Skip to main content

Full text of "NASA Technical Reports Server (NTRS) 19990110636: HSR Propulsion System Studies: A Status Report on the Down-Select Process"

See other formats


W.C. Strack 

NASA Lewis Research Center 
Cleveland, Ohio 

HSR Airport Noise Challenge 

-%/* 7 / 

First Wish: 

• Simple turbojet engines (V jet ~ 3200 ft/s) 

• Acceptable 20 + dB mixer-ejector nozzles 
1992 Status: 

sideline noise 
-123 EPNdB 

Stage m Goal 
102.5 EPNdB 

Stage IV 

Status with Generation I 
mixer-ejector nozzles 

without new operational procedures 

When the HSR program began there was widespread belief that a simple and 
familiar turbojet-like engine coupled to an advanced technology mixer-ejector 
nozzle was the propulsion system of choice for achieving FAR 36-Stage 3 noise 
requirements. Our ability to quickly demonstrate a practical 20 + dB suppression * 
nozzle was confidently presumed by many. Our rate of progress towards that 
objective, however, has been somewhat humbling. At the moment we are 
reasonably confident of achieving about 1 5dB suppression with a mixer-ejector 
nozzle designed for a high specific thrust turbojet-like cycle. Therefore, if we make 
no further suppression progress and conservatively assume no new operational 
procedures such as programmed lapse rate (PLR), then meeting the Stage III goal 
requires a large amount of engine and/or wing oversizing which is economically 
prohibitive. The scenario is further aggravated by the possibility of eventually 
needing to comply with even more stringent regulations (Stage IV). 

While this status may be somewhat disappointing to some, it must be remembered 
that the HSR program plan involves two generations of mixer-ejector nozzles 
beyond the current generation I nozzle designs. It is premature to conclude that 
we cannot design a practical 20 + dB mixer-ejector nozzle. On the other hand, it is 
prudent to consider alternative solutions to the noise problem. Thus, we are 
investigating four other propulsion system concepts. 


HSCT Noise Suppression Concepts 

Rear End 


Front End 

Variable Cycle Engine 


The highest specific thrust concept is the 1 -spool turbine bypass engine (a slightly 
modified turbojet) combined with a very large mixer-ejector nozzle requiring 
approximately 120 percent airflow augmentation during takeoff. The mixed flow 
turbofan (MFTF) and variable cycle engine (VCE) concepts have intermediate jet 
velocities because a low-spool driven fan absorbs much of the core energy. 
Consequently, much less secondary air is required in the miker-ejector nozzle to 
achieve low noise than the turbine bypass engine (TBE). The Flade engine is either a 
VCE or a MFTF with a third flowpath surrounding the fan and scrolled to the lower half 
of the engine. The fan driving this flowpath is modulated to absorb power during the 
takeoff and this provides a fluid acoustic shield underneath the mixer nozzle (no 
ejector). The TBE with an inlet flow valve (IFV) represents one member of the tandem 
fan class of concepts wherein a compression system reconfiguration can occur. 

During takeoff, auxiliary air is brought onboard and routed to the rear compressor 
while the normal inlet airflow is processed only by the front compressor before 
exhausting. In the cruise configuration, the auxiliary inlets are closed and the engine 
becomes a turbojet with an extra pressure loss due to the IFV. 

All of these candidate concepts achieve about 1 500 ft/s exhaust velocity during 
takeoff by raising the total airflow to about 1100 Ib/sec. They differ in where the 
airflow is introduced into the cycle and which technologies need to be developed to 
achieve success. 


Effect of Exhaust Velocity on Sideline Noise 

Mach 0.322, 689 ft., 650 lb/s 

Each of these five concepts can be characterized by its exhaust velocity and, 
therefore, its suppression requirements compared to a conventional unsuppressed 
nozzle. While a TBE presents a 20 + dB suppression problem to attain Stage III, the 
TBE/IFV can be designed to achieve Stage III without an elaborate suppression 
system, and the hybrid concepts fall somewhere in-between these extremes. 

There are, of course, other discriminating attributes to be considered such as 
weight, reliability, life, efficiency, thrust lapse, technology risk, tolerance to more 
severe noise constraints, installation drag, and climb noise. What is needed is an 
unbiased procedure to evaluate each of these concepts on a system basis that 
accounts for all of these criteria simultaneously. 

Overall HSCT Noise Issues 

1. Which propulsion concept best achieves a 
balanced compromise of performance, weight, 
size, noise, complexity, and life ? 

2. What price do we pay to achieve noise levels 
below Stage III ? 

issue 1 is important to resolve because the HSR program is resource-constrained to 
pursue technologies specific to only two concepts at most. This is also a difficult 
challenge to resolve with a high degree of confidence due to the large number of 
independent variables, the complex interactions between propulsion and airframe, 
multiplicity of merit criteria, and key technology and external uncertainties. 

