Author Topic: The Reaction Engines Skylon Master Thread (5)  (Read 441205 times)

Offline Hanelyp

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #700 on: 10/18/2015 08:33 PM »
My impression is that while Sabre 4 uses less hydrogen than earlier designs, it still uses an excess of fuel for cooling.  For the same amount of cooling methane will save little if any tank volume, and greatly increase fuel weight.

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #701 on: 10/18/2015 11:20 PM »
The last few pages of this thread (in which I've been lurking for years) have got me thinking. It seems as if the cooling requirements have decreased significantly since the change in engine design from sabre 3 to sabre 4, so much so that frost control is no longer an issue.

My question is if this decrease in cooling requirements could allow for a change to methane as a the cryogenic fuel. This could significantly reduce the size and weight of skylon's tanks (and reduce the developmental costs and headaches from dealing with liquid H2). Of course a smaller skylon would make re-entry more difficult - if you believe reaction engines understanding of atmospheric re-entry.

I know that the trades around reduced ISP and increased propellant density are complex. However there does seem to be a recent trend, given today's tank building expertise and from lessons learnt from designs like, for example, the x33 to compromise with a lower ISP fuel to gain a benefit from the reduction in weight of tankage and insulation etc.

Over a 200 cycle lifetime there would also probably be fairly significant cost savings not only in terms of the cost of fuel but also the systems and infrastructure required to store and pump the fuel at the various launch sites.

Is this an obviously answerable question? Or is it something that could go either way and would require a complete vehicle redesign and modelling to answer?

In any event I thought it was something worth thinking about - especially given how reaction engines have demonstrated great flexibility in their thought processes by deciding to ditch frost control - a technology which they have spent the better part of twenty years perfecting. It takes a lot of strength of will and character to ditch the fundamental, truly unique part of their business in order to optimise the final vehicle. If they are willing to ditch frost control perhaps they would be willing to ditch liquid hydrogen?

I've actually looked at this a bit, as much as I'm able to. I was interested in the possibility of using gelled hydrogen to improve volumetric isp, that's mixing liquid hydrogen with a second denser fuel to create a cryogenic slush that behaves like a solid in the tank but like a liquid when pumped.

I considered propane, methane and ammonia gels and found that optimised for payload fraction hydrogen performs better than either of the hydrocarbon gels but that that an ammonia/hydrogen gel might actually improve performance. However I'm not sure  if NH3/H2 gels are possible or how they behave.

Offline 93143

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #702 on: 10/19/2015 08:34 AM »
Keep in mind that Skylon's aspect ratio is already below the minimum-drag point, and it can't get any narrower because of the payload bay spec.  And since it airbreathes to Mach 5, drag losses are actually fairly substantial - even now I don't think the tankage uses all of the available space (don't quote me on that).  I'm not sure a change to a denser propellant would do much, other than reduce the Isp and perhaps complicate the system.

I might be wrong - how in-depth were your calculations?
« Last Edit: 10/19/2015 08:48 AM by 93143 »

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #703 on: 10/19/2015 10:21 AM »
Keep in mind that Skylon's aspect ratio is already below the minimum-drag point, and it can't get any narrower because of the payload bay spec.  And since it airbreathes to Mach 5, drag losses are actually fairly substantial - even now I don't think the tankage uses all of the available space (don't quote me on that).  I'm not sure a change to a denser propellant would do much, other than reduce the Isp and perhaps complicate the system.

I might be wrong - how in-depth were your calculations?
Not nearly well enough to convincingly answer the question.
I calculated the engine performance using propep for each combination at incremental mixture ratios then calculated the propellant load for each one over the C1 trajectory then generated a structural mass using some guesstimated MERS created from the C1 and D1 data we have and the papers on Skylon structure.
Because I was more interested in vehicle size than payload I abandoned payload bay specs and maintained aspect ratio recalculating bay size accordingly using the prop load to calculate tank volume and mass and to size the aeroshell volume and surface area (modelled as a sears-haack body). I was also modelling for a smaller gross take off mass as the SABRE cycles I created for these prop combinations ran at a 3:1 ratio with air and so would be lower thrust.
So as you can see lots of places for me to go wildly wrong in my assumptions.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #704 on: 10/20/2015 07:14 AM »
There is the benefit that you've lost a fourth liquid system from the engine which operationally must mean lower maintenance costs.
True, and not to be underestimated. But Methanol, unlike say the Anhydrous Ammonia on the Shuttle boiler, is fairly easy to handle at room temperature and pressure, so I'd guess the saving would be limited.  That said simpler is usually better.
Quote
AFRL is also looking at vehicle concepts.
I missed that, AFAIK the focus was the SABRE 3 cycle on the engine. Any vehicle studies would have presumably used that as their baseline.
Quote
No, I'm talking about the engine nozzle, the patent for it makes it clear. Pintle or centrebody is what I've read it as in ED papers.
Quote
"19. A nozzle arrangement according to claim 1, further comprising an actuator arrangement that is arranged to move the second portion of the nozzle between the two positions. "
Anything is possible but this sounds more like one of those "cover all the bases" clauses that patent drafters like to use.


