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Suborbital refuelling

Archibald

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You guys are great, really.

this morning I made an interesting exercise. Related to "carrier aircraft / airbreathing engine speed" vs "delta-v gain"
because, you see, the two are not proportional nor linear.

Let's take the case of Skylon. The airbreathing engines push rocket ignition to Mach 5.5. ok, then how much delta-v to orbit GAIN is mach 5.5 ?
Mach 5.5 is 1.87 km/s.
Now follow my reasonning: Clapp says going to Mach 5.5, airbreathing, lowered the delta-v to orbit to 20700 ft/s. Which is 6.3 km/s. From 9.3 km/s, that's a delta-v gain of 3 km/s. Problem: Skylon was supposed to gain only 1.87 km/s, not 3 km/s !

so the cool news is that the delta-v gain from carrier / airbreathing, grows faster than the carrier / airbreathing own speed.

In clear english: launch from a Mach 0.95 carrier provides a delta-v boost of much more than Mach 0.95. Well, here comes (again) Marti Sarigul Klinj
Mach 0.95 carrier brings +1100 m/s bonus on the way to Earth orbit (8.1 km/s left - from a ticket of 9.2 km/s, average)
Mach 2 carrier brings+1600 m/s bonus (so 7.6 km/s left)
Mach 3 carrier brings + 2000 m/s bonus (so 7.2 km/s left)

And we could add Burnside Clapp and Skylon coinciding results

Mach 5.5 = 1.87 km/s brings +3 km/s bonus (so 6.2 km/s left)

What is sure is
- Subsonic, supersonic, and low hypersonic represents three very different flight regimes, harder and harder.
- Meanwhile the rocket equation has a logarithm so it is non linear, too, just like the different mach regimes.

The basic rule is: the deeper into hypersonic, the harder, while the rocket equation non-linear nature just watch you heating the heat barrier, smiling.
High speed flight is as unforgiving as is the rocket equation.
airbreathing boost vs delta-v gain.PNG
 
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steelpillow

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Isn't the ISS replenished with LOX?
ISS needs oxygen only for the crew. It has various onboard systems for that, with storage being either chemical (e.g. as water) or gaseous. Replenishment is usually by sending up more water, although I suppose it is possible that gas canisters were once sent up when a lot was needed in a hurry after some mishap or other. Neither liquid nor high-speed transfer nor even fuel.
 

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Mach 0.95 carrier brings +1100 m/s bonus on the way to Earth orbit (8.1 km/s left - from a ticket of 9.2 km/s, average)
A heavily-laden subsonic carrier would start to create shockwaves around Mach 0.8, with say 0.85 the max to avoid significant interference with the separation dynamics. Moreover it would be lucky to reach 50,000 ft (15,000 m) where the speed of sound is 330 m/s (depending on local temperature), so separation would take place at around 280 m/s. (540 kn) Not sure how that impacts your numbers.
 

Archibald

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Good point. It is important to make the distinction between, as you say " A heavily-laden subsonic carrier " and a rocketplane with integrated jet propulsion (think Astroliner or Pathfinder or Black Colt). The second one would not give a rat about the issue you mention, obviously.
Of course it pays a weight penalty to that, since air-launch leaves the heavy jets and most of the undercarriage weight to the mothership instead of carrying that burden into orbit.
 
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Archibald

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ISS is refueled in *storable propellants* by Progress ships. A legacy of Mir and Salyut reaching back to 1978 and Salyut 6. First orbital prop transfer between spaceships.
Bar storables, and for all the intensive talk about prop depots from Apollo EOR to ULA and Jon Goff, nothing else has been achieved so far.
Elon Musk is presently the most serious contender to unlock the prop depot, and he will do it grand scale: in order to go anywhere beyond LEO, Starship will need a transfer of 1100 mt of methalox, both mild-cryogen around -180°C.
 

steelpillow

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ISS is refueled in *storable propellants* by Progress ships. A legacy of Mir and Salyut reaching back to 1978 and Salyut 6. First orbital prop transfer between spaceships.

LOX is not a "storable propellant" in that sense. Also, the propellant they replenish is only for the manoeuvring thrusters, it is not spaceship propellant within the meaning of the phrase. And one does wonder whether any of its predecessors might have had their thrusters topped up.
 