Since it is likely that noise regulations will be tightened sometime in the future it is 
important to determine the economic penalty associated with such an eventuality. 
Also, some propulsion concepts are able to accommodate severe noise constraints 
better than other concepts. Thus, this information could be a key discriminator 
during the concept selection process. It would be best if we generate a curve of 
penalty (e.g., aDOC) versus AdB below Stage III rather than presume a definition of 
Stage IV. Then, there could be more rational future rule-making. 


A Propulsion System Down-Select is Needed to Focus the 

Phase II Technology Program 

Late 1993 Down-Select 
Decision Gate 

In addition to the mainline environmental technology feasibility effort in HSR Phase 
I, a systems studies effort is also underway to address the issues listed on the 
previous chart. The schedule calls for a down-select to both a primary and backup 
concept by late 1 993. This will focus the technology effort in HSR Phase II. Note 
that this down-select pertains to the NASA sponsored technology thrust only-i.e., 
it is not a production engine down-select. The intent of the '93 down-select is to 
insure that the correct concept-specific technology is pursued in the earlier portion 
of Phase II to enable a low-risk final down-select in late '95. 


HSR Propulsion System Selection Process 

Concepts — ► 






Common — ► 

The process by which the down-select information is acquired begins with the 
establishment of a common set of groundrules for all participants to minimize the 
risk of disparate results. GE/PW are to perform a preliminary concept screening 
using takeoff gross weight as the prime evaluation criterion. This means that for 
each candidate concept the cycle will be optimized and representative airplane and 
mission models adopted. Propulsion-airframe installation (PAI) differences such as 
interference drag will not be captured, however. The output of this first level 
screening is passed to Boeing and Douglas for detailed comparative evaluations 
that include PAI effects. Boeing and Douglas have adopted somewhat different 
airframes and mission definitions (e.g., programmed lapse rate assumption) which 
means that somewhat different results may ensue. The merit criteria will be direct 
operating cost (DOC) and technical risk. 


HSR Propulsion System Concept Selection Criterion 

9 Prime discriminators: Direct operating cost (DOC) 

Technical risk ( uncertainty band) 

9 Risk to be incorporated into DOC when feasible 


Concept 1 Concept 2 Concept 3 Concept 4 Concept 5 

To avoid an unwieldy number of risks associated with the various technologies, 
risks will be incorporated into DOC wherever possible. For example, if an 
unconventional component such as a mixer-ejector nozzle requires an expensive 
R&D program to reach acceptable risk, then the cost of this element is 
incorporated into the DOC. The end result is an uncertainty band on DOC that 
reflects the agglomerated risks associated with each concept. Conceivably, the 
down-select process will produce results as depicted wherein the nominally lowest 
DOC concept (arbitrarily drawn as concept 4) also has the highest risk. In this 
case, the decision-maker will need to make a judgement concerning the balance 
between DOC benefit and increased risk. In the example shown, concept 3 might 
be preferred over concept 4 due to its lower technical risk. 


Monte Carlo Simulation Risk Analysis 

/ ' 

Cost Model 

- — 

/ s 




s > 

In order to determine the DOC uncertainties for each concept, GE/PW and Douglas 
are invoking a Monte Carlo simulation risk analysis. Component experts will 
estimate the probability of attaining several values of the key component criteria 
such as efficiency weight, acquisition cost, and maintenance cost. This will define 
sets of probability curves for each propulsion component such as the mixer-ejector 
nozzle, IFV, Flade fan, and mixed compression inlets. Random sampling of these 
component probabilities done many times, together with a propulsion systems 
model, will yield another (smaller) set of curves for each concept. These are 
confidence curves for the complete propulsion system. Combining this information 
with aircraft, mission, and economic models will lead to DOC confidence curves for 
each of the five propulsion concepts. Finally, these DOC confidence curves can be 
interrogated at three levels to yield the desired DOC uncertainty bands (e.g., 20 
percent, 50 percent, and 80 percent). 

Boeing prefers to do a more traditional risk analysis instead of a Monte Carlo 
analysis. They will interrogate a group of experienced technology experts to 
estimate risks associated with each candidate concept. This traditional approach 
will provide a check on the Monte Carlo simulation. 