Looking at the drawings you posted you're talking about a movable combustion chamber within an outer combustion chamber.

As a way to throttle thrust this seems be a)Mechanically very complex b)Lots of high temperature and pressure engineering combined with lots of cryogenic engineering. c) Large forces would need to be exerted to move one chamber inside another while operating.

On the basis that REL likes to avoid unnecessary complexity I  think they will stick to throttling by valves on the propellants or hot drive gas (which is inert, rather than super heated steam) supplies. There may be non obvious benefits to moving chamber design, but they'd have to be very substantial to justify the design risk.


"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #705 on: 10/20/2015 06:05 PM »

No, I'm talking about the engine nozzle, the patent for it makes it clear. Pintle or centrebody is what I've read it as in ED papers.
Quote
"19. A nozzle arrangement according to claim 1, further comprising an actuator arrangement that is arranged to move the second portion of the nozzle between the two positions. "
Anything is possible but this sounds more like one of those "cover all the bases" clauses that patent drafters like to use.


Looking at the drawings you posted you're talking about a movable combustion chamber within an outer combustion chamber.

As a way to throttle thrust this seems be a)Mechanically very complex b)Lots of high temperature and pressure engineering combined with lots of cryogenic engineering. c) Large forces would need to be exerted to move one chamber inside another while operating.

On the basis that REL likes to avoid unnecessary complexity I  think they will stick to throttling by valves on the propellants or hot drive gas (which is inert, rather than super heated steam) supplies. There may be non obvious benefits to moving chamber design, but they'd have to be very substantial to justify the design risk.




It's definitely the outer nozzle that translates.
Quote
FIG. 3 shows a representative one of the nozzles 10 in the rocket mode. In this mode, the second nozzle portion 40 is positioned in the rocket position. In this position, the second nozzle portion 40 is positioned relative to the inner nozzle portion 30 such that the annular throat 50 is closed. In other words, the second nozzle portion 40 is translated to the right in FIG. 3 relative to the first nozzle portion 30. This is such that the generally frusto-conical sections of the two nozzle portions 30, 40 no longer overlap and instead form a contiguous diverging rocket nozzle similar in shape to a conventional rocket nozzle (although it will be noted that the cylindrical section 43 of the second portion 40 still overlaps the first portion 30).

But re-reading the patent it's clear that the airbreathing chambers are fixed relative to the rocket chamber so it can't throttle by varying the throat diameter while operating, somehow in my memory it had been ambiguous.

Offline A_M_Swallow

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #706 on: 10/21/2015 01:59 AM »
{snip}
If you read the patent on SABRE 4 you'll see that in this cycle the precooler exit temperature is designed to never fall below 400K with the bidirectional valve in the cycle directing helium around the precooler to maintain that. 

400K is 126°C (260°F) which is above the boiling point of water. Could nitrogen be used instead of helium at those temperatures?

Offline 93143

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #707 on: 10/21/2015 06:21 AM »
Could nitrogen be used instead of helium at those temperatures?

Maybe, but it would have to reliably not plug a heat exchanger with liquid hydrogen on the other side, even during startup and shutdown.  Sounds potentially dicey to me.

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #708 on: 10/21/2015 02:54 PM »
Could nitrogen be used instead of helium at those temperatures?

Maybe, but it would have to reliably not plug a heat exchanger with liquid hydrogen on the other side, even during startup and shutdown.  Sounds potentially dicey to me.
The liquid hydrogen heat exchanger cools the helium down to 50k so if it was nitrogen it would freeze.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #709 on: 10/22/2015 07:18 AM »
Could nitrogen be used instead of helium at those temperatures?

Maybe, but it would have to reliably not plug a heat exchanger with liquid hydrogen on the other side, even during startup and shutdown.  Sounds potentially dicey to me.
Indeed. N2 is cheaper but for a relatively small volume closed cycle function like this it seems to add a lot of complexity for a very limited gain. 

It mightmake more sense if they switched to Methane but then you take a hit on Isp in rocket mode, which REL have stated is where Isp really counts.

