Author Topic: The Reaction Engines Skylon Master Thread (5)  (Read 422018 times)

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1380 on: 03/21/2016 07:28 PM »
@93143
I totally missed that the graph was referring to Sabre 4. Thanks for pointing that out. I know we all pretty much guessed that they were going to go with Sabre 4, AFAIK no official announcement has been made yet, but I guess going from that graph the decision has been made for Sabre 4.

It is interesting that in that talk he made no mention of the fact that the frost control system was probably not going to be needed and mentioned that one person nearly let out the secret of it without pointing out that the patents are public and the secret is revealed anyhow.   So why have the talks have continued following the "SABRE3" story?  One reason could be that it allows them to give an interesting talk, impress everyone etc without giving away too much about their current train of thought.

With regard to publicising talks, I can well imagine that they are not interested in publicity *that much* and are doing them for the money. If so it's a bit sad. I think fanbois are future shareholders :-)



The most likely reason that REL is giving these public talks now is simply recruitment, as was made clear in this presentation they're hiring 2-3 people every month, and in response to a question the audience was directed to the website for current job openings.
If you consider from REL's perspective they need to hire several hundred skilled engineers across a variety of disciplines  over the coming years and giving these lectures on a regular basis before a relevant audience does a lot to get the word out to potential employees.

Offline A_M_Swallow

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1381 on: 03/21/2016 10:34 PM »
If there are problems shrinking the engines a simpler vehicle could have a single engine. This may mean a tiny payload to LEO (e.g. Falcon 1 Vs. Falcon 9) or a sounding rocket (e.g. Masten Xombie). Both options allow testing of reentry.

Offline Hanelyp

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1382 on: 03/22/2016 02:00 PM »
As far as going single engine, it doesn't seem beyond consideration to divide the Sabre engine into an intake / turbopump assembly mounted centerbody, and rocket combustion chambers with nozzles in wing pods.  But that would require high pressure plumbing between the separated pieces.

Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1383 on: 03/22/2016 02:47 PM »
Original reply was lost in space/time :)

*REL itself, while rightly pointing out that the Skylon is DESIGNED for SSTO operations and is more "efficient" at that design point given it's design assumptions, explains that the Skylon AS DESIGNED is capable of and could be used as the suborbital first stage of a two-stage mission. It's right there in the Skylon Users Manual cited earlier. Page 8-9 give orbital figures while page 10 is for suborbital deployment. Note in either case for anything other than LEO all payloads have to include propulsion to deliver to their required destination. As noted above that's somewhere around 15mt to 18mt for Skylon delivery to orbit but the guide points out that Skylon can deliver up to it's maximum STRUCTURAL load of 30mt in a suborbital, two stage launch with payload supplied propulsion.  :)
That's the C2 manual, it isn't in the D1 manual, and was (if I recall correctly) disavowed by the REL team

It is in fact but still informative. If REL "disavowed" it they may want to take it off their website :)

Actually comparing both is instructive in and of itself:
C2: http://www.reactionengines.co.uk/tech_docs/SKYLON_User_Manual_rev1-1.pdf
D1: http://www.reactionengines.co.uk/tech_docs/SKYLON_Users_Manual_Rev_2.1.pdf

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- but I personally suspect it's more because such activity is more the nailbiting style operations of vertical launch vehicle instead of the dull airline style operation they aim to achieve than for any show stopping reason.

Actually there was something of a "show-stopper" in the operation description; The Skylon had to be TOWED home after the suborbital mission which would indicate it used all its propellant getting the 30mt to suborbital velocity.

Note in comparing the above manuals that the current D1 finally gets to a full 17mt "payload" by several fudges such as lower orbital altitude and mass capability in that the vehicle is now capable of handling less than 20mt overall with a specification of a maximum of 17mt to LEO (600km as opposed to 800km in the C2) shown.

Nothing 'wrong' with this in any way it's refinement of the Skylon design, but it does point out the issues being discussed about SSTO vehicles.

Quote
Take out the SOMA, fuel cells and heavily insulated orbital tankage, lengthen (and strengthen?) the cargo bay and add some batteries you've got a reusable first stage to a TSTO launch vehicle. Perhaps slap a conventional jet engine in the SOMA hole and some inlet ducting to allow RTB.

No the Skylon as designed isn't going to work for a first stage per-se. As noted even if you pull out the cited systems you still need to radically restructure the air/spaceframe to handle the new loads which is going to no longer make it a "Skylon" anyway.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Online adrianwyard

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1384 on: 03/22/2016 03:37 PM »
As far as going single engine, it doesn't seem beyond consideration to divide the Sabre engine into an intake / turbopump assembly mounted centerbody, and rocket combustion chambers with nozzles in wing pods.  But that would require high pressure plumbing between the separated pieces.

