rather than using photon pressure, as with a lightsail, the lasers would be focused to superheat the air under the craft, causing plasma explosions that accelerate it upward against an aerospike nozzle. Its nose would form an inlet to compress air into the heating zone like a ramjet.

There have been several proofs of concept constructed and tested, with decent degrees of success. It's a neat idea. An orbital version would use a huge laser to track the ascending vehicle, which would carry onboard propellant to release into the heating zone as the available atmospheric reaction mass decreased. The vehicle would also carry a small amount of onboard chemical propellant for final orbital insertion once it was out of range (alternately, an orbiting solar-powered laser array could be used in place of the ground-based array once out of range).

The idea of a spacecraft rising on a column of light, a sort of bifrost space elevator, is common in science fiction: 

It got me thinking. Suppose you had a "bifrost bridge" spaceport which could use lightcraft tech to lift payloads to the edge of the atmosphere, but could not effectively turn/focus to send them downrange. Thus, you could cheaply lift virtually any payload to 50-60 km but it would have negligible velocity. Under those circumstances, how would you design an SSTO vehicle to reach orbit? What engines and propellants would you use? How would it return to the surface? 

Edited January 23, 2020 by sevenperforce

[Wjolcz]

Wouldn't that be kind of like launching from a very tall mountain? As cool as that sounds it seems like a waste of electricity to power the laser. Building the laser wouldn't be cheap either. Why not have a mirror that focuses the beam as the spacecraft performs the orbital turn?

Edited January 23, 2020 by Wjolcz

[kerbiloid]

I would presume that aerobraking when the bottom is a wide aerospike is not a big problem.
The nozzle is heatshield itself, the throats (thanks for the term) can be easily closed by small caps, so just a ring around them should be heashielded, but it anyway would consist of thick and strong landing feet sticking down out of the hull.

The last 200 m/s of vertical speed look not like a problem for any engine.
Or for a landing laser, but this is not good on emergency landing. Also anyway it needs a jet at least to deorbit.

So, probably the only real problem is to accelerate horizontally at 60+ km.

I would presume a saucer with a radial thruster thrusted by the same plasma flow.
I.e. the saucer gets up on the laser pillar, opens a horizontal nozzle at one side (at CoM), and starts partially redirecting the gas flow to it, still being hold and fed by the laser.

Presuming 2 g of horizontal acceleration, the acceleration distance ~= 8000² / (2 * 2 * 9.81)  ~= 1600 km. For 4 g - ~ 800 km.
So, it need a chain of several tens of lasers.

On another hand, if accelerate it by the ground lasers up to 3 km/s, the distance is ~200 km, so just 2..3 lasers can be used, but then it should enable its own engine, powered either by a onboard reactor, or by an orbital laser power station, especially since its speed is closer to the orbital speed at 4+ km, and the fact that the faster it flies, the less power it needs thanks to the centrifugal acceleration.

To receive the power from top it maybe should be partially sheided from both top and bottom, but the top shield (the heat receivers) can be much lighter.

So, I would presume
a pan with wide aerospike below,
covered by a plate on top,
with low cylinderocone in between, with a nozzle at the rear side and windows around other its part.

It gets lifted by an on-ground laser,
then opens the rear nozzle and starts accelerating by a chain of several more ground lasers up to 3..4 km/s,
maybe opens some scoops and uses ramjet mode to reach 3..5 km/s, but not necessary,
receives the orbital laser beam, accelerates to 8 km/s, gets to the orbit.

Then it gets deorbited by an orbital laser, aerobrakes, lands by the ground laser, then uses emergency landing engines to finish.

The orbital laser can be a mirror for on-ground ones.

Edited January 23, 2020 by kerbiloid

[sevenperforce]

My question assumed you only had the ground laser, and that it could only accelerate the vehicle vertically, not horizontally.

4 hours ago, Wjolcz said:

Wouldn't that be kind of like launching from a very tall mountain? As cool as that sounds it seems like a waste of electricity to power the laser. Building the laser wouldn't be cheap either.

It would be more than launching from a very tall mountain because no mountain is as tall as you could achieve using this. You could also give it significant vertical velocity, enough to get well out of the atmosphere. Let's handwave the cost of the laser and electricity to run it.

The way I see it, you have two basic paths:

• Slow rise. By using the atmosphere as your sole propellant source on ascent, you need only carry engines for the boost into horizontal flight, and you need no particular aerodynamic shape. It is still an SSTO requiring 7.8 km/s of dV but that's easier than it sounds when you can optimize for vacuum at the outset and TWR is virtually no object.
• High boost. You can make your ship aerodynamic and carry inert propellant, allowing the laser to push you at high speed through the lower atmosphere and then heat your propellant to propel you much higher. Once your apogee reaches MEO altitudes, you can burn efficiently at apogee to lift your perigee out of the atmosphere, then burn at perigee to circularize.

 

[magnemoe]

I assume you would do an gravity turn, perhaps with an low profile to use the air as long as possible, and yes you probably need an secondary laser in space or far downrange, an mirror in space might be an option. 
It was an business idea about this using an space plane instead who would be easier to scale up one issue here would be takeoff. 
However this would be the easy part to solve. Tow it with an cargo plane, drop it from the stratolanch plane or use an powered trolley  

[sevenperforce]

1 hour ago, magnemoe said:

I assume you would do an gravity turn, perhaps with an low profile to use the air as long as possible, and yes you probably need an secondary laser in space or far downrange, an mirror in space might be an option. 

