The Reaction Engines Skylon Master Thread (1)

[MikeAtkinson]

Rob, what you are suggesting is basically HOTOL, which also used separate rocket and ramjet and a sled (although only up to 330m/s). I think you are suggesting rocket and ramjet at the back, this has severe centre of mass and centre of pressure problems over the flight portion of the envelope.

When looked at in detail this has always been discarded in favour of conventional rockets or air launch.

Astronautics has this to say. "The final design had serious operational disadvantages and a small payload. The only way the designers could continue to claim to put a reasonable payload into orbit was by specifying untried and speculative structural materials". I have heard Alan Bond ( one of HOTOL's designers) say something similar.

[RobLynn]

Skylon does not intend to be the 'operator of record' for this vehicle.  They intend to be Airbus/Boeing selling these ships to whoever is willing to operate them.  Tossing in a mult-million dollar, single location launch infrastructure is anathema to that goal.

In this far off future where there may be competitors able to sell for $1000/kg (ie SpaceX lowest possible price assuming some re-usability).

If each vehicle lasts for the estimated 200 flights @15000kg/flight then that is $3 billion revenue per vehicle.  If operational costs are $5million per flight then they could be selling the vhicles for $1-1.5 billion each, and they will need to to cover the $20billion that the project will owe investors at the end of development.

So if the vehicle costs the customer $1.5billion, then they will not be concerned about paying $100million for a launch sled, particularly if they only need to pay for a fraction of it due to cost sharing between several customers.

[simonbp]

As this is the Skylon thread, I should post their most recent animation. Turn up the sound; it's perfect...

http://www.reactionengines.co.uk/Skylon_16.9_450.mov

[RobLynn]

Rob, what you are suggesting is basically HOTOL, which also used separate rocket and ramjet and a sled (although only up to 330m/s). I think you are suggesting rocket and ramjet at the back, this has severe centre of mass and centre of pressure problems over the flight portion of the envelope.

Mmm, I'm begining to appreciate the benefit of wing mounted engines.

When looked at in detail this has always been discarded in favour of conventional rockets or air launch.

A sled launch is essentially the equivalent of an air launch for a winged LV, though with much lower costs and potentially higher launch velocities.

Astronautics has this to say. "The final design had serious operational disadvantages and a small payload. The only way the designers could continue to claim to put a reasonable payload into orbit was by specifying untried and speculative structural materials". I have heard Alan Bond ( one of HOTOL's designers) say something similar.

That is interesting - obviously COG/COP issues weighed heavily - not having the fuel tankage and payload evenly distributed about the vehicle COG.  From Astronautix the RB545 had poor air breathing Isp ~700s, really not in the same league as an LH2 ramjet with 2-4000s.
http://www.astronautix.com/engines/rb545.htm

Given the way COG has been addressed in Skylon and the seemingly low Isp of the RB545 I do question if Hotol unworkability is still a lesson applicable to a sled launched Skylon with ramjet engines.

Regardless of the workability of ramjet engines (I'm agnostic, but obviously interested in learning more) it does seem that sled launch could be an excellent way to save some dry weight in the wings and landing gear - 80tonnes and 540km/hr each vs 20 tonnes and 230km/hr per wheel set.  It is also possible that it may also further optimisation of the engine size and weight by removing the requirement to produce 1g runway accelerations.

[MikeAtkinson]

Rob, I think you missed my point about sled launch not gaining you much. Just as in a conventional vertical take off rocket fuel burned during hold-down, while the engines come up to full thrust, does not count to GTOW, so the fuel burned by Skylon on the runway does not affect its flight.

All that Skylon requires for the acceleration along the runway (as opposed to sled launch, for the same take off velocity) is slightly larger tanks and a more robust undercarriage. Most of the extra mass is in the undercarriage, but this undercarriage is desirable for many abort scenarios.

Those who have looked at sled launch in the past have usually given up and turned to air launch instead, that has all the advantages of a sled and none of its disadvantages. That is what HOTOL did, Interim HOTOL (also called HOTOL 2) used air launch from an An-225. Air launch is not easy however and it has proved difficult to design a cheap air launched system.

[RobLynn]

Rob, I think you missed my point about sled launch not gaining you much. Just as in a conventional vertical take off rocket fuel burned during hold-down, while the engines come up to full thrust, does not count to GTOW, so the fuel burned by Skylon on the runway does not affect its flight.

