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View Full Version : "The future of space exploration belongs to entrepreneurs and dreamers."



ChrisRT
2006-Jan-15, 04:50 AM
I've seen this said many times since the beginning of the millennium, yet it kills me.
The first step in the many of starting a private space venture is to create a means of getting into orbit... as seen in recent years, this cost hundreds of millions if not billions.

The way people spew about "The future of space exploration belongs to entrepreneurs and dreamers" makes it sound as if anyone can design, produce, and systematically launch rocket after rocket with paying citizens in it. In reality the first company to do such a thing will be Virgin Galactic. The man that started this company has billions at his disposal. The second he secures a position and starts launching citizens in 2008(?) he will be the monopoly of private space tourism.

My question is why do people say this? Is it just off the top of their heads or do they really think about what they are saying?
Say that you want to start your own private tourism company... Where would one get the funds? This is something few can do... The second that Richard Branson launches the first rocket; even if someone else magically came up with the billions to start his own venture there will be no room in the market. Richard will have the entire market share...
So is there really any space to dream in this market sector?

3rdvogon
2006-Jan-15, 10:30 PM
Whilst what Virgin are up to is an interesting development simply shooting up to a height of 60 or 70 miles and then dropping back down again in a short sub-orbital "hop" is a pretty small thing compared to getting into orbit.

I do not see any of these "garage" space programmes producing a vehicle that can achieve orbital velocities and withstand the higher stresses and temperatures of orbital re-entry. I think producing a vehicle that could achieve that would overstretch even Branson's funds.

Whilst I do not wish to unduly talk down the achievements made with Spaceship One I feel it is a little unfair for the popular media to try and compare what it can do with the Shuttle or even the Russian programme.

joema
2006-Jan-17, 09:29 AM
...I do not see any of these "garage" space programmes producing a vehicle that can achieve orbital velocities and withstand the higher stresses and temperatures of orbital re-entry. I think producing a vehicle that could achieve that would overstretch even Branson's funds...

It's great to see entrepreneurs pursuing manned space flight, however as stated above it's a huge leap from a suborbital hop to orbital flight. In fact orbital flight requires at least 65 times the energy, and similarly upgraded heat shielding. It seems very unlikely a manned orbital vehicle could be air dropped like SpaceShipOne or the X-15. The reason is required propellant mass is greater than any aircraft could lift, even a B-52. Remember you need 65x the energy, hence from the rocket equation much more propellant than 65x. That means a conventional surface launch, which in turn means a big rocket.

There's no clever way around that. You can make a super efficient airplane like Voyager, but you can't make a similarly efficient manned launcher. Getting to orbit is largely a brute force proposition involving huge amounts of energy.

Would be facinating to see what solutions Rutan, etc devise for those areas, but it will require a lot of money. More discussion in this thread: http://www.bautforum.com/showthread.php?t=19416

Glom
2006-Jan-17, 02:08 PM
X-15 was cool.

Some like to compare this to the early stages of civil aviation. It started out in the hands of only the rich and eventually expanded to all. But, in fact there was a difference. Civil aviation got people to where they wanted to go. Previously, they had been limited to trains and ships, which took ages. Civil aviation allowed travel to these places to be more convenient.

Currently, there is no where really to go in space other than space itself. It's a bit like the Concorde charter flights that British Airways did. Just for the fun of it. But that is hard to sustain into a large industry. Without stuff already there in space to go to, there will be limits to growth.

3rdvogon
2006-Jan-17, 03:04 PM
Surely the only efficient way to get to orbit:

If we assume for the moment that manipulating gravity remains in the realm of Sci Fi and if nanotubes prove inadequate to make the space elevator viable.

Then that leaves use with some sort of areodynamic vehicle driven by a reaction engine as the only option.

Logic dictates that lifting vertically off a pad is wateful - you probably expend half your propellant load just getting up to the cruising speed of a Concorde. How many tons of fuel as wasted getting the vehicle clear of the tower. Every vertical take-off aircraft we have built has been less efficient and had a shorter endurance than its conventional equivalent. Vertical take-off to orbit might be quick and simple but as everyone knows it is velocity - in particular, velocity along the line of your intended orbit that matters, not simply getting up to a certain altitude.

Using some sort of ground based electric catapult/ramp or whatever is a help but is any such system going to be capable of accelerating a large payload carrying vehicle much above Mach 1 - also do you want a vehicle weighing several hundred tons traveling horizontally that close to the ground.