Archibald

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I never said LOX was storable propellants. Nuance: I said the only in-space refueling done, past and present, had been with storable props.
 

Archibald

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Back to suborbital refueling. After that major blunder in the spreadsheet, a major shift has happened.

The Mk.1 keroxide bird can't go to orbit unless 3-FLOC (and even 4-FLOC with a valuable payload). Not practical.
So I have decided to *downrate* those birds to
- suborbital tourism, Blue Origin or SS2 on steroids (5 km/s rather than 1 km/s)
- suborbital P2P passenger transportation with ricochet trajectories
- satellite launch using expendable upper stage
- flight testing of Mk.2 technologies plus future pilots training
Still for these missions they remain civilian airports darlings, using kerosene, H2O2, and civilian turbofans like the BR715, LEAP, or JT8D. Those engines give a 1100 m/s boost, lowering delta-v to earth orbit from 9200 m/s to 8100 m/s. Still not enough to fill the "spreadsheet blunder" and going into orbit.

Now the case of the Mk.2 kerolox bird.
The blunder also affected it, in the 2-FLOC configuration it still can't make it to orbit. So I have decided to turn it into the military prefered bird. Which mean: LOX, but also, to compensate, trade the quiet, civilian turbofans for
- noisy, afterburning F-110s which give Mach 2 capability before rocket ignition. In turn, this lower the delta-v by 1600 m/s, from 9200 to 7600 m/s.
- J58 push that to Mach 3 and beyond, a gain of 2000 m/s+ to 7200 m/s.
- MIPCC and/ or SERJ push to Mach 4+ and get the delta-v to orbit even lower, -360 m/s which bring it to 6900 m/s

Finally is the Mk.3 hydrolox bird. Since it can use a SABRE, it can lower the delta-v to orbit to 6300 m/s.
But there is no asolute need to go so far, because its superior specific impulse saved it from the *blunder* - it still easily get into orbit as 2-FLOC with a decent payload. Lucky me !
So the Mk.3 also work with all the Mk.2 alternate engines - from civilian turbofans to MIPCC and SERJ.

In a nutshell, I shamelessly use the "air launch / airbreathing boost" to compensate for my "spreadsheet blunder". This is tolerable and not dishonest ONLY because I used proven data by respectable people (Clapp, MSK, also Sorensen / Bonometti air-launch study, DARPA and others).

Note that some criss-crossing between the three variants are possible: for example SERJ used peroxide, just like the Mk.1. But SERJ, like SABRE, must be insanely noisy. the military care less, but civilian airports... forget it.

Basically the Mk.1 keroxide bird has taken such a huge hit from the "spreadsheet blunder" I'm not sure even a Mach 2 or Mach 3 "airbreathing boost" (read, F110 or J58) could send it in orbit AGAIN, in a 2-FLOC configuration.
It would probably take a MIPCC or SERJ at Mach 4 to do it again.
But these engines open a new can of worm: Mach 4 in the atmosphere means the structure hit the heat barrier, even briefly - before lighting the rocket and getting out of the atmosphere; we are not discussing a LAPCAT long range cruise here.

So the philosophy is "forget going into orbit with the Mk.1, keep the civilian turbofans for airports even if all missions will be non-orbital". C'est la vie.
 
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Archibald

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And the Mk.2 spreadsheets, accordingly, become...

Geez that "airbreathing boost" really helps.
 

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steelpillow

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I never said LOX was storable propellants. Nuance: I said the only in-space refueling done, past and present, had been with storable props.

Bear in mind that you had just said that "Isn't the ISS replenished with LOX? I thought I'd read that somewhere but could be wrong that LOX was brought up on the Progress along with the RCS propellant." and the only current use for LOX in space is as a propulsion oxidiser. You may not have said "2+2=5" but taken together that was what your posts implied. Other readers may not be aware you didn't mean it that way.
 