My calculations suggest that a Mach1.3 sled launch would add 2-2.5 tonnes of payload, worth conservatively $2-2.5million per flight.

I found some gold:
http://www.reactionengines.co.uk/downloads/C1_trajectory_output.xls
This has a whole lot of numerical data on the Skylon C1 launch and reentry trajectorys - from it can calculate thrust, ISP, tank volumes and other useful performance info.
Interesting data extracted so far:
Airbreathing LH2 fuel use: 41800kg = 588m³
Rocket Fuel use: 175300kg = 518m³ at assumed 6:1 mix ratio
GTOW 275000 kg , LEO 56700 kg , estimate about 42000kg for vehicle and 15000kg for payload and 1200kg for brake cooling water dumped after liftoff.

I am starting some analysis of ramjet using this data, but in meantime the case for sled launch:

By the time skylon hits Mach 1, 330m/s it is 2 minutes into flight at 3400m altitude and has burnt 6960kg =100m³ of fuel, out of 1100m³ total fuel volume.  In energy terms that altitude+speed is about the same as Mach1.3 (420m/s) at ground level.

Fuel tanks weigh about 1000kg per 80m³ (based on Shuttle SLWT), with of course a bit extra for TPS and structure etc to support it, so that 100m³ burned represents about 1250-1500kg of vehicle dry weight that can be saved.

Landing gear is at most optimistic about 1.5% of landed weight - or 450kg for Skylon, however given that it needs to support and brake a 275tonne GTOW launch abort at up to 1g from 150m/s there is no way that it will weigh less than an insanely optimistic 1500kg (0.75%GTOW), 6 space shuttle tyres alone would weigh 600kg, and they operate at lower speeds and loads.  So at least a 1000kg saving can be had by sizing landing gear for landing only, though realistically it will be more than that.

Wings could probably be made smaller to save further weight, but lets ignore that - handling, reentry and landing might dictate no change.

So 2000-2500kg of dry mass saving from a Mach 1.3 sled launch, all of which can be turned into extra payload. At a very conservative $1000/kg payload that is $2-2.5 million more revenue per launch, or $4-500 million extra revenue over the 200 flight life of the vehicle.

Given such a huge payoff it is pretty hard to argue against sled launch.

[lkm]

Skylon is aiming more for 133-666 $/kg depending on payload value. And I very much doubt that that the SLWT is a good guide to mass, I  believe skylon uses a low pressure AL tank, no fancy AL-Li. Also I doubt Shuttle landing gear is a good guide to Skylon  either given the different operating regimes, different dry masses and 40 years of development.
If you're going to sled launch a Skylon it's going to be a stock Skylon as much as possible just like coventional aircraft are adapted for catapult launch on carriers, there's no sense destroying the economics of Skylon just for a little extra payload.
At least that way your sled can start out slow and ramp up as payload growth is required.

[93143]

Skylon's landing gear has an interesting feature - 1200 kg of water as a heat sink, that gets dumped right after a successful takeoff.  This is for a fully-loaded abort.  Coming down after a mission, the gear can be light.

As for the sled, HOTOL had one.  Skylon doesn't.  I can't recall whether I've seen a detailed rationale for this change, but it seems clear to me that there was one.  Perhaps you should be a little less confident...

One thing that strikes me immediately is that a launch sled is a pain logistics-wise...

...

Another point is that the SABRE 3 (the engine used in that spreadsheet) made substantial compromises to its airbreathing performance to support the rocket mode.  The SABRE 4 is a redesign that makes Skylon a much better airbreather, without impacting rocket performance.  This is part of how Skylon D1's performance went up ~50% over C1 while the GTOW only increased by 18%.

[MikeAtkinson]

Rob, I am puzzled. Is your idea of sled launch that no fuel would be expended until take off?

The engines would have to be at full thrust during a sled launch, as they will need to be checked out before take-off. Naturally this is going to be difficult with ramjets, their operation probably cannot be verified until the sled has reached a considerable speed.

From the spreadsheet we can see that 1613kN of thrust is needed at takeoff plugging that into the figures we used earlier T/W of a ramjet of 10 at Mach 1, we get total ramjet mass of 16.4 tonnes, add in the mass of the rocket(s) and this will be greater than the 19.6 tonne mass of 2 SABRE engines.

This is assuming that ramjet Isp is the same as SABRE during all stages of flight. On the contrary it is likely to be less, if ramjet Isp is in the range 1000-1500 and SABRE in the range 1500-3200 then ramjet fuel use is going to be twice as much as SABRE or ~80 tonnes.

