Transcript Route to success: Working together

Challenges of hypersonic air-breathing flight
Dr. David M. Birch
Head, Flow Control Research Group
Technical Director, Surrey Sensors LLC*
Access to space
Getting there is 90% of the problem.
Rosetta probe
1.5 tonnes
Ariane 5
775 tonnes
Access to space
Getting there is 90% of the problem.
Scania N230UD
18 tonnes
So why is it so difficult?
Aerodynamic drag:
minor problem.
Gravity:
Major problem.
Climbing out of Earth's potential well

m 
v  v0  u rel ln1 
t   gt
 m0 
Low-altitude ideal rocket equation
y   v dt
m  1 2
 m0  
 urel  v0 t  urel  t 
t   gt  y0
 ln1 
m   m0  2

Velocity at MES
Burn duration
Altitude at MES
Climbing out of Earth's potential well
It takes 50% of your
fuel to get to 12%
of your desired
altitude.
y
ymax
 t / mf
m
Although small, drag is still there
a / aSL
T / TSL
 /  SL
1
FD  U 2
2
Drag is proportional to
dynamic pressure
  e  ky
Density drops
exponentially with
altitude
P / PSL
ISO 2533:1975
ICAO Standard atmosphere
altitude  1000ft
Although small, drag is still there
5
1
U 2
2
4
3
2
M=1
altitude  1000ft
The verdict:
You want to light your main engines at as high an
altitude as possible.
• Carry rocket to altitude by other means
• Liftoff from "airborne launchpad"
Options available:
Current standard: launch from another rocket.
Saturn V stage 3 separation
Options available:
Launch from a
weather balloon?
UI "Rockoon" (1952)
Launch from an aircraft?
Scaled Composites SpaceShip One (2003)
NorAm X-15 (1959-1970)
Options available:
Fire out of a giant gun?
Jules Verne, De la Terre à la Lune (1865)
Options available:
Fire out of a giant gun?
Martlet 2G-1: 4-stage orbital
insertion variant (1969)
Prof. G. V. Bull
(1928-1990)
HARP main gun firing
(1965)
Options available:
Stick wings and a turbojet on your rocket,
and fly it to altitude.
Douglas D-558-2 "Skyrocket"
The "spaceplane" concept
The single-stage-to-orbit concept has been around
for a long time.
(1970)
Space Shuttle original concept (1970)
Convair Spaceplane (1961)
The "spaceplane" concept
The catch:
(a) Oxidizer mass is a killer.
Nearly half of liftoff mass is just oxygen
(in one form or another).
• OK, so harvest oxygen from
environment.
The "spaceplane" concept
The catch:
(b) Temperatures become a problem.
Lines of constant
burner inlet
temperature:
T ~ 1700 - 2000 C
Burner inlet
Mach number
Supersonic
combustion
Subsonic
combustion
Conventional
combustion
Flight Mach number
Engine will melt
before combustion
even begins!
The "spaceplane" concept
The catch:
(c) Flight envelope is limited.
Lines of constant dynamic
pressure
P = 90 kPa
M>6
Flight
Mach
number
Engine melts
P > 90 kPa
Aircraft breaks apart
20 kPa
P < 20 kPa
Aircraft drops out of sky
h > 90,000 ft
Engine suffocates
altitude  1000ft
The "spaceplane" concept
On the other hand...
But you no longer need mechanical compression!
• Ramjets, scramjets
10
4
• No moving parts!
Available ram
compression ratio
10
10
10
3
Engine melts
• Cannot self-start...
2
1
Conventional turbofans
10
0
0
2
4
6
Flight Mach number
8
Ramjets
Proven technology
Ramjets are ideal for Mach 1 ~ 3, and are a well-proven
technology.
LeDuc 010 (1949)
Ramjets
Proven technology
Ramjets are ideal for Mach 1 ~ 3, and are a well-proven
technology.
M 1
M 1
M 1
M 1
M 1
Fuel injectors /
flameholders
Inlet
Combustor
Nozzle
Ramjets
Proven technology
Scramjets are still highly experimental, but work best from
Mach 4 ~ 10
M  1
M 1
M 1
Fuel injectors /
flameholders
Inlet
Combustor
Nozzle
Ramjets
Proven technology
Scramjets are still highly experimental, but work best from
Mach 4 ~ 10
M 1
M 1
M 1
Fuel injectors /
flameholders
Inlet
Combustor
Nozzle
Supersonic combustion ramjets
Not-so-proven technology
Scramjets are still highly experimental, but work best from