I therefore think that such an electric launch system might be used to accelerate a vehicle up to the sort of velocities at which a RamJet might start operating but nothing more.

It would seem sensible that the vehicle should take advantage of aerodynamic lift to get it into the upper atmosphere where there is reduced resistance. Also we should attempt to achieve velocities as high as possible Mach 6/7 (which is half way to orbital) using an airbreathing engine. We should only start adding LOX to the combustion cycle at the point where the atmospheric supply becomes inadequate. Even so we should still draw atmospheric gases into the engine as when heated and accelerated they still provide a useful reaction mass, thus reducing the mass we have to carry on board. To put it simply we must extract everything we can out of the atmosphere to help us on our way rather than blindly fighting our way through it as all launch vehicles do today.

I have always believed that some form of single stage, largely air-breathing, spaceplane design with multi-modal engines is the only logical way go. The trouble is whilst this is the obvious solution the enormous R&D involved and long development programmed necessary will mean that I think the private sector will never put aside the funds for it.

Lets be clear - the massive growth in civil aviation that happened in the 1950s and the 1960s would never have happened without WW2 and the follow on cold War. Where would the Boeing Stratocruiser have been without the B29 before it. Could the 707 have emerged if Boeing had not had the benefit of Government funding from the B47 and the B52.

The private sector might have produced revolutions in micro-electronics and it may well do wonders in the field of genetics but I think it will not be able to have a big impact on getting into space without government funding. Unless I am wrong with my earlier assumption and some backyard inventor does discover the secret of an anti-gravity drive.

Glom
2006-Jan-17, 05:35 PM
Every vertical take-off aircraft we have built has been less efficient and had a shorter endurance than its conventional equivalent.

Specious. Of course, VTOL aircraft are less efficient than fixed wing aircraft, but that's for the job at hand, just like turbojets are less efficient than turbofans for jetliners. For spacecraft, bearing all the weight of aerofoils and control surfaces makes for great wasteage (see the Space Shuttle). The savings from getting to a relatively low speed and low altitude by aerodynamic means is more than lost by having to lug all the structure required for that stage up into orbital space.


Using some sort of ground based electric catapult/ramp or whatever is a help but is any such system going to be capable of accelerating a large payload carrying vehicle much above Mach 1 - also do you want a vehicle weighing several hundred tons traveling horizontally that close to the ground.

Yes. Trains have been accelerated to many a mach number by rail guns. A rail gun wouldn't be horizontal, but would be inclined to provide ballistic assistance in getting to altitude. I think X-15 esque craft could be launch in this way.

3rdvogon
2006-Jan-18, 10:00 AM
The savings from getting to a relatively low speed and low altitude by aerodynamic means is more than lost by having to lug all the structure required for that stage up into orbital space.


Yes. Trains have been accelerated to many a mach number by rail guns. A rail gun wouldn't be horizontal, but would be inclined to provide ballistic assistance in getting to altitude. I think X-15 esque craft could be launch in this way.

Improvements in materials technologies should reduce the deadweight of the aerodynamic surfaces, furthermore in an improved lifting body design is developed there is greater useable volume made available for other purposes.

I was not aware that any had yet built a supersonic maglev train or similar - I understood that such things were still operating at the cruising speed of the average turbo-prop. Unless you run your train in a vacuum tube no country is going to tolerate a large supersonic mass travelling at ground level. Remember they would not permit the Concorde to fly Supersonic over land. A railgun might accelerate a small solid projecticle up to hypersonic velocities for military purposes. That is quite different from doing the same thing with a large hollow vehicle designed to carry cargo or passengers. Getting up to say Mach 7 whilst in the "thick" air below 10,000 feet would surely demand that the structural strength of the vehicle and its ability to withstand extreme heat would both be greater than it would need for normal re-entry. That alone would add unessesary mass to the vehicle in much the same way as putting wings on it.

If the vehicle could take off climb and do its intial accelation using conventional jet fuel and atmospheric O2 - thus only switch to injecting cryogenic fuels into its engines after reaching Mach 3 then the wings would not be wasted they could be used to hold fuel. And if we were really smart and produce effective heat shielding any reserve jet fuel in the wing tanks could be used to restart the engines in sub-sonic mode to help with a more controlled landing.

I know this means developing an engine that can sustain combustion and provide thrust right through a speed range of Mach 0.2 throught to Mach 12 and that this a very tall order - but short of inventing the "mythical" anti-gravity such an engine must be seen as the holy-grail of aerospace.