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RanulfC would it be possible to run MIPCC on H202 rather than LOX & water ? At least SERJ has H2O2 right from the beginning (1967 !) which really help my case.
I personally like both of them very much. The neat thing with MIPCC, if you graft that on a stock F110, then you can turn a Mach 2, 1600 m/s delta-v boost into a Mach 4.5 one (around 2400 m/s) using the same basic engine. See my last post: in the case of the doomed Mk.1 bird, MIPCC would save the day and allow it back into orbit in the 2-FLOC scheme lost to my silly spreadsheet blunder.
 
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Archibald

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Yes, this one. Welcome aboard this ship, Michel Van.

I've redone the (doomed) Mk.1 keroxide vehicle with all the different engines. Basically, if the airbreathing boost doesn't carry it to Mach 3 (2000 m/s > 7200 m/s to orbit) then it can't make it. Zero boost (9200), subsonic boost (8100) and Mach 2 boost (7600) = it's dead, Jim.
Then Mach 3 barely makes it, leaving really MIPCC (Mach 4) and SERJ (Mach 4.5) as saviors. Can't use Mach 5.5 SABRE here, because LH2 & LOX.
 

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Archibald

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Variants of the ROLS – Reusable Orbital Launch System

ROLS Mk.1

Keroxide props for all of them. With BR715 or JT8D civilian turbofans it is very "airport friendly" but can't go into orbit even as 2-FLOC. Used instead as pilot trainer, tech demonstrator, space tourism, P2P passenger transport with ricochet trajectories, and satellite launch with expendable upper stage.

Mk.1A
In order to restore orbital capability, F110 turbofans from the F-15E push rocket ignition to Mach 2.5. Still marginal to get any payload into orbit, but way to go for both SERJ and MIPCC variants (see below)

Mk.1B
The same F110s modified with MIPCC push rocket ignition to Mach 4 and greatly improve payload to orbit. Problem: MIPCC eats LOX and water for lunch, not peroxide. Two solutions: solution 1, drop tanks (one LOX, one water, X-15 style). Second solution: put the tanks into the payload bay, and they eat into the payload to orbit.

Mk.1C
Alternative to Mk.1B with SERJ. Little gain to Mach 4.5 and, most importantly, runs on peroxide, just like the rocket engine. So no drop tanks nor payload bay tanks.

ROLS Mk.2
No need for the civilian turbofans, they don't like LOX plus can't go into orbit just like the Mk.1. So go for the military only. LOX allows specific impulse of 359 seconds instead of 327, greatly helping payload to orbit. Basic Mk.2 has F110, too, same reasons as Mk.1A, way to go toward MIPCC and possibly SERJ.

Mk.2A
The MIPCC bird, with an advantage over its Mk1B counterpart: MIPCC and the rocket both need LOX ! Still a need for a water tank, same places: payload bay or drop tank.

Mk.2B
The SERJ bird, with a mirror issue: H2O2 and LOX are dissimilar. Maybe a LOX SERJ is possible, Marquardt had a hydrolox one for NASA before switching to the keroxide one for the military.

ROLS Mk.3
Two variants here.
The basic Mk.3 has hydrogen-burning modified F-110 turbofans and, thanks to the extremely high specific impulse of 460 vs 320 – 360 for the others, don't need the advanced MIPCC or SERJ to make it to orbit as 2-FLOC.
And then there is the Mk.3A which takes profit of the... Skylon SABRE. This allows it to push rocket ignition to Mach 5.5, and gain a boost of 3 km/s on the way to orbit, lowering the delta-v from 9.3 km/s to 6.3 km/s. Combined with the other huge gain provided by suborbital refueling, its performance even as 2-FLOC is quite impressive. And it works in synergy with its Skylon great brother, eventually introducing Skylon to the art of suborbital refueling...
 

Archibald

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Seems that H2O2 can find its way into a MIPCC system. Water in the compressor, and peroxide in the afterburner. Pretty cool. Preston "Hypersoar" Carter once again, RASCAL, MIPCC. Those studies are godsends, really.

Since MIPCC works equally well with LOX and H2O2 it suits both Mk.1 and Mk.2 birds. Yet in both case they need a water tank in the payload bay or, alternatively, a water drop tank. I can really live with that to get the Mach 4 boost out of plain old military turbofans in large scale service - to you, F110.

