subsonic vs supersonic airlaunch

"Henry Spencer" wrote in message
...
In article ,
Pete Lynn wrote:
An airlaunch system which is truly less expensive will shift weight
and
complexity from the rocket to the aircraft wherever possible, even at
great expense.

I'm not so sure about that. Common superstitions notwithstanding,
aircraft design is harder, not easier, than rocket design -- there are
more things interacting. It looks to me like the right tradeoff is to
keep the aircraft as simple and straightforward as possible, even if
it
means making the rocket do a bit more work. (But then, I don't really
believe you need or want the aircraft at all...)

Currently, subsonic aircraft would seem less expensive at delivering
large payloads to high altitude than rockets, perhaps in the long term
this will change, though I am doubtful. I suppose my current attraction
to airlaunch is based on the potential for small scale, high flight
rate, low cost development. As such I might initially favour a large
motorised hang glider powered by a large propeller with tip rockets.

I have often contemplated a necessarily inexpensive high altitude
subsonic airlaunch system where not only is the rocket not weighed
down
by unnecessary aerodynamic structures, but where the aircraft carries
separate cryogenic tanks such that the rocket might be fueled at
altitude, avoiding the need for tank insulation on the rocket.

Such things have been suggested. Of course, if you don't use LH2, you
probably don't really need tank insulation on the rocket.

Yes.

LH2 tank weight is one of the primary disadvantages of LH2, as
technology progresses one would expect significant incremental reduction
in tank weight, this works in favor of LH2. As the dry mass penalty of
LH2 reduces, (assuming other dry mass does not similarly reduce), I
suspect that there might come a time, (not yet), when the hydrocarbon
vs. LH2 trade off moves in favour of LH2, I am wondering under what
circumstances, if any, this might come to pass, and if airlaunch might
change this balance.

Assuming cost effective airlaunch, gross rocket weight should not be
a
significant factor, not compared to rocket dry mass.

Even with airborne fueling, the aircraft does have to take off
carrying
the full gross weight of the rocket. It may be a little bit better
distributed, but it's all there.

You can escape that by using flight refueling, or doing your own LOX
manufacturing in flight, but either is a big added complication.
--

Yes.

Pete.

"johnhare" wrote in message
m...
With range regulatory issues, the aircraft looks like a possible win,
even
including development or modification costs. From a pure performance
standpoint, the pure rocket system wins. IMO, the extra regulatory and
range safety issues will be more expensive than the extra technical
aspects
of the airlaunch for early systems. When sufficient traffic builds up,
then
the more efficient rocket systems can operate from commercial launch
areas that keep the regulations handled in a timely manner. You have
made these points before on Sealaunch also.

I am not so sure, if we assume a high altitude subsonic airlaunch buys
the equivalent of 500m/s in delta v, then this might equate to something
like an extra 1% in payload fraction to LEO. If we assume that such an
aircraft might carry rocket at $500/ton then the extra payload to LEO
comes in at $50/kg. Ultimately I suspect an aircraft could do it for
near a tenth that price as fuel cost might only be around $10/ton
carried.

I suspect that it will be a long time before pure rocket vehicles are
down to $50/kg to LEO, let alone $5/kg to LEO, and so I think that
airlaunch might be justified. Obviously this analysis is a gross
approximation and other factors will decide, in the short term at least.

Pete.

Pete Lynn wrote:

"Henry Spencer" wrote in message
...
In article ,
Pete Lynn wrote:
I tend to think in terms of aerodynamically shielding the rocket and
launching at low dynamic pressure.

Not impossible, as witness the current DARPA work and some other
concepts.
It also removes any requirement that the rocket be aerodynamic
(assuming
that you don't plan to recover the whole thing).

Yes.

But it does add problems of its own, mostly in the form of still
greater
demands on the carrier aircraft. I think the B-58 was the only one of
the
big supersonics to ever do such a thing, and the B-58s are gone. So
you
need an all-new aircraft, and one with a sizable cargo bay, taking you
still farther away from existing design practice.

An airlaunch system which is truly less expensive will shift weight and
complexity from the rocket to the aircraft wherever possible, even at
great expense. And so the above infers that such airlaunch might not
actually be cost effective, independent of the other problems.