Glom
2006-Jan-18, 10:36 AM
Improvements in materials technologies should reduce the deadweight of the aerodynamic surfaces, furthermore in an improved lifting body design is developed there is greater useable volume made available for other purposes.

That still doesn't change the fact that in terms of fuel at least, it is more efficient to launch vertically off a pad and get through the atmosphere as quickly as possible than to waste time and fuel flying an aerodynamic regime.


Unless you run your train in a vacuum tube no country is going to tolerate a large supersonic mass travelling at ground level.

It won't be at ground level. By the time it leaves the tunnel, it will already have some significant altitude. Plus it would be near the coast anyway where it wouldn't be supersonic over land.


Getting up to say Mach 7 whilst in the "thick" air below 10,000 feet would surely demand that the structural strength of the vehicle and its ability to withstand extreme heat would both be greater than it would need for normal re-entry.

Who said it would need to reach mach 7 by the time is gets out?


If the vehicle could take off climb and do its intial accelation using conventional jet fuel and atmospheric O2 - thus only switch to injecting cryogenic fuels into its engines after reaching Mach 3 then the wings would not be wasted they could be used to hold fuel. And if we were really smart and produce effective heat shielding any reserve jet fuel in the wing tanks could be used to restart the engines in sub-sonic mode to help with a more controlled landing.

Designing an engine capable of accepting both liquid and cryogenic fuels is no mean feat. Further, turbojets are very guzzly.

3rdvogon
2006-Jan-19, 06:35 PM
That still doesn't change the fact that in terms of fuel at least, it is more efficient to launch vertically off a pad and get through the atmosphere as quickly as possible than to waste time and fuel flying an aerodynamic regime.

It won't be at ground level. By the time it leaves the tunnel, it will already have some significant altitude. Plus it would be near the coast anyway where it wouldn't be supersonic over land.

Who said it would need to reach mach 7 by the time is gets out?


Designing an engine capable of accepting both liquid and cryogenic fuels is no mean feat. Further, turbojets are very guzzly.

I simply cannot believe that lifting of vertically makes more sense - at the rate such rocket engines burn fuel nearly half the vehicles lift-off mass must have been burnt by the time it reaches Mach 3, when it is still probably below 60,000 feet alt. It would be possible to reach that speed and velocity using airbreathing engines for a fraction of the fuel mass expended.

As for reaching Mach 7 by electric propulsion - I chose that because that would mean the electric system would be getting you close to 50% of orbital velocity. If it only accelerates the vehicle to Mach 1.5 or 2.0 then quite frankly is it worth the massive infrastructure costs to built such a machine when you still have a hell of lot more speed to add to vehicle using its on board propulsion.

You say turbojets are guzzly - Yes but less guzzly than rockets - at least most of the reaction mass they push out of the back comes from the surrounding atmosphere and is not being hauled along on board.

Of course designing an engine that can be:
First a Turbofan
Then a Turbojet with AB the fan disk being feathered and becomming a baffle to slow down the air entering the AB chamber
Then close off the turbines to become a Ram Jet
Then SCRamJet with LOX injection to boost as required
Then finally switch over to pure rocket mode.
Would be a massive engineering exercise but once cracked it would be the perfect earth surface to orbit propulsion system - one that would extract every possible ounce of thrust that could be drawn from the air it passing through with minimal waste.

That is unless we can find some workable way of drawing in air molecules at the front of an engine and acclerating them electrically so that they emerge from the back at extreme velocities whilst at the same time "beaming" the energy to do this to the vehicle from either ground or orbital stations I do not see a more efficient way of achieving the high velocities required to get into orbit. Burning tons of on board LOX just to reach the same cruising speed and altitude of a Concorde is lunacy - the only reason we do it that way now is because it is the easiest proven engineering option not the most elegant or efficient solution.

Ilya
2006-Jan-19, 07:07 PM
Of course designing an engine that can be:
First a Turbofan
Then a Turbojet with AB the fan disk being feathered and becomming a baffle to slow down the air entering the AB chamber
Then close off the turbines to become a Ram Jet
Then SCRamJet with LOX injection to boost as required
Then finally switch over to pure rocket mode.