Is running that plumbing a show-stopper? As I think about the single SABRE integrated into a fuselage I can see a couple of benefits compared to Skylon in addition to the challenges. With Skylon the wings need to be strong enough to support the SABREs on the ground, and any loads the SABREs would apply when flying/gliding during turbulence. More strength often means more mass. Their shape is presumably driven and even compromised by this - shorter wingspan is better for loads.

The wings on an integrated SABRE/fuselage design can be shaped and sized for the aerodynamics alone.

You also have a much simpler, more conventional shape for re-entry. (Although we've heard from REL that Skylon's unusual shape can re-enter just fine.)

I'm confidently guessing drag would be lower too.

So picture something like super-sized X-37B SSTO that takes off and lands on a runway. Throw in the V-tail from the X-37 and FASTT design for good measure so you don't need the transpiration cooled canards.

Payload would be small compared to the twin SABRE Skylon, but perhaps a runway launched, responsive, reusable spaceplane would interest the USAF? Sort of like a next gen X-37.

But all of this does hinge on the practicality of running that plumbing.

Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1385 on: 03/22/2016 03:39 PM »
True. I think SSTOs would have a much stronger case if we had space tugs.

However, with electric propulsion now being available to raise satellite orbits...
The biggest change in REL's development plans was the introduction of the Skylon Upper Stage, which is a reusable space tug

Er, and "upper-stage" has ALWAYS been implicit in the design of Skylon you DO understand that right? :) Skylon is ONLY capable of reaching LEO so it would have been useless for it's "designed" job of delivering commercial satellites if it could not put them into the proper orbit(s) as required.

The required delta-v from LEO (800km with the C2 or 600km for the D1) was always implied to be part of the "payload" mass to LEO carried by the Skylon. I think that was always one of the biggest confusions.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline The Amazing Catstronaut

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1386 on: 03/22/2016 03:44 PM »

The required delta-v from LEO (800km with the C2 or 600km for the D1) was always implied to be part of the "payload" mass to LEO carried by the Skylon. I think that was always one of the biggest confusions.

Randy

Which in-turn leads to a smaller true payload. Shuttle also had the capability to have an extra upper stage as payload capability, if I'm reading this thread right.
Resident feline spaceflight expert. Knows nothing of value about human spaceflight.

Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1387 on: 03/22/2016 04:06 PM »
Secondly, there is what Zurbin is arguing against: let's do multiple middle steps. Let's push the final goal so far away that till this time will come, everything will change.

How's that worked out for Zubrin so far? He's now notorious for handwaving any problems in his own proposals while attacking any alternatives, to the point that I doubt anyone except a few rusted-on fanbois¹ take him at all seriously any more.

The most he's achieved is to reduce support for other technology development programs.

¹ Of course, one of those fanbois now owns his own rocket company. But ironically he practices the iterative development method I prefer.
That's not fair. What progress NASA achieved in their goals to go to moon and Mars and... Other tech programs, like VASIMIR, which is, as far as I read abut it, over promised even in theory. Additionally that was the goal of Zurbin to "don't waste resources on unknown purpose technology". Addionally that he is little aggressive and unfriendly in manner of his speeches have nothing to do with validity of his arguments.

Zubrin's more than a bit off-topic for this thread, as Skylon is one of those "technology development programs" he doesn't like because it doesn't get anyone (in his opinion) closer to Mars and never mind how much constant access does to ANY space program. But being honest Zubrin's trope has always been "don't waste resources on any ideas but mine" and he has made no bones about it. The fact that he is aggressive and unfriendly hasn't helped matters nor has the fact that he can't get everything but his exact ideas taken up as the one true way to Mars which makes him even more so.

Zubrin's goal sound laudable, at first, but the more you try to get into the details and the more questions you ask the less it seems so. Apollo is exactly the WRONG model to wish to emulate for a space program as it was a "one-trick" pony that did it's job but left very little to follow on and Zubrin simply won't except that HIS concept might have the same inherent flaw. The validity of his arguments beyond some of the more common sense ones, (ISRU was actually brilliant but it was already under consideration AND building up a Cis-Lunar ISRU and support infrastructure was 2/3rds of the cost of the entire VSI which Zubrin condemns) is suspect because of the assumptions they are based on.
A fact Zubrin refuses to even consider.

But lets get back to the actual subject which is Skylon...

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1388 on: 03/22/2016 04:07 PM »
To preserve the CoG protecting features you'd need to put it on top of the payload bay, like a sort of V1 cruise missile.
But that would leave the issue of how  you open the bay to get the payload out.