Kourou has on site LH2 facilities from the Ariane 5 programme (and it looks like they will be retained for the current Ariane 6 plan). My instinct is they will stay LH2
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #710 on: 10/22/2015 08:25 AM »
Reading the current report give a few more intriguing nuggets.

Quote
a change control board has been established and all of the requirements have been moved from paper
specifications to a computer based requirements tracking programme.
.....
The formal change control process was also required to enable compliance with the emerging
certification requirements.

The board consists of members of the Reaction Engines System engineering team and ESA personnel
representing the customers’ interests.

This suggests are REL are quite serious about firming up the design of Skylon and that any requirements that Skylon will have to meet will have an audit trail showing where they came from and why.

Note that ESA personnel are acting as customers to give the customer PoV. Ariane 6 remains the programme of record.

The SUS design has been revised

Quote
This design is shorter and lighter than previous versions increasing the allowable payload length from 4.4m to 8.6m and the reusable mode GTO payload from 6387kg to 7259kg. 


Which I think puts it on a level playing field with the Ariane 6 target.

The heavier SABRE design and reduced LH2 storage makes for a denser Skylon and it seems CoG limits the allowed AoA even more.

Quote
Both these factors were expected to increase the predicted skin temperatures. However the new
modelling also incorporated the results computational fluid dynamics analysis of SKYLON and in practice
the resulting temperatures similar to earlier C1 reentry models.

If they are referring to the fuselage temperatures from the engines during takeoff in the DLR CFD studies this suggests that they can keep skin temp in the C1 range by narrowing the allowed AoA, which it seems they have to do anyway.

On the foil blanket

Quote
The multi-foil blanket that lies between the aeroshell and truss structure has been looked at and a
new method of construction devised which makes its integration into the vehicle easier and more reliable.
Previously it was assumed that each foil layer would be laid one at a time. The new approach creates the
monolithic block of foils and spacers all of which are integrated in one piece.

This really surprised me as I'd always assumed these blankets were made as multilayer structures before being installed as complete units.

Making them off vehicle should simplify their mfg quite a lot. The only down side I can see would be if the edges are sealed they you have a thermal short cut from the front to the back faces. Obviously the bigger the pieces the fewer the number of these.

Another surprise was this.

Quote
An examination of the SKYLON power and data harness was started in April 2015. This project was
tasked to establish a realistic harness mass for the SKYLON airframe. This follows a late realisation that
the mass of the electrical and optical harness is likely to exceed the total mass of the units that it serves, and therefore it is an important component of system viability. 

This is quite extraordinary.

IIRC outside of the cutting edge low TRL stuff REL used scaled masses from the HOTOL programme for things like this.

This implies either the mass of LRU's to carry out the planned functions has gone down a lot in the years since HOTOL (plausible with the growth of MEMS sensors with an on chip CANbus interface for example) to the point where the cabling is now a significant fraction of the system mass, or so many more parameters have to be monitored the number of cables to carry this data has ballooned.

Historically ARINC and MilStd busses have been point to point with data rates in the 100kbs to 1mbs range. The newer 1773b is fibre optic and good to 20mbs and Spacewire to 400mbs. Of course now that 1Gbs star connected Ethernet is commonplace for offices and switches and routers are available in very small packages (some aerospace rated) is certainly an option. However given REL's aim to avoid cutting edge tech where possible I think they'll go with more (but not completely) conservative approaches.

While surprising I will note that so much of SABRE/Skylon has been tested in a computer I would expect this concern to be dealt with long  before any hardware is built.

"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #711 on: 10/24/2015 08:47 AM »
I understood REL's reluctance to be on making claims based on unproven technologies. SpaceX have demonstrated the use of networking technologies to reduce wiring complexity and weight.
Actually the use of Ethernet and small form factor industrial PC's (104 form factor, IE about 4 inches square) goes back to the Orbital Pegaus.

There are are recognized "Avionics Data Networking" standards that are good to at least 100mbs with full dual redundancy and controllable latency and timing variability .

The joker in the pack is radiation.

ELV's spend most of their time inside the Earths protective magnetic field. They don't complete a single orbit so don't have to face the radiation barrage of the South Atlantic Anomaly and they don't have a planned 200 flight life expectancy, which could cause hard errors to accumulate. The OBC's of the Dragon capsule are about the closest SX have come to this so far. AIUI these rebooted fairly regularly but there were enough of them running to maintain a working capsule at all times.  The most demanding systems will likely be the SUS, which will have to perform complex rendezvous motions after exposure to the radiation environment in near GEO orbit.