Would be a massive engineering exercise but once cracked it would be the perfect earth surface to orbit propulsion system - one that would extract every possible ounce of thrust that could be drawn from the air it passing through with minimal waste.
Well, as long as you are wishing for magic... I have a much simpler solution. Simpler in the sense of much fewer moving parts.

Compressed air rocket. Air in question is compressed almost to degenerate matter density. Open the valve at the rear and shoot to orbit. Simple, reusable, and completely non-polluting.

How to build the tanks to hold the said compressed air is left as an engineering exercise :)

Glom
2006-Jan-21, 11:36 AM
I simply cannot believe that lifting of vertically makes more sense

Well believe it. The upper limit of ATC airspace is a tiny fraction of the distance to orbital altitude. ATC airspace ends at 60,000ft, which is less than 20km. Space itself doesn't begin until 100km and it is isn't until 300-400km that you are in proper orbital space. Getting up to "high altitude" by aerodynamic means is only getting you less than 10% of the way there. And that's the easy bit! Even at say 2000kt, you're still only doing 1km/s. You need another 7km/s get to orbital speed. Drag at these speeds is huge so you want to get out of the thick atmosphere as quickly as possible. The problem here is that if you get out of the thick atmosphere, you are no longer in the aerodynamic regime. You're a ballistic missile.

You're aerodynamic regime gives you only a very small portion of the energy you need, but in return for that, you must haul the weight of aerodynamic surfaces and the mechanisms to operate them, all shielded against reentry heat, plus all the complications of air breathing engines, which with all their compressors and turbines tend to be more complicated than rocket engines.

Sorry, but at the end of the day, getting a bit of altitude and speed cheaply through aerodynamic flight might seem like a good way to go, but in the end, it just doesn't give you enough to make the added burden worthwhile. Rockets may seem guzzly on lift-off, but such is the business of spaceflight. At the end of day, it is cheaper to take a simple missile and blast it up through the atmosphere as quickly as possible than to faff around with a near useless aerodynamic regime.

joema
2006-Jan-21, 06:27 PM
...at the end of the day, getting a bit of altitude and speed cheaply through aerodynamic flight might seem like a good way to go, but in the end, it just doesn't give you enough to make the added burden worthwhile...
Exactly right. Airbreathing at first seems very compelling. E.g, consider the space shuttle orbiter/external tank. 75% of the entire assembly weight is just LOX. In theory an airbreathing vehicle would avoid this, thus being much lighter or having far more payload.

In actuality the drawbacks of airbreathing hypersonic flight eat away at the advantages until there's nothing left, plus you're saddled with a vastly more complex (and expensive) vehicle. That's one reason why the National AeroSpace Plane (NASP) failed.

It's not obvious watching a rocket launch, but most of the powered flight is essentially horizontal, building up orbital speed. It only ascends vertically a brief interval. The goal is get above the meaningful atmosphere fast, then pitch over and start building speed. That's by far the most efficient way.

A rocket burns a lot of propellent getting a few km up, but propellant is dirt cheap. E.g, LOX is virtually free and LH2 is just a few hundreds thousand $, even for the space shuttle.

Airbreathers must fly a "depressed trajectory" and stay within the atmosphere for much of the ascent. It's like a reentering space capsule but far worse. The heat shielding requirements are far beyond a space capsule or the shuttle.

As 777geek mentioned, you've got to have huge turbine engines to haul the entire vehicle off the runway and up to scramjet speed. In actuality no turbojet can do that, so you must have an interim engine to boost from turbojet max Mach no up to minimum scramjet operational speed. Then scramjets cannot achieve orbital speed, so you need yet another engine type (generally rockets) to boost from scramjet top speed to orbital speed.

Each of those systems requires separate fuel, separate structure, separate control systems, and shielding for the return trip.

Someday it may be done, but you don't get extra points for getting to orbit "the hard way", or getting to orbit the "coolest way". The goal is getting to orbit the simplest, safest and cheapest way. Rockets are very good at that, relative to the currently achievable alternatives.

For more info see: http://en.wikipedia.org/wiki/Rockwell_X-30#Advantages_and_disadvantages_of_hypersonic_airb reathing_orbital_vehicles

An alternative to horizontal launch airbreathing vehicles is airbreathing mother ship that carries a smaller rocket powered orbital vehicle.

However even the X-15 (which has many times the energy performance of Rutan's SpaceShipOne) wasn't remotely capable of achieving orbit. Yet the X-15 almost maxes out the B-52 payload capacity.