Without active cooling it is probably impossible to get ramjets up to Mach 5.4, active cooling would adversely affect ramjet T/W. A lower transition to pure rocket leads to much more fuel/oxidiser use due to the exponential nature of the rocket equation.

The tankage for the extra fuel and greater mass of the engines is considerably greater than saved due to sled launch. Without doing detailed calculations it is hard to be sure, but I suspect that sub-sonic sled launch would not lead to a positive payload and Mach 1.3 sled launch to a payload approximately half that of Skylon for the same size of vehicle.

[MikeAtkinson]

I also think you grossly underestimate how difficult a supersonic sled is going to be.

The Bloodhound SSC uses at least two techniques (negative lift and a very small clearance to the ground) that are inappropriate for a spaceplane.

The sonic boom will probably make co-locating with any current facility infeasible as well.

[Cherokee43v6]

Skylon does not intend to be the 'operator of record' for this vehicle.  They intend to be Airbus/Boeing selling these ships to whoever is willing to operate them.  Tossing in a mult-million dollar, single location launch infrastructure is anathema to that goal.

In this far off future where there may be competitors able to sell for $1000/kg (ie SpaceX lowest possible price assuming some re-usability).

If each vehicle lasts for the estimated 200 flights @15000kg/flight then that is $3 billion revenue per vehicle.  If operational costs are $5million per flight then they could be selling the vhicles for $1-1.5 billion each, and they will need to to cover the $20billion that the project will owe investors at the end of development.

So if the vehicle costs the customer $1.5billion, then they will not be concerned about paying $100million for a launch sled, particularly if they only need to pay for a fraction of it due to cost sharing between several customers.

Rob...

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

Besides.  Horizontal launch's purpose is to get rid of the launch gantry, not build one ten miles long in an isolated area where the sonic booms will be tolerated.

[93143]

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

I'm pretty sure you made that up.

[Cherokee43v6]

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

I'm pretty sure you made that up.

I'm a businessman... Even in 10 - 15 years, there's bound to be a larger market for fast document, light cargo delivery than orbital cargo delivery.

Skylon's design is neat, in that you can trade fuel for cargo and have a suborbital ship.  All you have to do is think about the capabilities and extrapolate based on what is in demand now.

[MikeAtkinson]

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

I'm pretty sure you made that up.

He did, Skylon is for acceleration missions to LEO, LAPCAT (http://www.reactionengines.co.uk/lapcat.html) is Reaction Engines Ltd idea for a long distance transport. This thread is for Skylon.

[Cherokee43v6]

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

I'm pretty sure you made that up.

He did, Skylon is for acceleration missions to LEO, LAPCAT (http://www.reactionengines.co.uk/lapcat.html) is Reaction Engines Ltd idea for a long distance transport. This thread is for Skylon.

My point was that Runway takeoff SSTO gives you everything below that as well, whereas having a massive launch infrastructure that requires the vehicle to be at mach speeds at ground level does not.

[RobLynn]

Skylon is aiming more for 133-666 $/kg depending on payload value. And I very much doubt that that the SLWT is a good guide to mass, I  believe skylon uses a low pressure AL tank, no fancy AL-Li. Also I doubt Shuttle landing gear is a good guide to Skylon  either given the different operating regimes, different dry masses and 40 years of development.
If you're going to sled launch a Skylon it's going to be a stock Skylon as much as possible just like coventional aircraft are adapted for catapult launch on carriers, there's no sense destroying the economics of Skylon just for a little extra payload.
At least that way your sled can start out slow and ramp up as payload growth is required.

I used SLWT as I thought that would produce the most conservative numbers (ie biased against sled launch).

A stock skylon does not make sense for sled launch as you are still paying exactly the same penalty in weight for stronger landing gear hard points, landing gear size, water tank, extra LH2 tanks (100m³), the saving made would be very small - the difference in lighter wheels and struts in the landing gear.

What I hadn't considered is the higher dynamic pressure for sled launch - probably double, but I don't know what if any impact that might have on aeroshell weight, possibly none possibly lots.

I am sure that they will initially charge whatever they can get away with for launches - undercut the competition but only just.  But ultimately the recurring costs of a sled launch should be less than $100k, so the economics will continue to pay off.

[RobLynn]

Skylon's landing gear has an interesting feature - 4 tonnes of water as a heat sink, that gets dumped right after a successful takeoff.  This is for a fully-loaded abort.  Coming down after a mission, the gear can be light.