MIPCC peroxide.png
 

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RanulfC

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Isn't the ISS replenished with LOX?
ISS needs oxygen only for the crew. It has various onboard systems for that, with storage being either chemical (e.g. as water) or gaseous. Replenishment is usually by sending up more water, although I suppose it is possible that gas canisters were once sent up when a lot was needed in a hurry after some mishap or other. Neither liquid nor high-speed transfer nor even fuel.

Ok, good to know

I never said LOX was storable propellants. Nuance: I said the only in-space refueling done, past and present, had been with storable props.

Bear in mind that you had just said that "Isn't the ISS replenished with LOX? I thought I'd read that somewhere but could be wrong that LOX was brought up on the Progress along with the RCS propellant." and the only current use for LOX in space is as a propulsion oxidiser. You may not have said "2+2=5" but taken together that was what your posts implied. Other readers may not be aware you didn't mean it that way.

That was me :)

RanulfC would it be possible to run MIPCC on H202 rather than LOX & water ? At least SERJ has H2O2 right from the beginning (1967 !) which really help my case.
I personally like both of them very much. The neat thing with MIPCC, if you graft that on a stock F110, then you can turn a Mach 2, 1600 m/s delta-v boost into a Mach 4.5 one (around 2400 m/s) using the same basic engine. See my last post: in the case of the doomed Mk.1 bird, MIPCC would save the day and allow it back into orbit in the 2-FLOC scheme lost to my silly spreadsheet blunder.

As Michel Van noted it's been suggested but keep in mind the LOX injection wasn't for cooling purposes but to stabalize the flame-front in the afterburner (and combustors at high-altitude) which is a boost you wouldn't get from peroxide unless it's decomposed. Using a keroxide SERJ would mean a bit bigger peroxide tank if you're using it for inlet and AB injection though on the other hand you should have to worry less about the compressor face heating issues since the compressor isn't designed the same as a standard turbojet. Above about Mach 1.5 the SERJ is supposed to transition to pretty much pure ramjet mode till around Mach 4/4.5-ish depending on the design. The MIPCC system would boost your initial fan/ram/rocket thrust, while adjusting 'oxidizer' levels in the rocket motor(s) would adjust the levels for the AB/ramjet section of the engine.

Usage in a 'standard' turbojet gets you both increased thrust and a wider mach range for the compressor face/afterburner system which is what the whole "Steamjet" concept came down to.

Randy
 

Archibald

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Thank you.

I'm desperately looking for some tech papers about running turbofans on hydrogen... preferably recent ones.
 

Archibald

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Just for you to know, Jeff Foust from The Space Review answered me, nice fellow, encouraging, but I have to rework the thing into something shorter. I'm a little too maniacal on details.
 

Archibald

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Send a much shortened and straightforward version. Fingers crossed, wish me good luck !!
 

steelpillow

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Congratulations!
I still think that carrying the tanker with you a la Space Shuttle external tank is more elegant, efficient and safer, but let the number-crunching now commence in earnest!
 

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I note that in the comments the obvious alternative of a bimese TSTO is repeatedly raised, just like in this thread.
 
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Archibald

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Il n'est de pire sourd, que celui qui ne veut rien entendre (sigh)
 

steelpillow

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I just posted a comment there, having realised that independently-launched tanker and orbiter craft need far less compatibility and integration testing than a biamese approach such as the Space Shuttle. Might make a difference in some markets.
 

Archibald

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fair enough, Martinbayer :D

Cross posting from NASAspaceflight.com

Bimese / trimese vs SOR

Bimese / trimese are linked on the ground and launch attached together. This mean their masses add, and on top of that, at the deep end of Earth steep gravity well.
By contrast the SOR rocketplanes launch separately, accelerates separately - they drag their own weight, not their twin brothers.
and when they link, it is not only very briefly and kind of "lightly" - it is also very high and very fast.


amind the posts so far, there is a fascinating exchange between "the radical moderate " and "Rob davidoff" over gravity losses.
This is the EXACT reason why I wanted to be published: to learn about GRAVITY LOSSES which are really the main unknown in the entire thing. The one that is the harder to grasp.

TheRadicalModerate

This is interesting, but I'm not convinced on the gravity losses. You'd obviously have to know the exact payload numbers to figure out the tangential velocity at rendezvous, but I'd be surprised if it were more than 3 km/s, especially since you need enough loft to do the whole refueling operation in vacuum. That gives you an effective gravitational acceleration of 8.4 m/s², which is still pretty hefty.