Therefore a manned orbital air launch vehicle would require a mothership much bigger than a B-52.

t/Space is thinking of having Rutan build a gigantic mothership to air launch a man carrying orbital vehicle. However the mothership would be titanic -- gross weight of one million lbs, payload 150 tons (3x a 747-400 freighter), wingspan 320 feet (1.5x a 747). It would the largest aircraft ever constructed.

That's one possible way and likely cheaper and more achievable than a hypersonic airbreathing orbital vehicle. However "cheaper" is a relative term. Building the world's largest heaviest aircraft wouldn't be exactly cheap, plus you have the additional development of the orbital vehicle.

Spherical
2006-Jan-22, 05:35 PM
The little booster that can:

http://www.spacex.com/updates.php

Also, I would not be so terribly quick to dismiss Rutan's fly back first stage design so quickly. A subsonic airframe with high aspect ratio wings is not such a difficult thing to design and build as it once was.

joema
2006-Jan-22, 08:11 PM
...I would not be so terribly quick to dismiss Rutan's fly back first stage design so quickly. A subsonic airframe with high aspect ratio wings is not such a difficult thing to design and build as it once was.
I'm not dismissing it, just saying it won't be easy, cheap or quick. It would be the worlds largest, heaviest aircraft, plus you'd have to separately develop a man-carrying rocket powered orbital vehicle MUCH larger and higher performance than the X-15 (which was already much higher energy performance than SpaceShipOne). IOW two major development efforts, vs a unitary surface-launched rocket.

Anybody that thinks building the worlds largest aircraft is not difficult should examine Howard Hughes's Spruce Goose, or Russian Antonov 225. Both were very expensive, had protracted development, and were produced in quantities of one.

As any aircraft designer will corroborate, there's nothing magical about using modern composite construction. It doesn't buy you huge gains in ease of construction, cost, or strength-to-weight ratio. All existing certified composite aircraft are ample testimony to that.

Basic calculations show the huge Rutan mothership likely would not have a high aspect ratio wing. If max gross weight is about 1 million lbs and span is 320 ft (as stated in the Jan 2006 Air & Space magazine), and if wing loading about equal to Rutan's Global Flyer http://www.answers.com/topic/virgin-atlantic-globalflyer (55 lb/ft^2), wing chord (fore-aft wing distance) would be 57 feet. That's an aspect ratio of 5.6, which is a broader wing than a Piper Cub. It would be an ungainly, chunky thing, not a thin tapered high aspect ratio wing.

Given sufficient funding Rutan could likely do it, do it much cheaper than the NASA space shuttle (for whatever THAT's worth), and operate it more cheaply and reliably.

But there's nothing unique about an air launch concept that facilitates those attributes.

A more traditional vertical launch rocket that emphasizes low cost, simplicity, reliability, and small support staff could also achieve the same goal, possibly at less cost and complexity than building the worlds largest, heaviest aircraft. E.g, something like the DC-X/DC-Y/DC-1 http://en.wikipedia.org/wiki/Delta_Clipper

My main points are (1) It's not easy to do (2) SpaceShipOne is less than 1/65th the energy capability and proves little vs a vastly larger orbital vehicle (3) There's nothing unique about the air launch approach that facilitates this (4) Compared to NASA, private enterprise may do it cheaper and faster, but that's such a low bar that any improvement would be easy.

Spherical
2006-Jan-22, 09:00 PM
Snipped:



A more traditional vertical launch rocket that emphasizes low cost, simplicity, reliability, and small support staff could also achieve the same goal, possibly at less cost and complexity than building the worlds largest, heaviest aircraft. E.g, something like the DC-X/DC-Y/DC-1 http://en.wikipedia.org/wiki/Delta_Clipper

My main points are (1) It's not easy to do (2) SpaceShipOne is less than 1/65th the energy capability and proves little vs a vastly larger orbital vehicle (3) There's nothing unique about the air launch approach that facilitates this (4) Compared to NASA, private enterprise may do it cheaper and faster, but that's such a low bar that any improvement would be easy.

No, if it were easy to do, I would have done it. I disagree with you about SpaceShip One. It proves a great deal. I am unconvinced about point three. As to four, who can argue?

Here is the point everyone should bear in mind. Private enterprise can do the job and it appears that there are private interests keenly interested in doing the job.

As for the straight up rocket launch, SpaceX shows that there are ways of doing that better as well.