I have often contemplated a necessarily inexpensive high altitude
subsonic airlaunch system where not only is the rocket not weighed down
by unnecessary aerodynamic structures, but where the aircraft carries
separate cryogenic tanks such that the rocket might be fueled at
altitude, avoiding the need for tank insulation on the rocket.

Actually, you might want to do this even more because of center of
gravity issues at takeoff (assuming it's launched off of the back of the
carrier aircraft) more than tank insulation....

"Pete Lynn" wrote in message
...
"johnhare" wrote in message
m...
With range regulatory issues, the aircraft looks like a possible win,
even
including development or modification costs. From a pure performance
standpoint, the pure rocket system wins. IMO, the extra regulatory and
range safety issues will be more expensive than the extra technical
aspects
of the airlaunch for early systems. When sufficient traffic builds up,
then
the more efficient rocket systems can operate from commercial launch
areas that keep the regulations handled in a timely manner. You have
made these points before on Sealaunch also.

I am not so sure, if we assume a high altitude subsonic airlaunch buys
the equivalent of 500m/s in delta v, then this might equate to something
like an extra 1% in payload fraction to LEO. If we assume that such an
aircraft might carry rocket at $500/ton then the extra payload to LEO
comes in at $50/kg. Ultimately I suspect an aircraft could do it for
near a tenth that price as fuel cost might only be around $10/ton
carried.

I suspect that it will be a long time before pure rocket vehicles are
down to $50/kg to LEO, let alone $5/kg to LEO, and so I think that
airlaunch might be justified. Obviously this analysis is a gross
approximation and other factors will decide, in the short term at least.

Pete.

For the near term, and modest payloads, I believe airlaunch can be effective
on the financial and technical scale. In the long term, a commercial
spaceport
that strongly addresses the regulatory side can swing the advantage to the
ground launched rocket. If the requirement is for a mach 3 or less launch
assist platform, I suspect that a reusable pure rocket vehicle can win.

Then there are the wild cards like the jet engine described in patent
#06430917 which might swing the advantage back.

"Pete Lynn" wrote in message ...
This is a trade off I have been trying to get a handle on for some time.

Firstly, very crudely assuming that the subsonic airlaunch vehicle
delivers rocket at $500/ton and the supersonic airlaunch vehicle
delivers rocket at $5000/ton. I am just guessing these numbers trying
to use the Concorde and 747 as rough guides, better estimates would be
appreciated, also, there are conceptual aircraft designs that might be
an order of magnitude cheaper.

Secondly, again assuming very crudely that payload mass fraction would
be 2% and 3% for subsonic and supersonic airlaunch respectively, (again
better estimates would be appreciated).

Then above a rocket cost only of $450/kg of payload to LEO supersonic
airlaunch is preferred, below this subsonic airlaunch is preferred.

Obviously this is very crude and specific designs, cost analysis and
comparison would be required for a more accurate result. However, I
find it very interesting that the question of subsonic vs. supersonic
airlaunch comes down to the cost of the rocket vehicle in terms of
cost/kg of payload to orbit.

If the above numbers are vaguely in the right ball park then in the
short term, while costs are high, supersonic airlaunch will be
preferred. In the long term, if costs decrease, then subsonic airlaunch
might become favoured.

Of course this is completely inaccurate, but I find the cost
relationship interesting and a little unexpected.

Pete.

I think if you were basing it on an existing plane, then subsonic
would win hands down. After all, how much would it coust to have two
strap on boosters to take the vehicle from 250m/s to 500m/s.

If you were designing the progam from scratech, then a Mach 4 launcher
might be a viable option. You could go even faster if you can get
SCRAMJets to work properly. However, expect the development costs of
your launcher alone to be of the order of several billion.

I think you can do better than a payload mass fraction of 2%. I was
estimating 5% with kerosene engines, though of course, the dry mass
fraction is a bit of a guess. I also assumed 2 stages after
seperation, with seperation somewhere between 5 and 6 km per second.

In article ,
Treez03 wrote:
*Could* be, not necessarily *would* be. There have been plenty of launch
systems designed to separate at quite sizable Q.

The point is that if high Q staging is a valid concern, not only *could* it be
designed out of a supersonically staged system, but it *would* be.

I think you vastly underestimate the level of hubris of the designers of
those systems, many of them hypersonic-flight people rather than launcher
people (that is, people to whom high-Q staging is an intriguing research
problem rather than a dangerous nuisance).