At a guess, you can get a decent payload for a total mission delta-v budget of 15 km/s. Assuming fuel transfer occurs after 5 km/s of delta-v and the aforementioned 3 km/s tangential velocity, that lets you lose roughly 5 km/s during refueling. That gives you 10 minutes for the whole refueling sequence.

Thinking about your post, Moderate, if the effective gravitational acceleration really is about 8.4 m/s^2, it seems like you'd have about 250 seconds of coasting time before you waste the entire 2 km/s benefit cited for the airbreathing & winged portion of the mission.

I think I may have messed this up. You really don't have any gravity losses in a ballistic arc--as long as you preserve your tangential velocity (which is easy), and don't have to add any additional radial acceleration (which is hard if you want to stay out of the atmosphere after restart).

I found some old Skylon flight profiles here (p. 7). I haven't done the slog through figuring out how much more payload you can carry with a launcher like this yet, but a better way of looking at how much time you have is:

1) Figure out how much more tangential velocity you need to get to orbital velocity after you've refueled, and how long it takes to acquire that.

2) Figure out how much loft (i.e. radial velocity) and altitude you have at burnout, and use those as initial conditions to determine when you're going to fall back to hit 60-70 km altitude (i.e. when drag will be a Bad Thing).

3) If (result of #2) - (result of #1) = (refueling time available) isn't enough time to be really sure that you can do the transfer, via whatever mechanism you choose, then you probably have a loser on your hands.

The big problem with this kind of computation is the same problem that Starliner had with the Atlas N22, though: If you have a lot of loft to maximize your refueling time, a failure means that you're likely to hit the atmosphere at an unsurvivable radial velocity. So you have to depress the trajectory more than you'd like, which reduces your refueling time, which...

You get the idea. It's like everything else in the history of SSTO, and especially with SABRE and Skylon: You're always right on the hairy edge between a viable and non-viable system, with a paper rocket that likely doesn't have enough design margin to be successful.

To be fully honest, I somewhat agree with the last sentence above.
The spreadsheets demonstrated to me, without any doubt, that
a) the margins are razor thin
b) the rocket equation is a twisted b*tch you can't really fool
c) keeping the parameters / spreadsheet on the RIGHT track is kind of running across a minefield.
 

steelpillow

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Bimese / trimese are linked on the ground and launch attached together. This mean their masses add, and on top of that, at the deep end of Earth steep gravity well.
By contrast the SOR rocketplanes launch separately, accelerates separately - they drag their own weight, not their twin brothers.
That is badly mistaken. It misses the frankly rather obvious point that bimese/trimese also add their engine thrusts. Same total thrust as SOR, so the gravity well argument is emotive rubbish. Moreover in biamese the lighter one transfers its excess thrust to the more critical heavier one, taking it beyond where its SOR twin can go until refuelling is complete. The net result is to help the critical case out of the gravity well, not to hinder it as suggested.
Or, to look at it the other way, SOR has to follow the trajectory of the heavier craft, minimising the time available for refuelling while maximising the subsequent path to orbit.
 

Archibald

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Moreover in biamese the lighter one transfers its excess thrust to the more critical heavier one

I thought they were identical ? otherwise you end with the space shuttle...
 

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Identical has the advantage of simplicity but I do not see why it should be definitive. The shuttle was almost there, but the external tank had no engine - or, in effect, its engine was located in the orbiter and became dead weight after separation. This was done so that the tank could be made lightweight and expendable, but I think today we would give it its engine back and make it re-usable.
Even if they are identical airframes, the equipment fit has to be very different. And even if they are identical weights at liftoff, the tanker will reduce faster as it spends the flight into space pumping fuel across to keep the orbiter topped-up. Why the whole flight? Because the longer it takes the smaller and lighter the pump can be.
 

Archibald

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Amid SSTOs and RLVs and TSTO that concept closely relates to bimese / trimese on one side (Mustard, Triamese, Delta IV Heavy, Falcon Heavy), and OTRAG / Microcosm - asparagus staging, on the other.