Belly doors, obviously. (Loading would be via an elevator-pit.)

As JS19 notes pretty SOP for bombers, and really MOST aircraft ordinance is bottom mounted, as well as various air-launched X vehicles and such. And with good reason because it greatly simplifies the mass shift and aerodynamic separation problems  of the vehicles involved. The original RASCAL concept assumed a mostly exo-atmospheric release would allow a simplified top-launched second stage with light weight doors but subsequent study by Georgia Tech showed this wasn't true and the vehicle would still require bottom mounted doors. The basic Skylon design would have to be radically re-designed to accommodate such a system so, again, I doubt it would be worth trying to adapt it for the job.

Something more akin to the University of Strathclyde CFASTT-1 with a recessed or belly bay.
http://strathprints.strath.ac.uk/41933/1/Brown_et_al_Towards_Robust_Aero_Thermodynamic_Predictions_for_Re_Usable_Single_Stage_to_Orbit_Vehicles.pdf

But that all depends on if you have to assume the wingtips are the only place to put the engines to address the CoG issues. (IIRC CoG was ONE issue that the wingtip engines addressed but there where supposed to be others, such as shock-interactions and keeping the intakes mounted as far away from the body as possible to reduce the air flow equations)
Frankly, while engine exhaust impingement is concerning a LOT of concepts used the aft end of the vehicle as an expansion surface for under-mounted engine exhaust. REL put the engines where they did because they are engine designers and not actually airframe designers.

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but dropping off the excess mass of the booster stage and continuing the mission profile with a more efficient second stage that is fully reusable is as valid an assumption as not.

If you're using SABRE, then you've got that backwards. You'd have a reusable first stage (the Skylon-derivative), with an expendable all-rocket, vacuum-optimized upper-stage.

No reason to assume it would be expended since there would be incentive to bring in a fully reusable design if possible. A vacuum optimized, "bare-bones" stage was something the AF played with for quite a while and they pretty much always 'base-line' an expendable upper stage for planning purposes but they would be satisfied with a reusable upper stage vehicle if they can get it. The rather obvious point that's been made over and over again in their studies is that barring someone building them a custom stage for no-money they are going to have to accept compromises in the design and expendable/reusable isn't as easy to call as was the assumption.

(There's a reason most recent studies showed an "X-37B-like" orbital stage boosted by various types of upper stages :) )

On cross-range, you folks are over-thinking it :)

I was pointing out that the Air Force (and hence AFRL) happens to like the ability to move both the launch point AND the launcher around to keep Op-For guessing as much as possible. You can't do that with a ground launch system and even most air-launch systems have issues due to range requirements and restrictions. (Yes in fact they can and do often apply to the military as well, especially in peace-time) SABRE "air-breath's" so that's probably part of what AFRL is considering.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline lkm

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1389 on: 03/22/2016 05:12 PM »
Original reply was lost in space/time :)

*REL itself, while rightly pointing out that the Skylon is DESIGNED for SSTO operations and is more "efficient" at that design point given it's design assumptions, explains that the Skylon AS DESIGNED is capable of and could be used as the suborbital first stage of a two-stage mission. It's right there in the Skylon Users Manual cited earlier. Page 8-9 give orbital figures while page 10 is for suborbital deployment. Note in either case for anything other than LEO all payloads have to include propulsion to deliver to their required destination. As noted above that's somewhere around 15mt to 18mt for Skylon delivery to orbit but the guide points out that Skylon can deliver up to it's maximum STRUCTURAL load of 30mt in a suborbital, two stage launch with payload supplied propulsion.  :)
That's the C2 manual, it isn't in the D1 manual, and was (if I recall correctly) disavowed by the REL team

It is in fact but still informative. If REL "disavowed" it they may want to take it off their website :)

According to Mark Hempsell it's out.
http://forum.nasaspaceflight.com/index.php?topic=34964.msg1272415#msg1272415

Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1390 on: 03/22/2016 05:21 PM »
As far as going single engine, it doesn't seem beyond consideration to divide the Sabre engine into an intake / turbopump assembly mounted centerbody, and rocket combustion chambers with nozzles in wing pods.  But that would require high pressure plumbing between the separated pieces.

Is running that plumbing a show-stopper?

As far as suggested above, probably. The SABRE is very much an "in-line" engine that would lose a lot of efficiency in trying to duct things around. A D-21 shape isn't really that bad of a basis for starting, it's been suggested for 2 or 3 combined cycle demonstrators that I can recall :)

Quote
As I think about the single SABRE integrated into a fuselage I can see a couple of benefits compared to Skylon in addition to the challenges. With Skylon the wings need to be strong enough to support the SABREs on the ground, and any loads the SABREs would apply when flying/gliding during turbulence. More strength often means more mass. Their shape is presumably driven and even compromised by this - shorter wingspan is better for loads.