ASEI
2006-Jan-22, 10:04 PM
Perhaps, if our nation pursued a moon-base and mars base aggressively, with no sign of packing up and going home, then this could serve as the place in space to go to, and to base buisnesses off of. Delivering supplies to the moonbase could be a serious deal for private space firms - an economic justification to build rockets, which would in turn provide cost saving innovations and economies of scale.
Running various utilities there, or construction to expand the base could also be spinoff industries.

Alas, that's a space program for a nation that's still largely active, industrial, and interested, as we were back in the 50s-70s. The modern space agency, government, and public would get bored halfway through the effort.

I agree with what is said here about rockets. There really is only one way to get into orbit using chemical propulsion. You can shape it like a traditional rocket, or try to pretend it's an airplane, like the shuttle, but the vehicle is going to end up being 90+% fuel, 10-% payload and structure. What we can do to ensure that these rockets are cheaper is to bring economies of scale into play in production, perhaps create a mass-producible liquid engine, or large scale hybrid boosters. After all, the vast majority of the cost of a rocket is development cost. If we would just make 10 times more of a particular rocket, it could be as much as 5 times cheaper.

BTW, though the memes "space-plane" and "SSTO" were heavily entrenched by a PR campaign in the 90's, there's nothing particularly useful about SSTO either. SSTO is very very hard to acheive with chemical rocket engines. Even with hydrogen/oxygen (and hydrogen carries it's own problems), an SSTO would have to have less than a 10% structural mass fraction. Most liquid rocket stages take 15%-20%. That's without any payload capacity. The only thing going for the Venture Star was a composite fuel tank that would just barely enable the vehicle to fly. The composite turned out to crack at the cryogenic temperatures that the hydrogen needed to be stored at, and it was porous, so the hydrogen leaked from the tank. The only way to justify an entire fuel tank made of composite materials (which have to be cured essentially in the shape and at the scale in which they're used, in gigantic autoclave facilities, and require a whole host of fastening gimmicks and fatigue lifespan limitations), is to make the vehicle reusable. Reusable vehicles need a lot of structure to be able to survive re-entry, and extremely expensive and complicated engines to survive more than one flight (and even then, they usually need to be repaired after 4 or 5). For a given pmf, two stage to orbit vehicles are far far less massive for a given payload than SSTOs. For missons involving greater dv, three stage to orbit vehicles become much less massive than two stage to orbit vehicles.

As the project progressed, the SSTO with the internal payload bay became the SSTO with an external payload bay, then the "One and a half stage to orbit" vehicle with an upper stage rocket, then a "technology demonstrator", and at last was relegated to a fancy escape pod for the ISS. Honestly, the people who engineered the Saturn V were closer to right than the space plane crowd has been historically. I wonder how the space program would have gone had we built Nova and continued to build improved version of the Saturn V?

joema
2006-Jan-23, 01:54 AM
...there's nothing particularly useful about SSTO either...

SSTO if implemented correctly could be extremely useful. E.g, the liquid fuel cost of a single shuttle is less than $1 million. The botched Venturestar program doesn't prove SSTO is not viable or unworkable, anymore than the botched Navy Vanguard program proved that orbital flight was unworkable. The program was flawed, not the concept.


Reusable vehicles need...extremely expensive and complicated engines to survive more than one flight...
Just because the resuable vehicle we're most familiar with (the shuttle) has very complicated, expensive engines doesn't mean all reusable vehicles must be so. The SSMEs were specifically designed for absolute maximum performance in the smallest possible space, which means tremendous combustion chamber pressures, great complexity and high development and operational costs. That is not representative of all reusable engines for all reusable vehicles.

In fact there are existing reusable engines such as the RL-10 that are vastly less complicated and expensive than the SSMEs. Another example is the Rocketdyne J-2S, originally planned for Saturn upper stages and later, X-33. It's much less complex and less expensive than the SSME: http://www.astronautix.com/engines/j2s.htm


an SSTO would have to have less than a 10% structural mass fraction. Most liquid rocket stages take 15%-20%.
Existing rockets not designed for SSTO have ALREADY achieved the required mass fraction.

The Titan II 1st stage's structural mass was only about 3.4%. It could theoretically be an SSTO all by itself. And that was with early 60's technology.


For a given pmf, two stage to orbit vehicles are far far less massive for a given payload than SSTOs...
There's no question SSTOs are challenging, but as stated above the required mass fractions were already achieved in the early 1960s. With a low mass fraction, SSTOs are very workable. You're right TSTOs have certain advantages, but it's difficult to reuse the 1st stage.