As for the sled, HOTOL had one.  Skylon doesn't.  I can't recall whether I've seen a detailed rationale for this change, but it seems clear to me that there was one.  Perhaps you should be a little less confident...

One thing that strikes me immediately is that a launch sled is a pain logistics-wise...

1200kg water according to that spreadsheet and other literature I've seen, but yes a good idea - although what is the mass penalty of that water cooling system?  the brakes still have to be able to arrest a 50000kg vehicle on landing.

I can't see why a sled would be a pain for logistics - the vehicle is precisely positioned in a cradle for fuelling and servicing, the landing gear does not need to be lifted in flight, simplifying that system.  If driven by a winch the sled should be very low maintenance - though a steam rocket might be similarly simple, (150 Isp).

Another point is that the SABRE 3 (the engine used in that spreadsheet) made substantial compromises to its airbreathing performance to support the rocket mode.  The SABRE 4 is a redesign that makes Skylon a much better airbreather, without impacting rocket performance.  This is part of how Skylon D1's performance went up ~50% over C1 while the GTOW only increased by 18%.

Good to know, I was wondering why the sabre Isp was so much worse than a ramjet.

[RobLynn]

Rob, I am puzzled. Is your idea of sled launch that no fuel would be expended until take off?

The engines would have to be at full thrust during a sled launch, as they will need to be checked out before take-off. Naturally this is going to be difficult with ramjets, their operation probably cannot be verified until the sled has reached a considerable speed.

A sabre engine will obviously be wasting far more fuel in starting and stabilising on the runway.  Sabre uses 100kg/s = 1.4m³ from high pressure insulated LH2 tank per second - and an involved start up procedure that will likely have a chill down process and delays as air compressing turbomachinery spools up this is unlikely to take less than 10-20 seconds, and at that could be on the order of 10m³ LH2 fuel use (or more).  A Ramjet by contrast takes probably 2-3 seconds of ignition and checkout (~160kg/s for 1600kN) that is really of no concern when all it is costing is a few hundred litres of kerosene - 0.6m³ total of simple uninsulated and probably low pressure fuel tank.

From the spreadsheet we can see that 1613kN of thrust is needed at takeoff plugging that into the figures we used earlier T/W of a ramjet of 10 at Mach 1, we get total ramjet mass of 16.4 tonnes, add in the mass of the rocket(s) and this will be greater than the 19.6 tonne mass of 2 SABRE engines.

This is assuming that ramjet Isp is the same as SABRE during all stages of flight. On the contrary it is likely to be less, if ramjet Isp is in the range 1000-1500 and SABRE in the range 1500-3200 then ramjet fuel use is going to be twice as much as SABRE or ~80 tonnes.

In looking at the sabre design I can see no reason why it would be lighter than a rocket+ramjet, when you look at all of the extra hardwear that sabre needs in the form of large heat exchangers brayton cycle machinery, air compressers etc. And remember that the external case/bypass duct that is always flowing air in the sabre is exposed to just the same temps as the ramjet, excepting the combustor and nozzle of course.

But this is where my handwaving falls over, without a specific design I cannot tell you how much a ramjet might weigh.  However simple analysis using the spreadsheet stated sabre inlet performance and assuming 90% efficiency in combustor and nozzle for a ramjet has the ramjet producing more thrust than the Sabre engine above Mach1, while Isp is on average almost 2x that of the Sabre (4000s ramjet vs 2300s sabre, both on LH2)  I admit that this is a very primitive analysis and so don't place too much faith in it.

Overall what matters most is of course the combined mass of rocket+ramjet+kerosene tank vs Sabre + LH2 tanks.

For the sabre I estimate the airbreathing portion 580m³ LH2 tanks weigh 7000kg (1000kg per 80m³ as for Shuttle SLWT) , while engines sabre engines are apparently 19600kg for about 27000kg total.

Using 60:1 thrust to weight rocket and 3600kN the rocket weighs 6 tonnes.  The integrated impulse for the airbreathing sabre engines from Mach 1 up to rocket ignition is about 780MN/s, now for a kerosene ramjet isp of 1200s average that would require about 780000000/9.81/1200 = 66000kg of kerosene, or  abourt 80m³=1000kg of tankage.

so 6000kg rocket + 1000kg kerosene tanks would allow ramjets weighing 21000kg while equalling the dry mass of the skylon with sabre, in reality as you suggest the ramjets are likely to weigh a lot less than that. 

