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#101 2012-01-01 17:53:41

Terraformer
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From: Lancashire
Registered: 2007-08-27
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Re: Reusable Rockets to Orbit

I'm with GW, unless someone provides an even more authoritive source tongue


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#102 2012-01-02 00:22:30

JoshNH4H
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Re: Reusable Rockets to Orbit

Hop, are you seriously not willing to accept an estimate of 9400 m/s +~100 m/s?  Seriously?  I'm doing a basic overview of major factors of interest in design.  100 m/s (my estimate of the potential variation due to different flight paths holding vehicle size constant) isn't that big of a deal, especially not for an initial overview.

When it comes to engineering (that's what we're doing now), you don't talk about "right" and "wrong," but rather low error, moderate error, high error, etc.  I rejected the idea of a two-stage rocket about four pages ago, for more reasons than the one that you've been tangentially talking about since I started the thread.  I refer you back to page 1.

I would also add that the model which I used to dismiss the two-stage to orbit model was not so far off; I could draw the same conclusions from the modified model that we've used over the past few pages. 

Anyway, back to heat shields:  Ideally, the wing loading/mass per area would be low enough that we would not need any additional shielding on the tank beyond that which would have been used to build the tank.  To be quite frank, that will not be the case.  Some kind of additional heat shielding will be needed.  It has to be durable, both in that it has a lot of margin on individual runs, and in that it will last for a large number of runs.  Our basic suggestions are:

-Inflatable/expandable
-Part solid part inflatable
-Broad side solid
-Narrow side solid

I suggested aerogel before; other materials which might be of use include carbon fiber cloth, as well as some kind of metal-carbon fiber composite, which would be useful because it would have fairly high strength up to the melting point of the metal, e.g., the carbon fibers would continue to have at least some strength for as long as the material remained solid.  I really like the idea of inflatable heat shields, I think they have a lot of promise.  We might want to look at an inflatable covered with carbon fiber cloth, or something of that nature.  A bit of aerogel thrown in never hurts either smile  haha.  Inflatable shields need more development but I think basic feasibility has more or less been demonstrated.

By the way, GW, just wondering:  Do you have experience in orbital rockets?  If so, which ones?


-Josh

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#103 2012-01-02 10:17:29

SpaceNut
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Re: Reusable Rockets to Orbit

Most errors are of the compounding round up or down of digits...
Heat shield materials are dictated by the temperatures that they must withstand for the period of time in use.
So for the reuseability it appears that the current descusion of design is utilizing wings, rather than to over come parachutes and recovery zones. But what is used must not require tons of man hours to review or to replace between useages.

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#104 2012-01-02 14:58:50

JoshNH4H
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Re: Reusable Rockets to Orbit

Spacenut, I very much agree.  Also, you bring up the very relevant point of how to slow the rocket down through the last mach or so, when the heat shield is no longer very effective at slowing down, but it is still going far too quickly to land, and it would be inconvenient to keep fuel aboard to just rocket away the velocity.  The idea that immediately occurs to me is to have inflatable wings or at least control surfaces to slow it down to a velocity where you can use the remaining methane in the tanks (just a bit, maybe as a gas for pressurization) to control yourself to a landing, be it vertical or horizontal.


-Josh

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#105 2012-01-02 19:41:20

Hop
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Re: Reusable Rockets to Orbit

JoshNH4H wrote:

Hop, are you seriously not willing to accept an estimate of 9400 m/s +~100 m/s?

Of course I'm willing to accept that estimate. That isn't the result I differ with.

This is what I object to:

JoshNH4H wrote:

rocketfreebodydiagram.png

The blue arrow is the length of the Green arrow minus the Red arrow.  The integral of this arrow with respect to time from launch until orbit is going to be equal to 2,370 m/s for a 300 km orbit.  2,370 m/s is 2.81 MJ/kg expressed as a velocity.  If you separate the two different vertical ⌂V components in this manner, the methods you would have to use to calculate both become much clearer.

I've seen several models I like that arrive at around 9.4 km/s.

GW has described two good models. One is taking small time slices and re-evaluating the thrust vector each slice, accounting for gravity and drag loss on each slice.