(snip some other good stuff)

Skylon has been compared to being a hypersonic/orbital airship at times and really the current wing size is enough for fully loaded take off so getting back down is going to be rather "fluffy" in any case :) One thing to keep in mind is the Skylon basic shape is designed to be as easy as possible to fabricate. Avoiding complex shapes or curves in favor of simple (and well understood) rounded tanks. Once you start trying to wrap tankage around a single engine you have to move towards a more complex design which complicates fabrication and increases cost etc.

Trade offs and all that :)

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Offline Space OurSoul

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1391 on: 03/22/2016 05:24 PM »
As far as going single engine, it doesn't seem beyond consideration to divide the Sabre engine into an intake / turbopump assembly mounted centerbody, and rocket combustion chambers with nozzles in wing pods.  But that would require high pressure plumbing between the separated pieces.

Is running that plumbing a show-stopper? As I think about the single SABRE integrated into a fuselage I can see a couple of benefits compared to Skylon in addition to the challenges. With Skylon the wings need to be strong enough to support the SABREs on the ground, and any loads the SABREs would apply when flying/gliding during turbulence. More strength often means more mass. Their shape is presumably driven and even compromised by this - shorter wingspan is better for loads.

The wings on an integrated SABRE/fuselage design can be shaped and sized for the aerodynamics alone.

You also have a much simpler, more conventional shape for re-entry. (Although we've heard from REL that Skylon's unusual shape can re-enter just fine.)

I'm confidently guessing drag would be lower too.

So picture something like super-sized X-37B SSTO that takes off and lands on a runway. Throw in the V-tail from the X-37 and FASTT design for good measure so you don't need the transpiration cooled canards.

Payload would be small compared to the twin SABRE Skylon, but perhaps a runway launched, responsive, reusable spaceplane would interest the USAF? Sort of like a next gen X-37.

But all of this does hinge on the practicality of running that plumbing.

I've considered similar notions, but always assumed REL must have considered and discarded the single in-line SABRE configuration. But now that the subject has been broached :-) ....

In my thoughts, I assumed there would be a single (set of) combustion chamber (s) at the tail, further reducing the drag/wing-structure issues. I note that the exact placement of the intake + precooler could be tuned in order to maintain optimal CoG.

Some other issues/advantages leap to mind:
-Where would the bypass burners vent? It doesn’t seem impossible to have bypass air ducted all the way to the back of the vehicle. Many smaller ducts seem preferable, in order to maintain as straight a path as possible.
-Heat transfer to the cryo tanks from that plumbing
-The current vertical stabilizer is over-sized merely to provide sufficient control authority for a single-engine abort. With in-line thrust, the tail could be significantly smaller.
-REL might want to maintain two completely independent engine systems anyway, but there would be an opportunity for some small cost, complexity and mass savings by having a single engine albeit probably 2X the size in most of its components.
-inline thrust seems to offer small advantages in terms of control stability, and the tail-mount nozzle offers greater authority from thrust vectoring, at the cost of differential throttle for yaw control
-no issues with exhaust impinging on the airframe (could be a big issue, as we saw way up-thread)
-The ducting that runs the length of the vehicle could double as structural members. On the face of it, it seems unwise to have structural members go through such extreme thermal and therefore dimensional transformations, but I also read that SR72 expects to have a “warm” structure, so maybe it’s not mad.
-Perhaps the hot piping could be jacketed with a co-axial pipe carrying LH2 forwards, although REL might actually be embarrassed to field such a low-tech brute-force HX. It’d be a poor substitute for the current design’s enthalpy-recovery HX.
-Could the bypass burners’ exhaust be ducted out the canard’s tips? Perhaps the canard could be raked more strongly in order to reduce the amount of kinetic energy lost by redirecting the bypass air sideways.
-All this ducting might force a change in the OML, with presumably a slightly wider fuselage at the base, making Skylon look more like a Dreamchaser and less like an F-104.
-There have been some experiments with drag-reduction via venting small amounts of air through tiny holes all over an airframe. Perhaps an alternate use for bypass air, but not a system that would play well with a TPS.