Level 2 HSR Program Schedule - Propulsion System Studies 

HSR Program Mtestone 

Propulsion Design Studies 
and Evaluations 

Airframer Evaluations 

NASA Design StucMes 
and Evaluations 

FY 91 




Propulsion Down-select 

m life Study Evaluations y 



“/ II 

Common Down-select Engine Engine 

Ground rules Process Cost 

\ \ \ \ \ 


\\\\ \ 


1 1 

Fcllb*± A 




& Evaluations . Fallback A 


LaRC JA ftCfe 


Having established common groundrules amongst GE/PW, Boeing, Douglas, and 
NASA, the propulsion system design studies are well underway within the 
propulsion community. Mach 2.4 data have been generated and delivered to the 
airframers for the TBE* MFTF (bypass ratios of 0.4, .63, 1.13), and the TBE/IFV. 
The VCE and Flade data are nearly complete. These data include performance, 
weights, cost, and acoustic information. In the spring of 1993, Boeing and 
Douglas will have completed a first pass comparison of all of the engine 
candidates. At this point, all first-order technical issues and concept-specific 
concerns will be identified. From then on, detailed analyses will be conducted to 
insure that each concept is fairly judged. This entails exploring ways to mitigate 
the weaknesses associated with each propulsion concept. The plan calls for 
sufficient information to be acquired by the beginning of FY94 to enable a credible 
down-select decision. NASA is also performing design studies and comparative 
evaluations to strengthen the overall effort. Because the technical challenges are 
complex and five organizations are involved in designing and evaluating numerous 
engine and nozzle concepts, there is also an enormous information management 
challenge to ensure effective use of available resources. 


Status of NASA In-House Comparative Propulsion Studies 

Mach 2.4, 100% supersonic 5000 n.mi. range, 292 passengers 
Stage III sideline constraint 

Assumes material goals and mixer-ejector nozzle goals are achieved 
LeRC results as of September, 1992 

Relative Takeoff 
Gross Weight 

Currently, there is not much comparative data to examine and what data does 
exist is quite tentative and laced with caveats. Nevertheless, this chart displays 
NASA's current state of understanding of three of the five concepts. Relative 
takeoff gross weight is shown assuming the EPM materials goals and the HSR 
mixer-ejector nozzle goals are achieved (e.g., the TBE nozzle delivers 18 + dB 
suppression in an acceptable size), and that no significant PAI penalties exist. 

Even with these assumptions the MFTF is superior to the TBE because its cruise 
efficiency is significantly better. If the PAI differences do not hurt the MFTF, this 
candidate appears to be very competitive. Because the VCE is essentially a MFTF 
derivative, it too is expected to compete well--especially on missions with large 
subsonic legs. The Flade is also anticipated to be quite competitive for similar 


Key Propulsion System Uncertainties 


1. Adequate mixer - ejector nozzle aero / acoustic performance 

2. Materials progress 


3. More severe airport noise regulations 

4. Operational procedure regulations (e.g., programmed lapse rate) 

5. Climb noise 

6. Mach number selection 

By itself, the previous chart depicts an oversimplified situation. In reality, there are 
a number of first-order uncertainties that need to be considered in the selection 
process. The sensitivity of the comparison with respect to these uncertainties 
needs to be determined to select wisely. 


Impact of Noise Suppression Technology and Noise Constraint 

Mach 2.4, all supersonic 5000 n.mi. range, 292 passengers 
Cycle, nozzle, wing hading, thrust hading vary along each curve 
Ignores aircraft instalhtion differences and possible climb noise constraint 
NASA Lewis results as of November 1992 

Relative Takeoff 
Gross Weight 

An example of the impact of the first three key uncertainties is displayed here in terms of 
takeoff gross weight relative to a TBE powered airplane with a 2900°F T 41 . (The TBE would 
yield lower TOGW if it were not constrained by turbine blade material limits and unavailability 
of a suitable high-suppression nozzle.) Note that the TBE is quite sensitive to the degree of 
success in achieving a quiet nozzle. If only 1 3dB of suppression is achieved, then the TBE's 
TOGW penalty is about 1 4 percent. The TBE curve represents various amounts of wing and 
engine oversizing to meet Stage III sideline noise. The MFTF (and the other concepts as well) 
offer more degrees of freedom in the form of cycle changes (BPR) to mitigate the adverse 
impact of a mixer-ejector nozzle technology shortfall. Hence the MFTF curve is less steeply 
sloped, and it could accommodate even a 10dB suppressor nozzle without a show-stopping 

The NASA results shown here also indicate that a 400°F material temperature shortfall would 
not be disastrous although 600°F would be. On the other hand, industry generated data show 
at least twice the sensitivity displayed here. These differences will be resolved soon. 

The impact of a Stage lll-5dB noise constraint may be determined by comparing results using 
the lower abscissa scale with results using the upper scale. For example, the aTOGW for a 
MFTF is about 7 percent for a 1 5dB suppressor nozzle. 

Finally, it should be understood that this figure is just the beginning. Undoubtedly it will 
change as more realism is added. Firm conclusions based on this alone are premature. 