The other is the quick and dirty (but effective) jiggered rocket equation.

John Shilling describes his methods of figuring ascent penalty here.

Depending on vehicle specifics, most of all these can give you about 9.4 km/s for reaching a 300 km altitude circular orbit?

I don't even know what your method is. So far it very vague and nebulous. How do you plug in the above 2.37 km/s to get 9.4 km/s?

Given the models you've volunteered so far, I don't give even odds you can write useful code to model ascent.

JoshNH4H wrote:

Seriously?

Seriously? Seriously -- I'm not contesting 9.4 km/s.  That you think I am demonstrates you didn't bother to even glance at what I actually wrote.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#106 2012-01-02 20:39:02

JoshNH4H
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Re: Reusable Rockets to Orbit

I understand what you wrote. What I don't understand is why you continue to talk about it when its relevance to the thread is minimal at best.  If you want to talk about trajectories and calculations, you are free to do so in a dedicated thread.  Seeing as it is a fairly interesting topic and one that is very relevant to orbital rocketry, I will absolutely participate in said thread if I have the time.  However, if we're sticking to a generally accepted figure of 9400 m/s delta V to orbit it is not relevant here.


-Josh

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#107 2012-01-02 23:11:11

SpaceNut
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From: New Hampshire
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Re: Reusable Rockets to Orbit

JoshNH4H wrote:

Spacenut, I very much agree.  Also, you bring up the very relevant point of how to slow the rocket down through the last mach or so, when the heat shield is no longer very effective at slowing down, but it is still going far too quickly to land, and it would be inconvenient to keep fuel aboard to just rocket away the velocity.  The idea that immediately occurs to me is to have inflatable wings or at least control surfaces to slow it down to a velocity where you can use the remaining methane in the tanks (just a bit, maybe as a gas for pressurization) to control yourself to a landing, be it vertical or horizontal.

The technolog that you refer to was in the airplane for mars thread that we had. It inflated a resin filled wing in the atmosphere that when struck by UV would harden.

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#108 2012-01-02 23:29:04

JoshNH4H
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From: Pullman, WA
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Re: Reusable Rockets to Orbit

That sounds like a very interesting technology; however, even if it didn't actually harden, simply using gas to inflate would probably be sufficient for the relatively low-force domain at the lowest velocities.


-Josh

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#109 2012-01-03 07:14:33

Hop
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From: Ajo
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Re: Reusable Rockets to Orbit

JoshNH4H wrote:

I understand what you wrote.

Not demonstrated. You still haven't answered my objections with numbers or equations.

Scratch that, you did toss out 9.4 km/s. If you did understand what I wrote, you'd know that was a straw man.

If you deliberately used a dishonest straw man argument, that would harm your credibility more than mere lack of understanding.

JoshNH4H wrote:

What I don't understand is why you continue to talk about it when its relevance to the thread is minimal at best.

(sigh....) Once again, ascent trajectory models are relevant to designing reusable rockets to orbit. They are the heart of matter.

Moreover your original post started with an attempt to model ascent.

What is irrelevant is an an avalanche of fan boy cheer leading. Cheerleading without valid equations or sound models. Such discussions will bring us no closer to achieving re-usable rockets.

Willfully ignoring the numbers actually subtracts from the credibility of such cheerleaders. If you want this New Mars to be thought of as an ineffective fringe group, ignoring math is a good way to achieve that.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#110 2012-01-03 13:47:06

RGClark
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From: Philadelphia, PA
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Re: Reusable Rockets to Orbit

GW Johnson wrote:

Hi gang:
This is GW Johnson the old aero engineer,  and ramjet expert from long ago.  I surely am glad to see the forums up and running again. 

In recent news:  I have picked up a consulting client for a possible ramjet launch effort.  And,  that client and I both think I may be just about the last living US all-around expert in ram propulsion (I seem to have outlived the rest). 