The fact comm sats operate for decades in this environment indicates the problems can be handled, at a price.

We will have to see wheather REL goes for a "rad hard by construction" design IE made on a rad hard process, like the 1750A's used by ULA, or a more rad tolerant by design approach using more mainstream parts.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online dchill

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #712 on: 10/24/2015 02:58 PM »
I understood REL's reluctance to be on making claims based on unproven technologies. SpaceX have demonstrated the use of networking technologies to reduce wiring complexity and weight.
Actually the use of Ethernet and small form factor industrial PC's (104 form factor, IE about 4 inches square) goes back to the Orbital Pegaus.

There are are recognized "Avionics Data Networking" standards that are good to at least 100mbs with full dual redundancy and controllable latency and timing variability .

The joker in the pack is radiation.

ELV's spend most of their time inside the Earths protective magnetic field. They don't complete a single orbit so don't have to face the radiation barrage of the South Atlantic Anomaly and they don't have a planned 200 flight life expectancy, which could cause hard errors to accumulate. The OBC's of the Dragon capsule are about the closest SX have come to this so far. AIUI these rebooted fairly regularly but there were enough of them running to maintain a working capsule at all times.  The most demanding systems will likely be the SUS, which will have to perform complex rendezvous motions after exposure to the radiation environment in near GEO orbit.

The fact comm sats operate for decades in this environment indicates the problems can be handled, at a price.

We will have to see wheather REL goes for a "rad hard by construction" design IE made on a rad hard process, like the 1750A's used by ULA, or a more rad tolerant by design approach using more mainstream parts.
Orion uses Time Triggered Gigabit Ethernet (TTGbE) for all its functions.  Here's an article that talks a bit about it (http://mil-embedded.com/articles/orion-avionics-designed-reliability-deep-space/#).  The TTGbE technology has now become an industry standard – SAE AS6802 and rad-hard ASICs are available.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #713 on: 10/25/2015 09:47 AM »

Orion uses Time Triggered Gigabit Ethernet (TTGbE) for all its functions.  Here's an article that talks a bit about it (http://mil-embedded.com/articles/orion-avionics-designed-reliability-deep-space/#).  The TTGbE technology has now become an industry standard – SAE AS6802 and rad-hard ASICs are available.
Noted but the standard only has 1 design win with only 1 test flight of hours.

I'll also note that in principle REL has access to ESA's experience of networks in deep space probes. These are units with years of exposure to deep space outside the Earth's protective magnetic field.

REL are aware their project is high risk so they aim to minimize risk wherever possible. Putting their faith in a bus design with such limited flight experience would be unwise. That position might change  if the design acquires more flight experience before they have to freeze the Skylon design.

Note that unlike the Shuttle Skylon is designed to be statically stable, needing (relatively) low actuator command rates to the flight surfaces.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline t43562

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #714 on: 10/26/2015 08:22 AM »
What's the main point of using ethernet though? Is it to reduce the number of cables or to increase the data rate?

I would imagine that you might have the essential flight controls and sensors on one low-latency, high reliability bus and the nice-to-haves on a high-throughput bus  e.g. cameras.

I also wonder how wireless sensor networks would function in space. e.g.
http://www.coventry.ac.uk/Global/05%20Research%20section%20assets/Research/Cogent%20Computing/PDFs%20for%20Publications/FMapNanotech.pdf
« Last Edit: 10/26/2015 11:21 AM by t43562 »

Offline tl6973

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #715 on: 10/26/2015 12:15 PM »
What's the main point of using ethernet though? Is it to reduce the number of cables or to increase the data rate?

I read somewhere that it significantly reduces the overall mass of cabling required, in comparison to serial cabling. As for a reduction in the number of cables, I have no idea.

I would imagine that mass reduction, though important, is a secondary benefit in comparison to cost/complexity/ networking benefits

Online dchill

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #716 on: 10/26/2015 02:27 PM »
Lockheed uses wireless sensor networks for DFI on the JSF and has proprietary results on the mass and labor savings from that, along with the benefits for not needing new wall penetrations, etc.   We've shared some of that info with Dream Chaser but it could also benefit other programs.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #717 on: 10/26/2015 11:27 PM »
Lockheed uses wireless sensor networks for DFI on the JSF and has proprietary results on the mass and labor savings from that, along with the benefits for not needing new wall penetrations, etc.   We've shared some of that info with Dream Chaser but it could also benefit other programs.
Interesting. Harvesting engine heat to power the sensors is a neat idea. The fact they are packed deep within the engine, itself packed deep within the fuselage, probably makes this a very cost effective option for the JSF design.