Variable Bypass Supercharged Core (VBSC) 

Low-Row Mode 
BPR -.60 

Uuwr tan 
Outer tan 

High-Row Mode 
VJ<1500 fps 



9 Core remains supercharged in high-flow mode 
9 Less pressure drop, better cruise TSFC 
9 Less auxiliary inlet air required 
9 Lower engine weight 

The previous chart contained several points for TBE/IFV and TJ/IFV engines. At 
the moment there is some controversy concerning whether the plotted points are 
too optimistic or not. Regardless of how that controversy is resolved, it is clear 
that, while such high flow concepts are appealing because they obviate the need 
for a high-risk mixer-ejector nozzle, they also suffer serious deficiencies. Namely, a 
non-supercharged core and consequently low thrust in the takeoff mode, large and 
heavy engines, and pressure drop through the IFV during cruise. These 
deficiencies may be partially alleviated by the new concept illustrated here. It is a 
turbofan/IFV with a flow splitter that keeps the core supercharged by the inner fan 
flow at all times. It also features a core-driven aft fan stage that prevents bypass 
ratio from rising at higher flight speeds (opposite of mission requirement). 



Takeoff and Climb to Cruise 

Jet Velocity, Vj, fps 


Another advantage of the variable bypass supercharged core (VBSC) concept is its 
ability to efficiently stay in the high-flow mode throughout climb. This may prove 
to be important to reduce climb noise. Shown here is the exhaust velocity V ( and 
net thrust F n during a typical climb path. Note the modest Vj throughout-rising 
from 1450 ft/s at Mach 0.3 to 2000 ft/s at Mach 0.9 at which point the mode 
switch occurs. NASA has conceptualized this engine very recently and has 
solicited industry feedback (pending) before adopting its inclusion into the down- 
select process. 


TOCW Sensitivity to Nozzle Performance and Weight 

Mach 2.4 HSCT 

Achieving high nozzle cruise efficiency is absolutely essential. A 1 percent C fg 
shortfall can increase TOGW by over 4 percent. Clearly, we need confidence in 
our predictive codes to substitute for lack of experimental data for many of the 
unconventional nozzle concepts. 

Nozzle weight is also a sensitive parameter. As the studies progress, the initially 
large spread in weight estimates has significantly diminished. 

Avoided on this figure is any mention of the takeoff nozzle performance which is 
often cited as important also. (E.g., it is one of the two merit criteria in the oft- 
used mixer-ejector nozzle technology goal charts.) This omission was deliberate 
because some of the high-specific thrust engines are top-of-climb sized and 
therefore do not suffer large penalties for takeoff C fg 's as low as 0.85 or so. There 
is also some evidence that even takeoff sized engines could tolerate relatively poor 
takeoff C fg 's if wing size is free to vary to compensate. 


Thrust Augmentation Issue 

Some cycles benefit from a 
mini-augmentor during climb 



0.4 BPR 


Top of Climb 
Nozzle Throat 
Temp., °F 


One of the more recently discovered issues is whether to use a mini-thrust 
augmentor or not in the high specific flow engines. For example, Boeing prefers to 
use a mini-augmentor during the upper climb path to offset marginal thrust levels 
that cause inefficient transonic system performance. A 3 percent TOGW reduction 
is possible using a mini-augmentor rather than a dry engine for a MFTF with 0.4 
bypass ratio. However, the use of augmentation also boosts the nozzle 
temperature levels about 600°F from the 1 200-1 400°F level to the 1700-2000°F 
level. The question is whether the TOGW payoff is worth the increased risk and 
maintenance associated with the higher temperature experienced during the upper 
climb. This issue is being investigated further. 


Nacelle Placement Restraints 

Another powerful influence on the down-select decision is propulsion-airframe 
integration (PAI). For example, the nacelle shape, which is driven by the 
propulsion geometry and changes significantly from one concept to another, and 
placement can dramatically alter the interference wave drag. Hence, to compare 
the alternative propulsion concepts we need to assure ourselves that PAI effects 
are properly determined even if this requires more than the usual analysis depth to 
understand. From early calculations it appears that some of the concepts do not 
integrate easily with the airframe and some re-design effort is warranted to avoid 
premature judgements. 



1. Late 1993 propulsion system down-select requires 
reliable M-E nozzle database 

— Adequate progress but not established yet 

2. Considerable concern exists about M-E nozzle risk 

— Aero / acoustic performance — Weight / size — Life 

3. Interest shifting toward low specific thrust cycle 
solutions to noise challenge 

4. Stage III-5 dB incurs approximately a 7% airplane 
takeoff weight penalty for mixed-flow turbofans with 
M-E nozzles 

At the moment, the propulsion system down-select process is hindered by our lack 
of an adequate experimental mixer-ejector nozzle database to enable high- 
confidence aero/acoustic/weight modeling. Progress in establishing the needed 
data base is progressing adequately but fitfully. Certainly there exists considerable 
concern about M-E nozzles--enough to spawn a new wave of interest in the high- 
specific flow alternatives. In the end, it is likely to come down to a matter of 
which technology challenges do we prefer to pursue. The decision may depend 
upon risks as much as on potential benefits.