I’m particularly glad to see LEO access under active discussion,  especially with Josh and Hop talking about reusable vehicles,  and perhaps ramjet assist.  The last stuff I had is a posting over at http://exrocketman.blogspot.com,  where I looked at horizontal takeoff and landing with a winged first stage using separate rocket and ramjet power.  That article is dated August 22,  2010,  so it’s way down the list (chronological,  latest on top).  There’s a navigation tool by date and title on the left,  under my photo.  It looked to me like a staging condition of near Mach 6 at around 60,000 feet altitude might well work out,  including booster flyback.  And,  it looks like ramjet might really pay off in this scenario.


  Hello again, GW. Perhaps you can help me with a question I'm working on. I want to calculate the fuel burn for an airbreather, such as a turbojet or ramjet, as it accelerating upwards, climbing to altitude.
What is usually given in the specifications of a jet engine is its specific fuel consumption. But I'm fairly sure this is based on the gross thrust. However, to calculate the acceleration of your vehicle you need to subtract off the ram drag to calculate the net thrust. How do you calculate this net thrust?
Also, I believe the specific fuel consumption also changes with altitude and speed. How do you take that into account?

    Bob Clark


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#111 2012-01-03 21:16:29

SpaceNut
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Re: Reusable Rockets to Orbit

Hop and JoshNH4H can we please reframe from any more of this back and forth on gravity, thank you...

The hypercone was to be just an inflated disk of material. Also there was some I-beam construction that were inflateable from the old threads but that was some time after the crash that we discussed them....

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#112 2012-01-04 14:14:45

GW Johnson
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From: McGregor, Texas USA
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Re: Reusable Rockets to Orbit

To answer a question Josh asked earlier,  I spent some time at what was then LTV Aerospace in Dallas working on the old "Scout" launcher.  I used a combination of jiggered rocket equation stuff and motor manufacturer catalogue data to set up the real trajectory code stuff.  The "gold standard" was (of course) the trajectory code.  My job was to determine feasible advanced configurations for "Scout",  and feasibility of some really unusual missions for it to do.  "Scout" was a 4 stage solid propellant vehicle.  They lost 1 of 4 in flight test,  then never another one in 30-some years. 

For Bob Clark:  airbreather thrust,  particularly ramjet,  is very strongly (dominantly) dependent upon flight speed and altitude air density.  The nozzle thrust is calculated same way as a rocket (chamber total pressure,  gas properties,  pressure ratio across the nozzle,  and nozzle geometry),  the pressure is just lower and the expansion ratio a lot less.  You do need to worry about the difference between static and total chamber pressure,  unlike most rockets. 

The ram drag is the drag of decelerating the ingested stream of air into the vehicle.  Its massflow multiplied by its freestream velocity (in appropriate units of measure) is the way that is done.  But,  nozzle force minus ram drag is only "net jet" thrust.  There are several more propulsion-related drag items to account.

There is spillage drag for subcritical inlet operation (which also means reduced inlet massflow!),  additive or pre-entry drag for ingested stream tubes in contact with the vehicle forebody,  and the drag of boundary layer diverters or bleed slots,  quite common with supersonic inlets.  None of those are simple to calculate "from scratch" (we use wind tunnel test data to correlate empirically a coefficient for each as a function of Mach and vehicle attitude angles),  and taken together they are often quite a significant force. 

If you subtract that sum of drags from net jet thrust,  you have the "local" or "installed" thrust,  corresponding with just plain airframe drag.  Most airframers work in that definition.  If you don't,  then you have to add that sum of propulsive drags to the airframe drag to get the corresponding proper drag for "net jet" thrust-drag accounting (not very popular outside the propulsion community). 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#113 2012-01-05 17:28:10

JoshNH4H
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From: Pullman, WA
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Re: Reusable Rockets to Orbit

Spacenut, gladly.  We have much more relevant things to talk about.

I also remember reports of paper airplanes which were to be launched from the ISS.  I don't know if it ever happened but it seems like a very interesting and very relevant experiment. 

I've always also wondered about the possibility for radiative cooling for re-entry vehicles.  Perhaps use inflatable boons to expand a carbon/glass fiber wing, use it to glide down and paint the backside black to help with the heat rejection.  This is one of those things, I think, where the only way to really know the specifications of the system is to experiment with different configurations, especially when we're talking about technologies which haven't seen a whole lot of development.  (Maybe SpaceX will take some interest in the context of reusable second stages, you never know).  It should be possible to come up with basic mass estimates, but which configuration is the best does not seem to be something which we can know for sure without some real-world testing.