However Skylon's highly accessible engine pods and (relative) ease of replacement would probably shift the cost balance. The real issues for this technology are likely to be a) Very limited operating experience b)Unknown radiation tolerance (while an F35 could theoretically find itself on a nuclear battlefield SkylonSABRE will be be exposed to higher levels of radiation as part of normal operation, although most of this will be on orbit when the main engines will be off.
c)ITAR issues. With what looks like being the first application of this tech being the JSF this will be on the "dual use" list if not outright under "munitions."

I think this is just too new and too loaded with red tape for a non US non defense company to use.

Part of the issue with existing systems is that point to point wiring needs 2 part connectors at either end of the cabling regardless of how long that cabling is. Since the all run in parallel that can mean a lot of connectors. The question is how much real redundancy does this buy? Moving to a proper bus IE a shared transmission medium, probably with 2 or 3 separate and separately routed channels eliminates a lot of that connector mass, while a paths capable of supporting 100s of mbs has at least 4x the bandwidth of all the 25 segments of Shuttle bus combined.

I think just moving to what is now standard practice in the industry (compared with the SoA when HOTOL was  designed) will reduce mass substantially.

REL does not appear to like novelty for its own sake. I suspect they will aim to to make the new systems as light as needed but avoid any cutting edge technology where possible. That said since the HOTOL design was last looked at in 1986 things which have a 10 year operating history will still be 20 years more advanced than HOTOL.

[EDIT The networking architecture of the recently launched LISA pathfinder mission would seem to be a good model to study as this will be in a radiation environment relatively close to Earth (no dives into the upper Jupiter atmosphere for example) but well outside it's protective magnetic field. Spacewire seems to have been adopted as the hardware standard across all main space agencies ]
« Last Edit: 10/27/2015 09:53 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline A_M_Swallow

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #718 on: 10/27/2015 07:04 PM »
{snip}
I think just moving to what is now standard practice in the industry (compared with the SoA when HOTOL was  designed) will reduce mass substantially.

REL does not appear to like novelty for its own sake. I suspect they will aim to to make the new systems as light as needed but avoid any cutting edge technology where possible. That said since the HOTOL design was last looked at in 1986 things which have a 10 year operating history will still be 20 years more advanced than HOTOL.

[EDIT The networking architecture of the recently launched LISA pathfinder mission would seem to be a good model to study as this will be in a radiation environment relatively close to Earth (no dives into the upper Jupiter atmosphere for example) but well outside it's protective magnetic field. Spacewire seems to have been adopted as the hardware standard across all main space agencies ]

In which case REL will be able to buy off the shelf TRL 9 Spacewire components. Obsolescence can be handed by using an interface compatible part; either at the chip or circuit board level.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #719 on: 10/28/2015 10:26 AM »
{snip}
I think just moving to what is now standard practice in the industry (compared with the SoA when HOTOL was  designed) will reduce mass substantially.

REL does not appear to like novelty for its own sake. I suspect they will aim to to make the new systems as light as needed but avoid any cutting edge technology where possible. That said since the HOTOL design was last looked at in 1986 things which have a 10 year operating history will still be 20 years more advanced than HOTOL.

[EDIT The networking architecture of the recently launched LISA pathfinder mission would seem to be a good model to study as this will be in a radiation environment relatively close to Earth (no dives into the upper Jupiter atmosphere for example) but well outside it's protective magnetic field. Spacewire seems to have been adopted as the hardware standard across all main space agencies ]

In which case REL will be able to buy off the shelf TRL 9 Spacewire components. Obsolescence can be handed by using an interface compatible part; either at the chip or circuit board level.
Provided it uses Spacewire, which I think is likely.

Note that since HOTOL never got into detail design both SABRE and Skylon will be clean sheet in this regard.

The challenge will be to balance what is needed with what you could have, swamping the system with data which is not strictly necessary to carrying out the core tasks. The classic mistake in this area was during Shuttle testing where there were issues around the tail flap. Unfortunately the instrumentation engineers had only budgeted about 1 sample per second for this to measure angle ( I think this was it could only be commanded to change angle quite slowly). However aerodynamic forces were large and the actual movement (as forced by the airflow) could be much faster.

In principal modern bus speeds should mean there will always be spare capacity to increase data rates if something needs faster sampling below the hardware design limit of the ADC. More precision will likely need a new converter board.

But at the back of my mind I'm thinking "640KB, more than enough for any  computer program home users will ever need"  :( .
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

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