Honestly, the people who engineered the Saturn V were closer to right than the space plane crowd has been historically. I wonder how the space program would have gone had we built Nova and continued to build improved version of the Saturn V?
The Saturn V was a magnificent machine, but it was very expensive, about $2 billion per launch in current dollars. Nova would have been much more expensive. However an uprated Saturn V using F-1A engines was easily achievable, in fact the F-1A was already developed. It would have had significantly more payload at little more cost.

ASEI
2006-Jan-23, 02:24 AM
but it's difficult to reuse the 1st stage. Shouldn't recovering the first stage be much easier than recovering the second? You have far less of a re-entry problem for the first stage of a launch vehicle. They're usually going much slower, and are closer to their home launch base. They could theoretically parachute into the ocean, couldn't they?

Even if you do get cheap reusable engines that are robust enough to actually live up to reusability, wouldn't it still make more sense to do TSTO for the relaxed allowances for structural margins and increased payload capacity? If you're going to be building and refurbishing stages anyway (which is the majority of the cost, independent of how much of it you're rebuilding and refurbishing) wouldn't it make more sense to be working with more robust, easier to engineer stages than flimsy, absurdly tight performance envelope SSTOs?

Can anything with a 10% structural mass fraction (including payload) survive re-entry anyway? That's a lot of tank to shield with a very small mass. Perhaps it would make sense to jettison the tank and re-enter only the engines in some sort of engine landing pod?

joema
2006-Jan-23, 05:39 PM
Shouldn't recovering the first stage be much easier than recovering the second? You have far less of a re-entry problem for the first stage of a launch vehicle. They're usually going much slower, and are closer to their home launch base. They could theoretically parachute into the ocean, couldn't they?
With fully reusable TSTO, the big problem is recovering the 1st stage. Optimal performance design means the 1st stage will be much larger (maybe 5 times the mass) vs. the 2nd stage. It's just how the math works out.

That in turn means the staging velocity will be very high, maybe 8,000-12,000 ft/sec, which in turn means the 1st stage will be many hundreds of miles downrange, at high altitude, and moving away at high velocity.

How do you recover a large liquid fueled 1st stage under those conditions? It's very hard. It would essentially have to reenter, albeit at lower than orbital speed, so would need heat shielding. Parachuting into the ocean isn't compatible with delicate liquid fueled engines. In theory you could put wings on it and fly it back under remote control. In that case it would need jet engines, fuel, structure and control systems. If you wanted to land it horizontally you'd also need landing gear. All those things add complexity, development cost, and hurt mass fraction.

Despite the difficulties of SSTO, that's why fully reusable TSTO isn't an easy solution. They both have problems, and while different, the TSTO problems are formidable.

The mass fraction benefit of TSTO is significant but not huge. E.g, a kerosene-LOX SSTO requires 94% mass fraction (max 6% structural mass), while a TSTO of similar technology requires 90% (max 10% structural mass). For more details see (PDF):

http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf


Even if you do get cheap reusable engines that are robust enough to actually live up to reusability, wouldn't it still make more sense to do TSTO for the relaxed allowances for structural margins and increased payload capacity?
As stated above, the structural margins and mass fraction benefits of TSTO vs SSTO are not huge.

The question really is, why consider fully reusable TSTO since the mass fraction advantages aren't big, yet it introduces another set of problems, plus requires developing two separate vehicles.


Can anything with a 10% structural mass fraction (including payload) survive re-entry anyway? That's a lot of tank to shield with a very small mass. Perhaps it would make sense to jettison the tank and re-enter only the engines in some sort of engine landing pod?
Actually having a high mass fraction (say 5% empty structural mass) is an advantage on reentry, not a disadvantage. Vehicles like the Apollo CM and shuttle are very heavy relative to their surface area. That in turn means much greater heat shielding requirements. A reentering SSTO would be very light relative to its surface area. It's like the difference between a falling leaf and a falling rock. It wouldn't take nearly as much heat shielding, and aerodynamic stresses would be spread across a broader area. Specifics:

Aerodynamic "wing" loading on reentry:
Apollo CM: 487 kg/m^2
Shuttle orbiter: 331 kg/m^2
Venturestar: 143 kg/m^2

For a 95% mass fraction vehicle (5% empty structural mass), structural forces on reentry are actually quite small relative to the structural forces of holding the propellant on liftoff. A vehicle with a high mass ratio (hence large wing-area-to-empty-mass ratio) also doesn't need as robust heat shielding.