-Josh

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#114 2012-01-06 09:33:22

GW Johnson
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Re: Reusable Rockets to Orbit

Paper airplanes from ISS.  Interesting.  The ultimate in low wing loading.  Did they ever run this experiment?  Especially since the ignition point for paper in air is about 451 F or 233 C? (not as exciting in metric,  thanks to Ray Bradbury). 

I think I need to amplify for Bob Clark on airbreathers.  Ramjet and gas turbine are quite different,  even though they may share the same kinds of inlet components.  Gas turbine air massflow is set by the engine speed and the subsonic air density at the compressor face.  In essence,  it's almost a constant volume flow rate at any particular rotor speed,  and that volume flow is more-or-less proportional to the rotor speed (which is your throttle setting).  The inlet has to be operated subcritically in order to match the massflow it scoops up with the massflow demand of the engine.  The usual specifications give the maximum static thrust of the installed engine,  and a minimum TSFC figure,  which is not obtained at that thrust,  but at around 2/3 thrust or thereabouts.  Afterburner complicates things further.  In-flight figures are different yet. 

Ramjet does not have a massflow demand inside to match.  The inlet airflow maximizes at whatever the cowl lip can sweep out,  or can be less due to spillage,  but never greater.  Ramjets can (and were) designed to operate subcritically for maximum pressure recovery,  but most modern all-supersonic systems are supercritical inlet for maximum massflow.  If the internal flame stabilizer is not too lossy,  then max massflow is more important than max pressure recovery.  Systems that fly subsonically or transonically still design and operate the old way. 

For ramjet,  the internal fluid mechanical conditions from forward and aft must match up at the nozzle entrance:  the amount it can flow must match at recovered pressures to the amounts scooped up and injected as fuel.  If the nozzle can flow more,  then you reduce pressure recoveries by going further supercritical.  If the nozzle cannot flow enough,  then you reduce massflow by going subcritical and spilling air.  These calculations can get quite complicated,  especially if the nozzle unchokes,  or a C-D nozzle separates due to overly-high backpressures.  All of them are best done with compressible flow models. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#115 2012-01-06 14:24:00

Hop
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From: Ajo
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Re: Reusable Rockets to Orbit

GW Johnson wrote:

Paper airplanes from ISS.  Interesting.  The ultimate in low wing loading.  Did they ever run this experiment?  Especially since the ignition point for paper in air is about 451 F or 233 C? (not as exciting in metric,  thanks to Ray Bradbury).

The 8 km/s re-entry is a major problem. They have a thread on RLVs at NasaSpaceFlightForum. Danny Dot noted the difficulty of finding durable materials that can withstand the temperature. He said the shuttle's leading edge was 3000 degrees F. He complained of a TPS material being softer than chalk.

It seems to me one of the problems is achieving an FMR of 16:1 and having a spacecraft strong and temperature resistant enough to endure re-entry.

Watching Musk's Grasshopper video it looks like he hopes to use reaction mass to shed re-entry velocity in addition to aerobraking. If he hopes to achieve some re-entry delta V with propellant, this makes his FMR even more challenging. Get the FMR too high and you have a very tenuous, fragile vehicle even less able to endure re-entry. I'm not giving Musk's TSTO RLV even odds.

However, given propellant in orbit, I believe it's quite doable to decelerate the upper stage and land it intact on the launch pad. Thus I believe lunar supplied propellant depots would enable TSTO RLVs.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#116 2012-01-06 14:50:10

GW Johnson
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Re: Reusable Rockets to Orbit

Shuttle leading edges went to 3000 F and required carbon-carbon composite precisely because the wing loading (vehicle weight divided by wing planform area) was up around 1 or 2 hundred pounds per square foot,  just like a fighter jet,  and all the space capsules.  The white tiles on the sides and upper wing surfaces,  and the black ones on the belly and lower wing surface,  were low-density alumino-silicate,  with a solid phase change that causes cracking at 2300 F.  Those were restricted to peak 2000 F skin temperatures on the shuttle.  Carbon-carbon is weak enough structurally,  to be sure.  Those tiles were far more fragile yet. 