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Offline Space OurSoul

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1392 on: 03/22/2016 05:27 PM »
A question that’s unrelated to this layout scheme that occurred to me while wondering what to do about the bypass burners: With sufficient additional LH2 could the bypass air be utilized TAN style to enable the main nozzles’ expansion ratio to be tuned closer to vacuum optimum? It may not produce much (any) more thrust, the stoichiometry being only that appropriate to air not pure O2, but might it address the stability issues that would otherwise plague an over-expanded nozzle?
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Offline RanulfC

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1393 on: 03/22/2016 05:53 PM »
Original reply was lost in space/time :)
As noted above that's somewhere around 15mt to 18mt for Skylon delivery to orbit but the guide points out that Skylon can deliver up to it's maximum STRUCTURAL load of 30mt in a suborbital, two stage launch with payload supplied propulsion.  :)
That's the C2 manual, it isn't in the D1 manual, and was (if I recall correctly) disavowed by the REL team

It is in fact but still informative. If REL "disavowed" it they may want to take it off their website :)
Having both versions shows how the design has been revised, so while removing it would be for claity I sincerely hope they don't.

You actually can't seem to access FROM their site, I found it in a round-a-bout fashion. The only one listed is the D1 manual.

Quote
Quote
Actually there was something of a "show-stopper" in the operation description; The Skylon had to be TOWED home after the suborbital mission which would indicate it used all its propellant getting the 30mt to suborbital velocity.
I'm not sure why that's a show stopper? I've not seen an operational mode for Skylon that relights the SABREs (other than transitioning from air to LOX). Towing is their preferred method of site-to-site transport for Skylon

Really? I hadn't seen that anywhere as I understood the preferred method was self ferry.

Quote
Quote
Take out the SOMA, fuel cells and heavily insulated orbital tankage, lengthen (and strengthen?) the cargo bay and add some batteries you've got a reusable first stage to a TSTO launch vehicle. Perhaps slap a conventional jet engine in the SOMA hole and some inlet ducting to allow RTB.

No the Skylon as designed isn't going to work for a first stage per-se. As noted even if you pull out the cited systems you still need to radically restructure the air/spaceframe to handle the new loads which is going to no longer make it a "Skylon" anyway.

I'm sorry that I was unclear, I was expressing how I though a SABRE-based TSTO craft could be designed - I quite agree you couldn't take a delivered Skylon D1 and turn it into one in the same way a mechanic may turn a hummer into a limo. From the design PoV I'm not quite sure why strengthening and lengthening would require the design would need to be radically restructured - in the same was as the D1 was not radically restructured from the C2.[/quote]

Sorry "I" wasn't clear either, I understood the intent but I was pointing out it's actually the opposite; The length is decreased with a smaller propellant load and the entire structure has to be reinforced to carry a significantly higher mass for a second stage system. And that's before we even open up the can or worms on how the second stage is deployed from the first :)

Quote

The required delta-v from LEO (800km with the C2 or 600km for the D1) was always implied to be part of the "payload" mass to LEO carried by the Skylon. I think that was always one of the biggest confusions.

Which in-turn leads to a smaller true payload. Shuttle also had the capability to have an extra upper stage as payload capability, if I'm reading this thread right.
As I understand it the Shuttle generally used the IUS, and the Centaur-G was designed to be flown (in Challenger or Discovery) but wasn't

In general I was pointing out the assumption was always there.

According to Mark Hempsell it's out.
http://forum.nasaspaceflight.com/index.php?topic=34964.msg1272415#msg1272415

Yup saw that. It's interesting the reasons for it but it's why AFRL is more interested in SABRE than in Skylon.

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

Online adrianwyard

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1394 on: 03/22/2016 10:49 PM »
As far as going single engine, it doesn't seem beyond consideration to divide the Sabre engine into an intake / turbopump assembly mounted centerbody, and rocket combustion chambers with nozzles in wing pods.  But that would require high pressure plumbing between the separated pieces.

Is running that plumbing a show-stopper? As I think about the single SABRE integrated into a fuselage I can see a couple of benefits compared to Skylon in addition to the challenges. With Skylon the wings need to be strong enough to support the SABREs on the ground, and any loads the SABREs would apply when flying/gliding during turbulence. More strength often means more mass. Their shape is presumably driven and even compromised by this - shorter wingspan is better for loads.

The wings on an integrated SABRE/fuselage design can be shaped and sized for the aerodynamics alone.

You also have a much simpler, more conventional shape for re-entry. (Although we've heard from REL that Skylon's unusual shape can re-enter just fine.)

I'm confidently guessing drag would be lower too.

So picture something like super-sized X-37B SSTO that takes off and lands on a runway. Throw in the V-tail from the X-37 and FASTT design for good measure so you don't need the transpiration cooled canards.

Payload would be small compared to the twin SABRE Skylon, but perhaps a runway launched, responsive, reusable spaceplane would interest the USAF? Sort of like a next gen X-37.

But all of this does hinge on the practicality of running that plumbing.

I've considered similar notions, but always assumed REL must have considered and discarded the single in-line SABRE configuration. But now that the subject has been broached :-) ....