I'm not saying SSTO is easy or the only right way, but fully reusable TSTO has it's own difficulties which are essentially just as formidable.

As shown in the above article, large air launch orbital vehicles have yet another set of difficulties, plus the performance benefit of air launch is less than you'd think.

Personally I think the best approach is either a well designed SSTO focusing on simplicity and low operational cost (e.g, DC-X/DC-Y/DC-1), or an expendable multistage booster focusing on simplicity, low manufacturing costs and low operational costs.

publiusr
2006-Jan-25, 10:06 PM
Given sufficient funding Rutan could likely do it, do it much cheaper than the NASA space shuttle (for whatever THAT's worth), and operate it more cheaply and reliably.

Rutan has no experience with large aircraft. The AN-225 is based on the AN-124--and there are quite a few of those. Branson needs to buy the second un-assembled AN-225, and let Rutan build an interim HOTOL or MAKS type craft of a smaller, more manageable size that fits his lvele of production.

Then Branson would have a VIRGIN airlift and space division. VLA will be of far less general utility than AN-225.

Until then, space still belongs to the Soviet Chief designers and their offspring--whose big gov't rocketry efforts were every bit the sucess that collective farming was a failure.

Space flight is blue-collar work, not white collar sport.

Less like MSN--more like TVA.

joema
2006-Jan-25, 10:24 PM
Rutan has no experience with large aircraft...
If you read the above thread, you'll see that's essentially my point.

publiusr
2006-Jan-25, 11:21 PM
You and I understand that--but the cool-aid drinkers don't.

ASEI
2006-Jan-26, 01:00 AM
an SSTO would have to have less than a 10% structural mass fraction. Most liquid rocket stages take 15%-20%. .


Existing rockets not designed for SSTO have ALREADY achieved the required mass fraction.

The Titan II 1st stage's structural mass was only about 3.4%. It could theoretically be an SSTO all by itself. And that was with early 60's technology.


Perhaps the estimates for mass fraction that I was given were overly conservative. I'll look into this.

As far as spreading the aerodynamic forces out over a larger structure, you still get more moment and less "beam" mass to deal with it. A 500 kg ceramic rock is virtually indestructable aerodynamically. A 500 kg, 1/2 inch wide ceramic sheet with some aluminum supports thrown in is far more flimsy. (stress = My/I, M = integral(shear, x) that sort of thing). Though you have a point that they would slow down more rapidly in the upper atmosphere, and re-enter the lower atmosphere slower.

joema
2006-Jan-26, 05:02 AM
...A 500 kg, 1/2 inch wide ceramic sheet with some aluminum supports thrown in is far more flimsy...
While correct, my statement about wing loading may have been misleading.

It's counterintuitive, but the qbar (dynamic pressure) is much lower on reentry than ascent. Therefore the aerodynamic loading and resultant structural stress is much lower on reentry. A large, lightweight object is just not under that much physical stress during reentry.

The problem on reentry is heat, not aerodynamic stress. You have a given amount of kinetic energy to radiate. IOW so many megawatt hours to dissipate. A small surface area will therefore be under much higher thermal stress than a large surface area, all other things (mass, velocity, etc) constant.

It's like the difference between an 1 kilowatt electric stove eye that's 8 inches diameter vs a 1 kilowatt stove eye that's 4 inches diameter. If power input is the same, the smaller one will get much hotter.

So in that sense it's similar to aerodynamic wing loading (mass/wing area), but the energy equivalent: kinetic energy / wing area.

The bottom line is for a given vehicle mass and reentry speed, the one with larger "wing" area will be under much less thermal stress per unit area. That in turn means it requires less robust, more lightweight thermal protection.

Back to aerodynamic stress: a good example of a large, very lightweight structure that endures much aerodynamic stress is the space shuttle external tank. It weighs about 66000 lbs (29940 kg) empty, and loaded with propellant 1.65 million lbs (748400 kg). Thus the ET mass fraction is 96% (4% structural mass). Yet on ascent it undergoes several times the dynamic pressure as upon reentry. It burns up on reentry because it has no heat shielding at all, not because it can't take the aerodynamic force.

For details on space shuttle ascent/reentry dynamic pressure, see: http://spaceflight1.nasa.gov/shuttle/reference/green/