If the vehicle has a much larger aerosurface for its weight (low wing loading,  say 10-20 pounds per square foot),  peak skin temperatures reduce to under 2000 F,  although total heat to be absorbed and disposed of actually increases.  Skin temperature drives the material selection problem,  the other is handled fairly easily.  You will be decelerating at higher gees to make this happen.  Plus,  it's all a transient. 

Myself,  I rather like the idea of enduring a bit rougher ride in order to make my re-entry vehicle out of simple aluminosilicates,  perhaps even plain old fire curtain cloth on a steel tube frame.  I think it is very funny that the better,  less fragile reentry vehicle might actually be built similar to the venerable old Piper Cub of the 1930's. 

And,  low density ceramics should be fiber reinforced as ceramic-ceramic composites,  not that fragile stuff on the shuttle.  I have done this,  a quarter century ago.  They are still extremely low density,  yet fairly tough structurally.  Really tough compared to that fragile nonsense they flew on shuttle.  I used mine as a ramjet liner,  which survived hours of burn and hundreds of excursions into very violent rich-blowout instability.  The only reason I quit then was the project was done.  Could have gone on for many more hours. 

As it turns out,  the materials I used then are still available.  I checked just the other day. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#117 2012-01-07 00:25:22

JoshNH4H
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From: Pullman, WA
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Re: Reusable Rockets to Orbit

Hop- Assuming that FMR stands for functional mass ratio (I had to google it and look at a list of possible meanings, that one made the most sense in context), I quite agree.  It's difficult, though certainly not impossible, to get your mass ratio down to that area anyway (It has certainly been done; It is my understanding that several off-the-shelf rocket stages have the mass ratio to function as SSTO launch vehicles.  At the very least I can point to the Centaur upper stage as having this theoretical capability, seeing as I've actually done the calculations for it, though this is saying nothing of thrusting capabilities).

GW- Lower wing loading would be nice.  Using an empty rocket as your aeroshield seems to be a decent way to get a pretty low mass per area, as I said before perhaps enough with just a tiny bit of augmentation to do it with just a thin heat shielding layer over the plain metal.  I don't know.  I also remember I once suggested using a magnetic field to deflect the hot plasma.  It's a wacky idea, but if it would work at all I think it would work best in the context of an empty cylindrical rocket tank, which after all could function as a giant solenoid.  Anyone have ideas on that?  It might be too far term to consider for this rocket, but it is physically possible.


-Josh

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#118 2012-01-07 08:30:35

Rune
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From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Reusable Rockets to Orbit

JoshNH4H wrote:

GW- Lower wing loading would be nice.  Using an empty rocket as your aeroshield seems to be a decent way to get a pretty low mass per area, as I said before perhaps enough with just a tiny bit of augmentation to do it with just a thin heat shielding layer over the plain metal.  I don't know.  I also remember I once suggested using a magnetic field to deflect the hot plasma.  It's a wacky idea, but if it would work at all I think it would work best in the context of an empty cylindrical rocket tank, which after all could function as a giant solenoid.  Anyone have ideas on that?  It might be too far term to consider for this rocket, but it is physically possible.

Well, looking at the history of reusable first stages, it seems you need a very sturdy solid motor casing, and a low staging speed, to survive the pressure loads if you just use "an empty rocket stage". Ask SpaceX, it seems they also overestimated their chances (F9's first stage is covered in cork to heat-shield it through reentry, but they have only recovered little pieces of it and telemetry that indicates it blows up when it hits the atmosphere).

The plasma idea needs a powersource, never mind the magnets (cryocooled if superconducting, mind you). An incredibly dense one that doesn't create more waste heat that it deflects. Good luck with that one.

Me, I lean on the idea that if you get it sturdy enough to survive aerodynamic loads (and control the heck out of it the whole way so you choose the precise angle of attack), then you can heat-sink your way through it with little added effort. Pressure-fed tank sturdy, for example. If necessary, an active cooling loop can work wonders (the shuttle used one with freon, that's where the heat went in the end), and an open one (I'm thinking perspirating heatshield) even more so.