Incidentally, I never claimed this system would throw 60 years of rocketry under a bus. It is rather complementary from all the others.
Some advantages are
- flying out from runway to orbit without too complex engines (just dumb rockets and dumb jets)
- adding more vehicles into the FLOC - 2 to 3, 3 to 4, and 4 to 8 to raise payload into orbit
- also using straightforward propellants (kerosene and H2O2 are liquid at room temperature) to get closer from airline-like operations. In the sense that cryogens like LOX / LH2 tends to segregate rockets away from aviation.
 

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I thought they were identical ? otherwise you end with the space shuttle...

That was the original idea but reality quickly points out that to do so then ALL the vehicles have to be built as orbiters rather than boosters and that runs into problems, as noted, with higher 'dry' mass from the start. And since it was unlikely that simply sticking 'extra propellant' into the cargo bay of the "booster" vehicle was ever going to work...

And also as mentioned you "booster' has to be able to actually 'boost' which requires more optimized engines and the list goes on. As the size of the "booster" grew commonality went out the window and concept fell by the wayside. One interesting thing to note is this is where the initial idea of "external tanks" came into play as the 'booster' was replaced by "cheap, expendable" propellant tanks and more broad range engines on the orbiter.

Identical has the advantage of simplicity but I do not see why it should be definitive. The shuttle was almost there, but the external tank had no engine - or, in effect, its engine was located in the orbiter and became dead weight after separation. This was done so that the tank could be made lightweight and expendable, but I think today we would give it its engine back and make it re-usable.

"Slightly" incorrect :) The engines were on the orbiter BECAUSE it was recoverable and a key point was to get the engines back rather than expend them like say, was done on Buran. Without on-board propellants the orbiter simply was a recoverable structure that contained the payload, crew and engines so in theory as a 'smaller' vehicle it would be less expensive to build. (Didn't work out that way)

Recovery of the external tank would be problematical due to the needed mass of the recovery system which would detract from the systems payload-to-orbit and as noted incresae the price significantly. Recovery of the engines in a recoverable pod, (ala Vulcan for example) has been suggested but I get the feeling you are talking about a recoverable 'stage' rather than an external tank correct?

Further you can in fact recover external tanks with some effort, in fact the Shuttle ET was induced to tumble on release to help ensure it broke up upon reentry since the later models were light enough and had sufficient insulation that there was a high probability of them reaching the Earth's surface. There are a number of suggested methods for recovering jettisioned external tanks of various types and dimensions, the main question is if it is economical or not and I'd think that would be highly dependent on the overall flight rate of the system.

One intereresting concept is external tanks that singely or seperately hold combinations that increase the overall effectivness of the LV such as would feed tri-propellant or Thrust Augmentation (TAN) propellants that would make the LV much more efficeint both during launch and at higher altitude.

Even if they are identical airframes, the equipment fit has to be very different. And even if they are identical weights at liftoff, the tanker will reduce faster as it spends the flight into space pumping fuel across to keep the orbiter topped-up. Why the whole flight? Because the longer it takes the smaller and lighter the pump can be.

From what I've seen/read weren't most propellant cross-feed concepts to feed all the connected stages engines from a single tank not to actually try and put propellant back into a tank that's being drained? I know that's how "asparagus staging" is supposed to work but from what I understand that's even more difficult than feeding seperate thrust-structures and engines with their own vibration patterns and other issues. It would seem self-evident that in order to 'refill' the tank the new propellant would have to be flowing at the same rate and pressure, (actually IIRC a slightly higher pressure to surpress surge and sloshing?) that that tanks propellant is being drained so the pumps would be nominally the same?

Randy
 

martinbayer

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If I understand Archibald's concept correctly, all separately launched suborbital refueling vehicles would be *completely* identical and therefore all designed and built for flight into and return from orbit, so all the arguments made against completely identical bimese/trimese stages in terms of higher dry mass due to things like TPS and RCS, the likelihood of being able to accommodate extra propellant in the booster/tanker cargo bay, and rocket engine design optimization and required complexity ("just dumb rockets") apply accordingly. I also see no reason to assume a crossfeed system that cycles the booster propellant through the orbiter tanks first instead of one that feeds all engines of both connected stages directly from the booster tanks to keep the orbiter tanks filled up from launch until stage separation.
 