In my thoughts, I assumed there would be a single (set of) combustion chamber (s) at the tail, further reducing the drag/wing-structure issues. I note that the exact placement of the intake + precooler could be tuned in order to maintain optimal CoG.

Some other issues/advantages leap to mind:
-Where would the bypass burners vent? It doesn’t seem impossible to have bypass air ducted all the way to the back of the vehicle. Many smaller ducts seem preferable, in order to maintain as straight a path as possible.
-Heat transfer to the cryo tanks from that plumbing
-The current vertical stabilizer is over-sized merely to provide sufficient control authority for a single-engine abort. With in-line thrust, the tail could be significantly smaller.
-REL might want to maintain two completely independent engine systems anyway, but there would be an opportunity for some small cost, complexity and mass savings by having a single engine albeit probably 2X the size in most of its components.
-inline thrust seems to offer small advantages in terms of control stability, and the tail-mount nozzle offers greater authority from thrust vectoring, at the cost of differential throttle for yaw control
-no issues with exhaust impinging on the airframe (could be a big issue, as we saw way up-thread)
-The ducting that runs the length of the vehicle could double as structural members. On the face of it, it seems unwise to have structural members go through such extreme thermal and therefore dimensional transformations, but I also read that SR72 expects to have a “warm” structure, so maybe it’s not mad.
-Perhaps the hot piping could be jacketed with a co-axial pipe carrying LH2 forwards, although REL might actually be embarrassed to field such a low-tech brute-force HX. It’d be a poor substitute for the current design’s enthalpy-recovery HX.
-Could the bypass burners’ exhaust be ducted out the canard’s tips? Perhaps the canard could be raked more strongly in order to reduce the amount of kinetic energy lost by redirecting the bypass air sideways.
-All this ducting might force a change in the OML, with presumably a slightly wider fuselage at the base, making Skylon look more like a Dreamchaser and less like an F-104.
-There have been some experiments with drag-reduction via venting small amounts of air through tiny holes all over an airframe. Perhaps an alternate use for bypass air, but not a system that would play well with a TPS.

Routing the hot bypass air does sound like the thorniest problem. It may not be worth it, and instead you dump it through bypass doors - a la SR71. But that comes with a performance penalty (no thrust from bypass burners, plus maybe drag) and complexity.

It's a good guess that Bond et al considered this sort of configuration in depth after the HOTOL cancellation, before coming up with Skylon.

IIUC the only thing that's a little different now is the temperature/pressure of the cooled air is less extreme with SABRE 4.

Offline 93143

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1395 on: 03/23/2016 12:21 AM »
Note in comparing the above manuals that the current D1 finally gets to a full 17mt "payload" by several fudges such as lower orbital altitude and mass capability in that the vehicle is now capable of handling less than 20mt overall with a specification of a maximum of 17mt to LEO (600km as opposed to 800km in the C2) shown.

Nothing 'wrong' with this in any way it's refinement of the Skylon design, but it does point out the issues being discussed about SSTO vehicles.

Look closer.  Both designs have a payload of exactly 15 tonnes at 300 km altitude and 0° from an equatorial launch site.  D1's payload charts go to lower altitudes; C2's go to higher altitudes.

The "refinement" here is that the latest design work showed that reentry from 800 km might be a little optimistic, at least for the D1 airframe, and that there may be some advantage to be had in using orbits as low as 160 km vs. 250.  The nominal payload isn't part of the "refinement"; it's from market studies.  It's what the design is built around and hasn't changed.  If anything, D1 has better payload performance, because Skylon C was a rough exercise that apparently turned out to be a little short of its target when analyzed in depth, whereas Skylon D was supposed to establish a for-sure buildable design.

REL put the engines where they did because they are engine designers and not actually airframe designers.

Or maybe they put the engines where they did because it's actually the best place to put them if you aren't overly concerned about your design looking "futuristic" (ie: like the X-30).  These aren't really "hypersonic" engines, and may be better served by more traditional placement.

issues with exhaust impinging on the airframe (could be a big issue, as we saw way up-thread)

I thought we established that the heating rate was probably way too low?

Though with an engine stuck right to the fuselage, it might not be...  furthermore, an engine on the bottom would probably require a significantly heavier undercarriage to maintain ground clearance, and an engine on the top might have issues with being in the wake of the fuselage.

On the subject of running an engine through the fuselage, I always figured that would make the vehicle both larger and heavier than it needed to be, partly from volume and structure considerations and partly due to the extra insulation required.  Simplicity of manufacture would also be ill served by having to build the tankage around an engine duct.  And putting the precooler and turbocompressor at the front of the vehicle to manage CoG would multiply the issues because now the duct has to be both hot and high-pressure...