So, my two cents can be summed by up by this idea that just popped in my head: spaceX got a real nice engine by forgetting their initial dreams of cheap(er) pressure-fed rockets... but their stages ended up failing structurally. With great empty weights, of course. That their are going to have to kill with structural reinforcements.


Rune. That's what happens when you forget the BDB creed. XD


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#119 2012-01-07 08:37:29

Rune
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From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Reusable Rockets to Orbit

GW Johnson wrote:

Paper airplanes from ISS.  Interesting.  The ultimate in low wing loading.  Did they ever run this experiment?  Especially since the ignition point for paper in air is about 451 F or 233 C? (not as exciting in metric,  thanks to Ray Bradbury).

IIRC, and I might not, it was not exactly paper. Or not any paper. And some numbers run by someone at JAXA said it should work. As far as I know, no one called the phone number written in the little things, which doesn't really prove much. Oh, and they were supposed to take their sweet time going down, on the order of days.

That's what I remember of the story, anyway. Luckily, we no longer need to trust our faulty organic brains these days. Here's what google spits, dated 2009, a year before they let them loose:

http://www.airspacemag.com/space-explor … plane.html


Rune. Quite cute in any case.


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#120 2012-01-07 10:29:07

GW Johnson
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From: McGregor, Texas USA
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Re: Reusable Rockets to Orbit

If these paper airplane things really were released,  I suspect most of any survivors went into the ocean.  I never heard about any results though.  Fascinating experiment!

As for Falcon tanks "exploding" during entry,  I am not surprised.  Heat shield cork or not,  with inert mass fractions in the 4-5% range,  these items are quite fragile.  A tumbling cylinder is going to get crushed from the side by stagnation pressures as it tumbles broadside.  High internal pressure could stave that off a bit,  but stopping the tumble with a drogue to take the loads end-on is actually more effective,  and the toughest,  heaviest part of your heat shield can be smaller. 

Spacex's cost reductions come from a smaller logistical support tail,  not from reusability.  Falcon-9 at 10 metric ton payloads has the same price per unit mass delivered as Atlas-5's max 20 ton payloads (both near $2400/pound,  if memory serves).  Falcon-Heavy will beat Atlas 5 by about a factor of 3 on unit price,  and deliver more than twice the mass at 53 metric tons.  And that's without effective reusability.  Small logistical tail is the real driver for cheaper access to LEO,  not reusability.

Reusability might help,  though.  (But,  I really doubt it would be more dramatic than what Spacex achieved with its smaller logistical tail.)  Maybe the first step is detach and save the engines only.  Use the tankage sacrificially to protect the engines during reentry,  then detach and parachute the engines to the sea.  They'll have to be tough engines,  sea water does bad things to hot metal.  They'll need a float,  too. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#121 2012-01-07 11:05:30

Hop
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From: Ajo
Registered: 2004-04-19
Posts: 146
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Re: Reusable Rockets to Orbit

JoshNH4H wrote:

It's difficult, though certainly not impossible, to get your mass ratio down to that area anyway

The mass ratio is only part of what I said. You need a large ratio and having a spacecraft strong and temperature resistant enough to endure re-entry.

Whether that's possible, I don't know. It has yet to be demonstrated.

JoshNH4H wrote:

At the very least I can point to the Centaur upper stage as having this theoretical capability, seeing as I've actually done the calculations for it, though this is saying nothing of thrusting capabilities).

Needless to say, I don't regard your math as consistently sound. I would need to see a cite and the work behind your calculations.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#122 2012-01-07 12:29:07

RGClark
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From: Philadelphia, PA
Registered: 2006-07-05
Posts: 501
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Re: Reusable Rockets to Orbit

Hop wrote:
GW Johnson wrote:

Paper airplanes from ISS.  Interesting.  The ultimate in low wing loading.  Did they ever run this experiment?  Especially since the ignition point for paper in air is about 451 F or 233 C? (not as exciting in metric,  thanks to Ray Bradbury).

