Space Forum / Space Flight / November 2005

There was quite a detailed discussion of this many years ago.  Back then, I
remember it being called "pogo".  If you do a Google Groups search for "pogo
jet engine first stage", you'll find news articles about this concept.

The above led me to here:

POGO (a.k.a. Jet Engine Launch Assist Concept (JELAC))
http://www.alt-accel.com/pogo/pogo.htm

Jeff

Yes, but with the exceptions of air launch from an existing aircraft, and
systems which use the jets for some other purpose as well, using rockets
is better.

Jet engines and their air intakes are complex and heavy, work only over
limited ranges of speed and altitude, and are quite fussy about the
smoothness of the incoming airflow (which means you can't just hang them
on anywhere).  Rocket engines are light and compact, don't care about
speed or airflow, and work *better* as altitude increases.

Why is that an advantage?  Liquid oxygen is compact, relatively easy to
store and handle, and so inexpensive that it's nearly free.  It *is*
heavy... but with rockets, extra thrust is cheap.

Design group after design group has come up with an elegant jet/rocket
design, and as an afterthought compared it to an all-rocket approach...
and been startled to discover that the all-rocket system looked to be
simpler, more capable, and cheaper both to develop and to operate.

If you go the first 5 miles straight up with jet engines, you only have 195
miles left to low orbit. I heard it's an insignificant amount and a total
waist of time.

Not really, except some people do use jet aircraft for airlaunch.

Jets aren't much good for VTOL expendables, too much weight and too little
thrust, performance falls off at ~15 km/45,000 ft and low mach, and they
cost a lot too.

What could be useful though is a reuseable piloted winged rocket booster
with jets and wheels for the first stage of a TSTO, and just use the jets to
get from the ground to and from the upper atmosphere.

I usually use the idea of a 747 with rockets engines added. MTOW 450 tons,
flies to 15 km high and a convenient place on jets, then the main rocket
burn takes it to 120 km altitude and 2 kps horizontal, dropping off a 60 ton
second stage on the way.

This has several advantages, first you start the main burn at 15 km and say
250 m/s - that's not a lot in terms of getting to orbit, although it helps a
little - but now you can optimise your rocket engines for vaccuum operation,
you don't have the disadvantage of trying to operate then at sea level.

You can also get a bit of lift on main burn while keeping the g load down -
in an ordinary rocket accelerating upwards at 3 g perceived (and using 3 g's
worth of fuel) is actually acclerating at 2g relative to the Earth. 1 g's
worth is used just to keep you where you were.

I don't suggest trying to optimise for supersonic flight performance, and
you would be flying upwards at ~ 45 degrees while you were still in
atmosphere, but any L/D at all is a bonus [*].

The wings and jets allow safe flyback, safe operation if the main engines
fail in a non-catastrophic way (and even some catastrophic ways, depending
on design).  It adds many safe aborts - in fact the whole flight can have
safe aborts all the way, assuming the known, proven aircraft technology does
not fail - five or six orders of magnitiude better safety than present
rocket technology.

It allows quick turnaround and short flight times, as you can arrange to
land at the airfield where you took off from, then refuel, load another
second stage and boost again - no ferrying back is needed.

You can also split the second stage mission into two - cargo stages and a
people stage. That way you can get more flights out of one booster, and if
it flies say 6 flights per day - then it gets very cheap to operate per
flight, around $1 million per boost (including generous capital repayment
and some profit).

If you have some jet flight range before burn (which costs only the price of
the jet fuel, the mass cost is limited to jet fuel tankage) it also allows
you to start the main burn below the orbital track you want - and this
allows first orbit docking.

If you can dock on first orbit, your "capsule/CEV" can be a whole lot
lighter - it does not need life support for several days in orbit waiting to
meet the track you want, room to move about, loos and seperate sleeping
areas - you can just pack the passengers in like sardines, or aircraft
passengers, they do not need to move around for an hour or so. That way you
get a lot more passengers in. It can be more than double the number in fact,
halving the cost per passenger.

You can enclose the second stages on ascent and seperate in vaccuum through
"bomb-bay" doors - so a non-returning cargo stage need never see any
aerodynamic forces at all, and can be designed and built lighter. Even if
the second stage is carried externally the maxQ is much lower due to the
flight profile.

And it doesn't need a launch pad - just a hanger with a LOX supply in an
airport.

And you can fly off from a coastal airport and only start the rockets over
the ocean, making licencing a whole lot easier.

And - well I can go on at length, easy ferrying, use of existing jet engine
and airframe maintenance facilities and trained staff, available COTS
aircraft parts and existing stores - but that's enough for now.

Note that Henry doesn't like the idea - I think it's because it would take a
whole lot of capital to build, and would need a high flight rate to be
cheap, and he believes that he could use that capital better. Afaik he
doesn't have any other major objections. Henry?

[*] any L/D at all is a bonus if the drag is the same - not an unreasonable
assumption, as it is going slower at the same altitude/air density.
Otherwise any L/D > 1 for any extra drag is useful.