Without active cooling it is probably impossible to get ramjets up to Mach 5.4, active cooling would adversely affect ramjet T/W. A lower transition to pure rocket leads to much more fuel/oxidiser use due to the exponential nature of the rocket equation.

The tankage for the extra fuel and greater mass of the engines is considerably greater than saved due to sled launch. Without doing detailed calculations it is hard to be sure, but I suspect that sub-sonic sled launch would not lead to a positive payload and Mach 1.3 sled launch to a payload approximately half that of Skylon for the same size of vehicle.

The ramjet combustors and nozzles are exposed to roughly the same temperatures regardless of speed - though the combustor will not have inlet air cool enough to keep it's walls cool.

The Sabre engine with it's bypass ducts always flowing air is exposed to temperatures and pressures just as high as a ramjet, prior to the combustor and nozzle and passive cooling is sufficient in these areas even at Mach 5+ (1120°C peak for a few 10's of seconds from spreadsheet) using appropriate superalloys.  I agree the combustor and nozzle may need active cooling but if cooling is required the fluxes and pressure are low and so relatively easy, also there is almost 100kg/s of kerosene fuel available for cooling.

Given the short life required - 30 hours for 200 flights there may be a better alternative:  Ceramics
like SiC are unsuitable for gas turbines because high pressures + moisture lead to unacceptable erosion over required 20-30000 hour life required particulalrly given tight tolerances and aerodynamic intricacy.  Also flakes or brittleness is a problem in rotating turbines.  But a ramjet has very low air pressure (2-3bar) and so erosion rates will be tiny, while large dimensions mean loosing a mm or 2 is irrelevant to flow geometries.  SiC can be used in such an environment at temperatures up to 1900°C, and so a combustor and nozzle with air film cooling would be pretty straight forward - even if that cooling air is at 1100°C.

It seems you are assuming a very low Isp for an LH2 ramjet, however it is more like 3500s Isp for LH2 ramjet than the 1400s typical of a kerosene ramjet owing to higher calorific value of hydrogen (120MJ/kg vs 43MJ/kg).  If you don't believe me check out diagram on page 4 of following paper with Isp 4500-3000 s at Mach3 to Mach 6 as engine runs in ramjet mode:
http://smartech.gatech.edu/jspui/bitstream/1853/8416/1/aiaa_99-2354.pdf

That is far higher than the < 2000 Isp average that the sabre manages over the Mach3-Mach5.4 (from spreadsheet)

As I showed above a kerosene ramjet (or perhaps propane for simple self pressurisation) creates a massive saving in tank volume, so regardless of higher fuel load the vehicle dry weight will drop greatly, giving more payload to orbit.

[RobLynn]

I also think you grossly underestimate how difficult a supersonic sled is going to be.

The Bloodhound SSC uses at least two techniques (negative lift and a very small clearance to the ground) that are inappropriate for a spaceplane.

The sonic boom will probably make co-locating with any current facility infeasible as well.

Bloodhound SSC cannot be positively located on a rail, and is running on an unprepared surface.  As it stands this is pretty well developed technology as rocket sleds have been operated up to mach 8.5

The Skylon on sled is clamped down and released once engines are lit.

I agree noise will be an issue, but there are a lot of uninhabited desert areas are available for landing strips and rails

[RobLynn]

The initial market for these vehicles is not going to be to orbit.  Yes, there will be some of that, but the primary use for this type of vehicle is going to be antipodal transport of cargo (and possibly people).  The fact that it can do orbit is all well and good extra, but TARDIS Express is a far more likely business model for this than servicing orbital facilities...

Besides.  Horizontal launch's purpose is to get rid of the launch gantry, not build one ten miles long in an isolated area where the sonic booms will be tolerated.

4 miles should be enough
There would need to be a lot of Skylons sitting around awaiting deployment, and you would also need massive LH2 infrastructure everywhere you might want to go.

Also commercial supersonic flight over land or even near land has long since been banned in most inhabited parts of the world, Nimbys are everywhere.

Given that the payload would need to be delivered to a special skylon launch site probably by plane why not just keep that plane flying to wherever it is needed for 1/10th the price and probably only a few more hours?  The messing around in loading and unloading, customs checks etc is where the majority of the time is lost anyway.

I really cannot think of any time critical cargo that would be able to pay for even an optimistically low $5 million flight to save at best perhaps 5 hours.  Skylon is just too big and expensive to operate for point to point.