The trick here is that you can't do a gravity turn because you can't deviate from a vertical ascent using the laser. The laser can't follow the vehicle downrange.

It seems arbitrary, yes, but I wanted to do a thought experiment about what kind of rocket you would design if you couldn't do a gravity turn.

[kerbiloid]

Spoiler

  

24 minutes ago, sevenperforce said:

It seems arbitrary, yes, but I wanted to do a thought experiment about what kind of rocket you would design if you couldn't do a gravity turn.

A curved
.

 

[magnemoe]

1 minute ago, sevenperforce said:

The trick here is that you can't do a gravity turn because you can't deviate from a vertical ascent using the laser. The laser can't follow the vehicle downrange.

It seems arbitrary, yes, but I wanted to do a thought experiment about what kind of rocket you would design if you couldn't do a gravity turn.

You will need multiple lasers downrange and you need an gravity turn. Else the project is junk. 
You will get stuff like starship and new glen to fight against. 
Designing another sail powered wooden ship of the line in 1855 or an America liner passenger ship in 1955 is a lot of the same, the world is changing. That microwave pumped space plane looked descent however it needed more ground infrastructure than you can get today. 

[Guest]

Actually, you don't need a gravity turn at all. You just have to get your lightcraft to escape velocity, or close enough for a small onboard propulsion system to do the insertion burn. Orbital velocity at extreme altitudes is tiny.

[James M]

Have a second Laser in orbit pointed toward the target using reaction wheels. Fire away!

[wumpus]

You'd get 1000m/s delta-v (just from the altitude) and should be able to eliminate all aero delta-v.  I doubt it would be worth it.

This is pretty much the whole problem with stratolaunch.  That and it greatly limits the size of the rocket it has to carry.

[sevenperforce]

On 1/24/2020 at 4:08 PM, wumpus said:

You'd get 1000m/s delta-v (just from the altitude) and should be able to eliminate all aero delta-v.  I doubt it would be worth it.

This is pretty much the whole problem with stratolaunch.  That and it greatly limits the size of the rocket it has to carry.

If you can lift something to the edge of space with NO regard to cost or size or shape, it would open up a lot of additional options, OMLs, and so forth.

[wumpus]

24 minutes ago, sevenperforce said:

If you can lift something to the edge of space with NO regard to cost or size or shape, it would open up a lot of additional options, OMLs, and so forth.

I'm pretty sure you would still need lots of thrust as your acceleration would suddenly change to straight down (unless you could be supported while building up lateral velocity).  You might not need quite as much thrust, and this might mean that SSTO might even work, but I'd doubt it.  You could use only vacuum optimized engines, but you could do that from air (and possibly mountain) launch as well.

I'd also wonder if the trip down might be more valuable.  If you could kill all your lateral velocity in the "right part of the atmosphere" (and then not care about parachutes or retrorockets) you might save more than 1000m/s worth of mass.  I'm also unsure of "the right part of the atmosphere": is it the mesopause (lowest temperature, about 200K) or the stratopause (higher temperature 275K, but more than twice the pressure).  I suspect the stratopause, considering that all that heat comes from adiabatic heating.  If they reach nearly the same pressure, you will be cooler in the stratopause (don't know if they will reach the same pressure).

[Flibble]

14 hours ago, sevenperforce said:

If you can lift something to the edge of space with NO regard to cost or size or shape, it would open up a lot of additional options, OMLs, and so forth.

OMLs?

All lifting something up to the edge of space gets you is something that about to fall back down. You still need to build up orbital velocity, which is what the majority of the fuel a rocket carries does. If you can't track the object, then as soon as it lights its engine to insert it will fall back down. That makes your earlier idea about TWR being irrelevant untrue - you're going to need enough TWR to offset gravity and also start accelerating the ship. Existing LVs can get away with low TWR upper stages because they already have enough orbital velocity to partially offset gravity.

[sevenperforce]

7 hours ago, Flibble said:

OMLs?

All lifting something up to the edge of space gets you is something that about to fall back down. You still need to build up orbital velocity, which is what the majority of the fuel a rocket carries does. If you can't track the object, then as soon as it lights its engine to insert it will fall back down. That makes your earlier idea about TWR being irrelevant untrue - you're going to need enough TWR to offset gravity and also start accelerating the ship. Existing LVs can get away with low TWR upper stages because they already have enough orbital velocity to partially offset gravity.

OML = outer mold line.

And yes, I am quite aware of the basic underlying principle of orbital flight. TWR may not be entirely irrelevant, but it is not nearly as big of an issue as it is on the ground. Lower TWR requirements, higher vacuum efficiency, and the obviation of the bulk of atmospheric and gravity drag bring the dV requirements down to manageable levels for SSTO -- you can achieve orbit with an 82% propellant ratio if you use RL-10s. Large, draggy tanks aren't an issue and provide a huge advantage during re-entry.

Hence the question -- what vehicle shape, engine configuration, and EDL profile would be favored if you could "jump" to the edge of the atmosphere for free?

[έķ νίĻĻάίή]

I’d say no to the concept already, as the amount of energy needed just to turn on the laser... damn.

[sevenperforce]

1 hour ago, έķ νίĻĻάίή said:

I’d say no to the concept already, as the amount of energy needed just to turn on the laser... damn.

Not proposing the concept. Thought experiment.

[Flibble]

One advantage would be you could use large spherical tanks to improve mass ratio I suppose, since you don't need an aerodynamic geometry.