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Archibald

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If I understand Archibald's concept correctly, all separately launched suborbital refueling vehicles would be *completely* identical and therefore all designed and built for flight into and return from orbit

...so all the arguments made against completely identical bimese/trimese stages in terms of higher dry mass due to things like TPS and RCS, the likelihood of being able to accommodate extra propellant in the booster/tanker cargo bay, and rocket engine design optimization and required complexity ("just dumb rockets")

...apply accordingly.

I also see no reason to assume a crossfeed system that cycles the booster propellant through the orbiter tanks first instead of one that feeds all engines of both connected stages directly from the booster tanks to keep the orbiter tanks filled up from launch until stage separation.

I have difficulty understanding the beginning and the end of your sentences there. No malice, really, it is unrelated to the "intensity" of the debate here. I tried to cut them into more manageable bits...
 

martinbayer

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If I understand Archibald's concept correctly, all separately launched suborbital refueling vehicles would be *completely* identical and therefore all designed and built for flight into and return from orbit

...so all the arguments made against completely identical bimese/trimese stages in terms of higher dry mass due to things like TPS and RCS, the likelihood of being able to accommodate extra propellant in the booster/tanker cargo bay, and rocket engine design optimization and required complexity ("just dumb rockets")

...apply accordingly.

I also see no reason to assume a crossfeed system that cycles the booster propellant through the orbiter tanks first instead of one that feeds all engines of both connected stages directly from the booster tanks to keep the orbiter tanks filled up from launch until stage separation.

I have difficulty understanding the beginning and the end of your sentences there. No malice, really, it is unrelated to the "intensity" of the debate here. I tried to cut them into more manageable bits...
Archibald,

I responded to Randy's criticisms of bimese concepts by pointing out that your concept, which he also favors, has the very same design conditions for completely identical vehicles as the ones he specifically identified for bimese stages, and I agreed with him that refilling the orbiter tanks of a bimese TSTO in flight (which nobody has however proposed or postulated in this thread anyway) instead of using the booster tanks only for feeding all rocket engines directly during combined ascent does indeed not appear to be meaningful.

In terms of a rocket powered RLV that is able to operate from conventional runways, a corresponding HTHL TSTO concept has been proposed and repeatedly discussed by Ramon Chase, see for example https://www.yumpu.com/en/document/r...erm-reusable-launch-vehicle-strategy-chapters as well as AIAA 1991-2388, AIAA 2004-3731, AIAA 2006-4606, and AIAA 2009-7417, albeit not specifically as a bimese system. I do however see no reason why such a design would be precluded in principle. Whether using existing airports would justify any potential drawbacks (such as the dreaded interference drag ;)) is in the eye of the beholder. Note however that the Delta Clipper VTVL demonstrator was already designed for operations in austere environments almost three decades ago, and looking for example at SpaceX, VTVL RLVs should only need a concrete platform the size of a helipad for launch and landing operations rather than a whole runway like HTHL concepts. And of course it's a logical step from a system like the Falcon Heavy to a bimese or trimese VTVL RLV. I still prefer winged vehicles though since I'd rather put up with the additional dry mass and moderately increased overall complexity of some (mostly passive) aerodynamic structure than waste valuable rocket engine burn life on deceleration, descent and landing in a substantial atmosphere.

Who knows, perhaps a bimese could even be based on a winged vehicle with both airbreathing and rocket propulsion along the lines you favor and might end up looking somewhat like a concept included in the 1964 book "Man and Space" authored by none other than Sir Arthur C. Clarke, see https://www.google.com/imgres?imgurl=https://live.staticflickr.com/4042/4625428477_b8fbd26a86_b.jpg&imgrefurl=https://www.flickr.com/photos/x-ray_delta_one/4625428477&tbnid=j96k-D-tUS1EpM&vet=12ahUKEwjJpPjvr8DnAhUMlJ4KHXIcA6wQMyg2egQIARB1..i&docid=Lvy7ATNFuLcfDM&w=1024&h=800&q=valigurski&ved=2ahUKEwjJpPjvr8DnAhUMlJ4KHXIcA6wQMyg2egQIARB1...
 
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