Mach 4 ~ 10
Internal compression
and expansion
Spike nozzle profile
external expansion
Contoured inlet diffuser
Rocketdyne RS2200
(1998)
Wall-mounted hypermixing
fuel injectors
MiG-21 (1959)
M = 2.05
Supersonic combustion ramjets
Not-so-proven technology
Scramjets are still highly experimental, but work best from
Mach 4 ~ 10
Spike nozzle profile
external expansion
Supersonic combustion ramjets
Not-so-proven technology
Scramjets are still highly experimental, but work best from
Mach 4 ~ 10
NASA X-43 (2004)
Boeing X-51 "Waverider" (2010)
Supersonic combustion ramjets
So why the problems?
The practical problems of employing supersonic combustion are very great: It is
necessary to capture a stream tube of supersonic air, inject fuel, achieve a fairly
uniform mixture of fuel and air, and carry out the combustion process--all within
a reasonable length, and preferably without causing a normal shock within the
engine. There is currently no conclusive evidence that these requirements can
be met.
Weber, R. and McKay, J. S. (1958) "Analysis of Ramjet Engines
Using Supersonic Combustion," NACA TN 4386
There are still serious questions as to whether or not stable supersonic
combustion is possible over the required range of burner entry conditions.
Heiser, W. and Pratt, D. (1994) "Hypersonic Airbreathing
Propulsion," Washington:AIAA
Supersonic combustion ramjets
So why the problems?
Combustion is just an exothermic chemical reaction with a reasonably
low activation energy. For it to happen,
No problem
Big
problem
No problem
Medium
problem
a)
 b)
 c)
~ d)
the fuel and oxidizer must be combined in the
appropriate relative quantities;
the fuel and oxidizer must be mixed;
the activation energy must be exceeded, and
the fuel and oxidizer must remain together for
some finite time, since the reaction takes some
time to occur.
Problems so far:
• Must increase speed with altitude, to hypersonic range
• Must combust supersonically for M > 6 or engine will melt
• Alternatively, must pre-cool inlet air before combustion
• Turbocompressors won't work above M > 2, and ram
compression won't work for M < 2
• Thrust available is limited by melting point of materials
Combined cycle propulsion
One option
Rocket
Scramjet
Flight
Mach
number
Ramjet
Turbojet
altitude  1000ft
Combined cycle propulsion
One option
Turbojet
M < 0.8
Variable geometry inlet
0.8<M<2
Turbojet inlet closed
2<M<5
Ramjet mode
Ram inlet open
Turbojet exhaust closed
Combined cycle propulsion
One option
Variable combustor nozzle geometry
5<M<10
Scamjet mode
Oxidizer injection
M>10
Rocket mode
Ram inlet closed
Aspirating turbopumps
Another possibility
Conventional staged
combustion turbopump
Fuel in
Oxy in
Exhaust
Comp
Comp
Turb
Burn
Burn
Rocketdyne RS-25
Aspirating turbopumps
Another possibility
Fuel in
Oxy in
Air-breating, staged
combustion turbopump
Mix
Comp
Comp
Turb
Rich exhaust
Air in
Burn
BIG
PROBLEM
Liquid air
Burn
Balepin 2003
More problems...
Fundamental problems of supersonic flows
Supersonic heat exchanger
• Cross-flow fins for best heat exchange, but powerful shocks and
losses (both potential and mass flow) result.
• Wall-flush heat exchangers minimally intrusive, but minimally
effective.
• Heat exchange rate linked to cryogenic fuel flow rate: required fuel
flow rate dictated by engine design, exchange rate dictated by
altitude.
• Inlets must be HUGE for useful amount of air ingestion at altitude
• Again, the required oxidizer flow rates are HUGE.
So...
What we have learned
We need high speed air-breathing launch platform for stepchange in orbital insertion cost.
• High speed means high altitude and therefore low thrust;
• High speed means either supersonic combustion or heat exchange.
We're getting there.