...not to mention that separating the precooler and compressor from the combustion chambers by some substantial fraction of 80 metres might make for an interesting engine integration problem...

With sufficient additional LH2 could the bypass air be utilized TAN style to enable the main nozzles’ expansion ratio to be tuned closer to vacuum optimum?

I believe the SABRE 4 uses low-expansion annular nozzles for airbreathing mode anyway.  The high-expansion-ratio central nozzle is only for rocket mode, which starts at around 28 km altitude.

I understood the preferred method was self ferry.

Not for Skylon C it wasn't.  The airbreathing Isp was too low.  Even now it isn't clear what's "preferred", but SABRE 4 and the D1 airframe changes do seem to enable much longer range.

it's actually the opposite; The length is decreased with a smaller propellant load

Keep in mind that the airframe size is already not driven by the propellant load.  D1.5's fineness ratio is a compromise between optimizing for low drag and optimizing for low mass; earlier iterations of D1 had a lot of wasted space inside.  With the payload bay as wide as it is, their options for scaling down are limited.

...

Also, this has been bugging me for a while:

I still see no reply to the very informative post of 93143. Any one with better numbers? let's have a polite discussion on how much off the design must be not to deliver. We all want to see the calculations of those claiming the margins are really small.
You need the numbers, the ACTUAL numbers to base such a conversation on. The numbers provided by 93143 are the best estimates available from the estimates and assumptions that REL has done so far which is all well and good. But they are still not actual performance figures.
If we had actual performance figures, there would be no room for said conversation.  The whole point is that we don't; the viability of the design is not known yet and depends on how close they can get to their estimates.  The question is: how much margin for error in those estimates do they have?  If my math is right, it seems they have quite a bit...
If their math is correct then your math is probably correct to their estimates... Which still doesn't eliminate the possibility that everyone's math isn't going to be met by actual performance. Doubtful but the chance is non-zero to a significant degree.

You're missing the point.  I wasn't calculating the performance of the vehicle.  I was calculating how badly wrong REL's estimates would have to be to kill their idea completely.  Saying "yes, but their numbers might not be right" doesn't engage the actual question.
« Last Edit: 03/23/2016 03:59 AM by 93143 »

Offline Paul451

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1396 on: 03/23/2016 04:23 AM »
but dropping off the excess mass of the booster stage and continuing the mission profile with a more efficient second stage that is fully reusable is as valid an assumption as not.
If you're using SABRE, then you've got that backwards. You'd have a reusable first stage (the Skylon-derivative), with an expendable all-rocket, vacuum-optimized upper-stage.
No reason to assume it would be expended

(People are reading too much into a throw-away line.)

I didn't mean that it had to be expendable, but that the expendability or reusability of the upper-stage is completely independent from the first stage.

I just meant that if you are using a SABRE-derived TSTO, then it will never be the second stage that is based on Skylon with a dumb booster stage. TSTO-Skylon would be the first stage, the second stage will just be a boring old rocket US.

I wouldn't expect a reduced SABRE propellant load. I'd expect the second stage to be pushed out of the cargo bay at around 200km altitude during a ballistic coast, doing around 6km/s

That was my image for STSO, except no payload bay, dorsal-mounted second stage/payload. (And probably lower staging, 80-100km.)

I wasn't calculating the performance of the vehicle.  I was calculating how badly wrong REL's estimates would have to be to kill their idea completely.

The issue isn't whether Skylon is possible, but whether it's practical. We don't need another unaffordable launcher. It doesn't take much to blow out the already-extraordinary design costs. Nor to blow out the ops costs.

Offline john smith 19

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Re: The Reaction Engines Skylon Master Thread (5)
« Reply #1397 on: 03/23/2016 04:04 PM »
The issue isn't whether Skylon is possible, but whether it's practical. We don't need another unaffordable launcher.
And yet national governments keep funding them. While the LV is tied to a 2000Km artillery range they probably always will.  :(
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It doesn't take much to blow out the already-extraordinary design costs. Nor to blow out the ops costs.
Not even a "IMHO" to qualify that statement?

Aircraft cost models are basically built on mass, not complexity. Most of values for big ones will be derived from passenger aircraft. Such vehicles are very complex internally due to all the passenger support hardware. The models say nothing about this. Just that an aircraft that big is likely to cost this much. And Skylon is the size of an A380. Thrust wise its 2 1/8 bigger.

Keep in mind the equivalent cost models in the LV business predicts the total cost of the F1 & F9 programmes to first F9 launch about 10x what SX actually spent with NASA BAU processes and 6x with "more commercial" procurement.  ???