The 8 km/s re-entry is a major problem. They have a thread on RLVs at NasaSpaceFlightForum. Danny Dot noted the difficulty of finding durable materials that can withstand the temperature. He said the shuttle's leading edge was 3000 degrees F. He complained of a TPS material being softer than chalk.
It seems to me one of the problems is achieving an FMR of 16:1 and having a spacecraft strong and temperature resistant enough to endure re-entry.
Watching Musk's Grasshopper video it looks like he hopes to use reaction mass to shed re-entry velocity in addition to aerobraking. If he hopes to achieve some re-entry delta V with propellant, this makes his FMR even more challenging. Get the FMR too high and you have a very tenuous, fragile vehicle even less able to endure re-entry. I'm not giving Musk's TSTO RLV even odds.
However, given propellant in orbit, I believe it's quite doable to decelerate the upper stage and land it intact on the launch pad. Thus I believe lunar supplied propellant depots would enable TSTO RLVs.

I don't think it's that hard. We already have two examples of reusable spacecraft, the space shuttle and the X-37b. The percentage weight of the TPS for the space shuttle of the max landing weight of the shuttle is about 8%:


Space Shuttle thermal protection system.
3.3 Weight considerations.
http://en.wikipedia.org/wiki/Space_shut … iderations

Reportedly the TPS on the X-37b is lighter weight but tougher and requiring less maintenance:

X-37B Orbital Test Vehicle.
http://www.boeing.com/defense-space/ic/ … b_otv.html

You would need to add wings for this approach but a common estimate is that wings account for 10% of the landing weight which wouldn't be too much. And with modern materials the wing weight probably can be cut to half that.
Elon doesn't like wings though. An estimate of the fuel requirements for a powered vertical a la the DC-X is 10% of the landed mass:

Horizontal vs. vertical landing (Henry Spencer; Mitchell Burnside Clapp).
http://yarchive.net/space/launchers/hor … nding.html

For TPS, the PICA-X material weighs half that of the AVCOAT material used on the Apollo heat shield:

Re: Dragon v/s Orion.
http://forum.nasaspaceflight.com/index. … #msg754168

The Apollo heat shied was 15% of the landed mass:

Apollo Command/Service Module.
2.7 Specifications
http://en.wikipedia.org/wiki/Apollo_Com … ifications

So the Dragon heat shield can be 8% of the landed weight. You still have to figure out how to get the heat shield in place to cover the engines on reentry while uncovering the engines on ascent and for the short time when you are making the powered portion of the landing. Not a trivial task.
One solution is to do head first reentry. This requires high heating on the pointy end of a SSTO. But the PICA-X material is even supposed to be able to withstand the reentry speeds from Mars flights, so it could work for this.
For the TSTO's SpaceX is planning, you could have the blunt heat shield at the top of each stage. I thought this was would be unstable with the heavy engines at the top during reentry. But apparently this was a solved problem during the DC-X planning because that was the preferred reentry method since it the offered for the Air Force extended cross-range.
Another solution that was suggested for the DC-X was for base first landing with an aerospike nozzle. The high temperature resistant aerospike was supposed to act as a heat shield, perhaps in addition with some low thrust firing of the engines.


  Bob Clark

Last edited by RGClark (2012-01-07 17:17:11)


Nanotechnology now can produce the space elevator and private orbital launchers. It now also makes possible the long desired 'flying cars'. This crowdfunding campaign is to prove it:
Nanotech: from air to space.
https://www.indiegogo.com/projects/nano … 13319568#/

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#123 2012-01-07 13:08:27

Rune
Member
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Reusable Rockets to Orbit

GW Johnson wrote:

Spacex's cost reductions come from a smaller logistical support tail,  not from reusability.  Falcon-9 at 10 metric ton payloads has the same price per unit mass delivered as Atlas-5's max 20 ton payloads (both near $2400/pound,  if memory serves).  Falcon-Heavy will beat Atlas 5 by about a factor of 3 on unit price,  and deliver more than twice the mass at 53 metric tons.  And that's without effective reusability.  Small logistical tail is the real driver for cheaper access to LEO,  not reusability.