Rule #1 of successful launcher design is to never, never, never let the
hypersonics people near your drawing board.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

"Alex Terrell" wrote in message
m...
I think if you were basing it on an existing plane, then subsonic
would win hands down. After all, how much would it coust to have two
strap on boosters to take the vehicle from 250m/s to 500m/s.

I expect quite a lot. :-)

If you were designing the progam from scratech, then a Mach 4 launcher
might be a viable option. You could go even faster if you can get
SCRAMJets to work properly. However, expect the development costs of
your launcher alone to be of the order of several billion.

I suspect that it is fairly safe to assume that scram jets will always
be more expensive than the equivalent rocket.

I think you can do better than a payload mass fraction of 2%. I was
estimating 5% with kerosene engines, though of course, the dry mass
fraction is a bit of a guess. I also assumed 2 stages after
seperation, with seperation somewhere between 5 and 6 km per second.

I was assuming an assisted SSTO approach, (showing my prejudices).
Higher payload fractions do tend to reduce the cost advantage of
airlaunch, though not sufficiently to make it uneconomic, in the ideal
sense.

Pete.

Rule #1 of successful launcher design is to never, never, never let the
hypersonics people near your drawing board.

Couldn't agree more. The space launch problem is not the same as supersonic
cruise. (Hypersonics? Let's not even go there!) Range and economy are
paramount for supersonic cruise. Maximum velocity at maximum altitude is the
design driver for supersonic launch. Rapidly climb, accelerate, zoom, and then
come home. It's a brute force problem, not a nuance problem where another half
percent drag reduction or inlet recovery is appreciated.

This brings me back to my original question. Why should a supersonic launch
platform cost appreciably more to build than a subsonic one made of the same
material? I suspect the answer may lie in the lower L/D of supersonic plan
forms. If supersonic L/D is half the subsonic case, it follows that the
aircraft would be roughly twice as big to carry the same payload. Add a larger
fuel load needed to accelerate out to supersonic speeds, and there would be a
lot more airplane to build in the supersonic case. On the other hand, for a
given payload the rocket stage gets smaller due to higher staging velocity.
Studies I've seen indicate minimum takeoff weight with staging somewhere
between M=2 and 3 along a fairly flat curve, all other things being equal.
This assumes new-build, mission specific aircraft in both the supersonic and
subsonic cases.

In the end, I don't think the trade would be technology driven. Other "little"
things like desire for secondary use of the aircraft, intended launch site,
availability of supersonic corridors, etc., would probably be the deciding
factors.

"Alex Terrell" wrote in message
om...
Perhaps. I was thinking that if SCRAMJets can be made to work,
there may be a market for a high Mach vehicle with varients for
1. Cargo, 2. Passengers, 3. Space Launch.

Cost increases sort of exponentially with speed while efficiency
decreases sort of exponentially with speed. If your base rocket costs
$10000/kg to LEO then a SCRAM jet might be justified, as it might still
be able to get more delta v for your buck than either the rocket or a
subsonic launch system, (although subsonic launch would be orders of
magnitude cheaper it can not deliver as much delta v, which is worth
more in this case).

If your base rocket costs less than $500/kg to LEO then I doubt even
supersonic launch would be justified, as the rocket could get the delta
v cheaper. Supersonic launch systems are ultimately less efficient and
more expensive than a developed rocket system, they will not ultimately
be cost effective if rocket costs reduce to the necessary levels.
Subsonic launch systems should always be cheaper than a pure rocket
system as they are inherently more efficient and cheaper over that first
atmospheric stage, everything else being equal.

Even if you had a SCRAM jet for free, it would still be more expensive
to run than a number of rocket systems that are already in existence.
The choice of preferred airlaunch system comes down quite directly to
the cost of the rocket system.

Pete.

(Treez03) writes:

Assuming a new aircraft would be built from scratch in either case, why does it
cost more to build a supersonic aircraft? The supersonic case would certainly
burn more fuel, maybe doubling direct operating cost. Beyond that, I've never
seen why a supersonic aircraft should cost significantly more than a subsonic
aircraft, as long as speeds are low enough that they can both be built of the
same materials.

Because it takes significantly more thrust to go supersonic. Because
the aerodynamics become harder (you have to be stable in both subsonic
and supersonic regimes). Because if you go much faster than the speed
of sound, heating becomes a big enough issue you *can't* use the same
materials and techniques.

In short, because it's harder.

Jeff
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