So I'll guess that SABRESkylon would have to have very bad project management to go over that level (run as a government cost plus contract, which is where most of the cost data for the LV business came from) and I would expect it to be somewhat below that level.

Physically Skylon is big, but internally it's relatively simple, once you get past the construction materials. Built in a clean sheet factory it should be possible to mfg it with a high degree of automation at a reasonable pace.

The thing I would love to know is how much has all the work REL put in already lowered those cost figures? My guess is that what they've done (over the last decades) has knocked quite a lot off that $12Bn  estimate, by turning "possibly, probably, maybe" into "yes, if we use parts of this diameter it will work" or "no, we'll have to revise the parameters for the D1.5 revision."
But that would disconnect their estimate from the numbers funding organizations would get when they run the cost models, so they won't talk about them.  :(
"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 #1398 on: 03/23/2016 04:05 PM »
Er, and "upper-stage" has ALWAYS been implicit in the design of Skylon you DO understand that right? :) Skylon is ONLY capable of reaching LEO so it would have been useless for it's "designed" job of delivering commercial satellites if it could not put them into the proper orbit(s) as required.
Implicit certainly, but it was only relatively recently they started discussing the Skylon Upper Stage in detail and adding it to their main budget.
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The required delta-v from LEO (800km with the C2 or 600km for the D1) was always implied to be part of the "payload" mass to LEO carried by the Skylon. I think that was always one of the biggest confusions.
True. 15 tonnes was always the mass to LEO, and it was expected "something" in that mass would handle the getting to GEO part.

The required delta-v from LEO (800km with the C2 or 600km for the D1) was always implied to be part of the "payload" mass to LEO carried by the Skylon. I think that was always one of the biggest confusions.

Randy

Which in-turn leads to a smaller true payload. Shuttle also had the capability to have an extra upper stage as payload capability, if I'm reading this thread right.
15 tonnes is the true payload to LEO.  SUS is sized to put (IIRC) a 6 000 Kg comm sat on it's way to GEO, which is at the top of currently projected comm sat sizes.
"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 #1399 on: 03/23/2016 04:05 PM »
A D-21 shape isn't really that bad of a basis for starting, it's been suggested for 2 or 3 combined cycle demonstrators that I can recall :)
Agreed. If you must have a demonstrator logically you either a)Go with a scaled down version of your core design, to leverage all your CFD and wind tunnel work or b)Go as simple as possible, like an X plane. A test vehicle to demonstrate 1 key capability, with everything else kept as simple as possible.

I think scale models are tricky. They have most (all) of the difficult bits of the full size machine but only needed in 1 of or 2 of quantities, so as far as possible you want to get OTS stuff for the non tricky bits.

Personally I think you need to size the model to the sizes of the OTS bits you've got and avoid the special orders unless absolutely necessary. I'm especially thinking of the undercarriage. I think we all recall the Dream Chaser test vehicle using landing parts from a stock fighter and it's roll over when the landing gear door bay jammed.  :(
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Skylon has been compared to being a hypersonic/orbital airship at times and really the current wing size is enough for fully loaded take off so getting back down is going to be rather "fluffy" in any case :)
That's a bit unfair (and IIRC there really is a story about a hyper dirigible  :) ). Skylon's  structural loading is like that of an airship, but materially it's a very different beast.
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One thing to keep in mind is the Skylon basic shape is designed to be as easy as possible to fabricate. Avoiding complex shapes or curves in favor of simple (and well understood) rounded tanks. Once you start trying to wrap tankage around a single engine you have to move towards a more complex design which complicates fabrication and increases cost etc.
Very true, especially if LH2 is the fuel. Again it's back to keeping a demonstrator as cheap as possible. Cylindrical tanks are still the way to go.
I was pointing out that the Air Force (and hence AFRL) happens to like the ability to move both the launch point AND the launcher around to keep Op-For guessing as much as possible. You can't do that with a ground launch system and even most air-launch systems have issues due to range requirements and restrictions. (Yes in fact they can and do often apply to the military as well, especially in peace-time) SABRE "air-breath's" so that's probably part of what AFRL is considering.
I'll note Skylon is restricted by it's need to use a runway capable of handling a B36, of which the USAF owns 3 in CONUS. Naturally you take a payload hit for non equatorial launch but not letting anyone know you've launched may outweigh this.

However because Skylon is about 210 tonnes lighter landing would require much more minimal facilities. For the really impatient Skylon user I think a launch from one coast to full orbital velocity might be possible, deployment, then reentry in time to hit a runway on the other coast for landing, loading of LH2 and self ferry back to main operating base.

It sounds a bit involved to me but technically the launch asset never leaves the US, rather than wait a day for it to return to track over the original launch site (although Skylon's X-range is 2000Km+)
"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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