Reusability might help,  though.  (But,  I really doubt it would be more dramatic than what Spacex achieved with its smaller logistical tail.)  Maybe the first step is detach and save the engines only.  Use the tankage sacrificially to protect the engines during reentry,  then detach and parachute the engines to the sea.  They'll have to be tough engines,  sea water does bad things to hot metal.  They'll need a float,  too.

I'm with you on the logistics tail part, of course. And part of that success stems from a simple design (one fuel, one engine) that allows to simplify production. The thing is, they started out with a simpler design still, pressure-fed engines without turbopumps, but got away from it early on grounds of performance. Of course they came up with a pretty amazing engine instead, but I'd bet a Merlin without turbopump would be even cheaper to build, even if it doesn't break any records. Dunno, maybe the isp and weight fraction hit makes TSTO completely impractical, and they really wanted to keep the number of stages to recover controlled. But I "somehow" doubt it's because stronger tanks made of inexpensive steel, not those fancy lithium alloys, are more expensive...

Also, I'm a bit dubious of ocean recovery, since it requires both specialized equipment and stress on the hardware (read: corrosion, but also the water impact) leading to additional work to "process" the stage for a new flight. Every engineer hour spent refurbishing a rocket could be spent building a new one. So in order to have a truly effective reuse you have to minimize the processing effort. Ideally, that process would be as simple as hooking a computer so the internal systems verify everything is OK, doing a test fire, stacking, filling the tanks and charging the batteries, and nothing else, at least not for dozens of flights. Well, maybe the traditional visual inspection before flight and some ground transport by other means. I see that happening only if either the stage lands horizontally on a runway or vertically under rocket power, and since wings are for airplanes and not for rockets, vertical landing it is, IMO.


Rune. Short of a low-tech staged DC-X. As many stages as it takes to make it cheap and lasting.


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#124 2012-01-07 19:27:29

JoshNH4H
Member
From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
Website

Re: Reusable Rockets to Orbit

wrt the paper airplane re-entry, it was my understanding according, I believe, to that "Scientific American" article that the material being used was paper coated in glass nanofibers; Not exactly paper, per se, but still not something that could exactly be described as "not paper."  I'm not surprised that it did not survive re-entry, but on the other hand that they thought it might work at all I think says something about the potential that this kind of material has.

Hop:

Needless to say, I don't regard your math as consistently sound. I would need to see a cite and the work behind your calculations.

Were you polite, I would give you a citation and tell you to work the rocket equation out for yourself.  Seeing as you've instead taken it as an opportunity to flame me, I don't see why you would expect any response.  Whatever impatience I may have had in posts towards you in the past, I do not intend to continue because this is not in accordance with forum rules or simple matters of respect.  I will instead treat future flames on your part as rule violations and respond as appropriate.


-Josh

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#125 2012-01-07 21:05:15

louis
Member
From: UK
Registered: 2008-03-24
Posts: 5,466

Re: Reusable Rockets to Orbit

JoshNH4H wrote:

wrt the paper airplane re-entry, it was my understanding according, I believe, to that "Scientific American" article that the material being used was paper coated in glass nanofibers; Not exactly paper, per se, but still not something that could exactly be described as "not paper."  I'm not surprised that it did not survive re-entry, but on the other hand that they thought it might work at all I think says something about the potential that this kind of material has.

Hop:

Needless to say, I don't regard your math as consistently sound. I would need to see a cite and the work behind your calculations.

Were you polite, I would give you a citation and tell you to work the rocket equation out for yourself.  Seeing as you've instead taken it as an opportunity to flame me, I don't see why you would expect any response.  Whatever impatience I may have had in posts towards you in the past, I do not intend to continue because this is not in accordance with forum rules or simple matters of respect.  I will instead treat future flames on your part as rule violations and respond as appropriate.

Well I for one don't want to lose Hop's input. I've been flamed by him - and lived to tell the tale.  smile [ And I'm still awaiting his answer on why with more meteorites we have higher meteorite prices on Earth...] Best to ignore the flaming or respond in kind - banning would diminish the site.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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