NASA/TM—2013–216590

Proposed Flight Research of a Dual-Bell Rocket 

Nozzle Using the NASA F-15 Airplane

Daniel S. Jones and Trong T. Bui

Dryden Flight Research Center, Edwards, California

Joseph H. Ruf

Marshall Space Flight Center, Huntsville, Alabama

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September 2013
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NASA/TM—2013–216590

Proposed Flight Research of a Dual-Bell Rocket 

Nozzle Using the NASA F-15 Airplane

Daniel S. Jones and Trong T. Bui

Dryden Flight Research Center, Edwards, California

Joseph H. Ruf

Marshall Space Flight Center, Huntsville, Alabama

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National Aeronautics and

Space Administration

Dryden Flight Research Center

Edwards, CA 93523-0273

September 2013
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7115 Standard Drive

Hanover, MD 21076-1320

443-757-5802
1

Abstract

For more than a half-century, several types of altitude-compensating rocket nozzles have been 

proposed and analyzed, but very few have been adequately tested in a relevant flight environment. One 

type of altitude-compensating nozzle is the dual-bell rocket nozzle, which was first introduced into 

literature in 1949. Despite the performance advantages that have been predicted, both analytically and 

through static test data, the dual-bell nozzle has still not been adequately tested in a relevant flight 

environment. This report proposes a method for conducting testing and research with a dual-bell rocket 

nozzle in a flight environment. We propose to leverage the existing NASA F-15 airplane and Propulsion 

Flight Test Fixture as the flight testbed, with the dual-bell nozzle operating during captive-carried flights, 

and with the nozzle subjected to a local flow field similar to that of a launch vehicle. The primary 

objective of this effort is not only to advance the technology readiness level of the dual-bell nozzle, but 

also to gain a greater understanding of the nozzle mode transitional sensitivity to local flow-field effects, 

and to quantify the performance benefits with this technology. The predicted performance benefits are 

significant, and may result in reducing the cost of delivering payloads to low-Earth orbit.

Nomenclature

ACN altitude-compensating nozzle

A* nozzle throat area

CB conventional-bell

CDE Cone Drag Experiment

Cf thrust coefficient

CFD computational fluid dynamics

DFRC Dryden Flight Research Center (Edwards, California)

F thrust

LEO low-Earth orbit

MSFC Marshall Space Flight Center (Huntsville, Alabama)

NASA National Aeronautics and Space Administration

NPR nozzle pressure ratio (PC/Pamb)

NTF Nozzle Test Facility

Pamb ambient pressure

PC nozzle chamber pressure

PFTF Propulsion Flight Test Fixture

TRL technology readiness level

USAF United States Air Force

Introduction

Background on the development and performance of the conventional-bell (CB) nozzle will first be 

provided. This background will serve to unveil the impetus behind improving nozzle performance, a 

motivation that can be traced back to national technological roadmaps. Several types of 

altitude-compensating nozzle (ACN) concepts have been proposed to improve nozzle performance, one of 

which is the dual-bell rocket nozzle. A brief background on these concepts will be provided, along with 

more detail on the dual-bell nozzle and the current technology readiness level (TRL) of this technology.

CB Nozzle Background

In 1882, the Swedish engineer Carl G. P. de Laval first introduced his impulse steam turbine (ref. 1).

He later displayed a significantly improved version of his single-stage steam turbine at the 1893 World 
2

Columbian Exposition in Chicago, Illinois; a system which utilized a unique series of 

convergent-divergent nozzles (ref. 2). Later, in 1915, Robert H. Goddard was the first to utilize the 

de Laval nozzle with early rocket experiments, demonstrating a significant efficiency improvement in 

converting the fuel’s chemical energy into the kinetic energy of the rocket (ref. 3). Ever since this time the 

CB nozzle has been the gold standard for the architecture of virtually all rockets.

A CB nozzle is the most efficient means by which to convert the high pressure and temperature gases 

within a rocket’s combustion chamber into thrust, but only at its design altitude. A CB nozzle is most 

efficient when it is ideally expanded, where the nozzle exit pressure is equal to the ambient pressure. 

Since the ambient pressure decreases as a rocket ascends, ideal expansion can occur at only one altitude. 

All rockets with a CB nozzle will experience significant inefficiencies in plume expansion throughout 

most of the rocket’s trajectory, since near-ideal plume expansion can only be attained at one specific 

altitude. Figure 1 illustrates this phenomenon, with three distinct phases of a CB nozzle plume through its 

trajectory: (left) at sea level (inefficient); (center) at its design altitude (optimal efficiency); and (right) at 

a high altitude (inefficient) (ref. 4). Intuitively, a rocket nozzle has its greatest performance when every 

location of the nozzle plume across the nozzle exit plane has a velocity vector perpendicular to that plane, 

resulting in the greatest imparted momentum on the rocket. 

Figures 2(a) and 2(b) also help to illustrate the inefficiencies incurred by a CB nozzle during a 

rocket’s ascent, by plotting the thrust coefficient and nozzle efficiency of three different CB nozzles, each 

with a fixed area ratio, during operation at various altitudes. The area ratio of a rocket nozzle is defined as 

the area at the nozzle exit plane divided by the area at the throat of the nozzle. From these figures, it is 

evident that each of the CB nozzles can achieve optimal flow conditions only at one altitude during the 

rocket’s ascent. Figure 2(b) reveals that each of the CB designs incur greater performance losses when 

operating farther from this design altitude, in either direction.

The thrust coefficient is used by nozzle designers to help evaluate nozzle performance. Equation (1)

shows the connection between the thrust coefficient (Cf) to the rocket’s thrust (F), chamber pressure (PC), 

and the area at the throat of the nozzle (A*

). 

 = 

 (1)

Figure 2 can be used in combination with equation (1), helping to illustrate that performance (thrust) 

is directly related to the thrust coefficient, and that a CB nozzle experiences considerable performance 

losses throughout most of a rocket’s trajectory. Utilization of some type of ACN will enable the actual 

thrust coefficient to be closer to the optimized thrust coefficient, directly increasing the thrust over the 

rocket’s integrated trajectory. This overall increase in thrust results in a rocket with greater performance, 

which can deliver higher mass payloads to low-Earth orbit (LEO) for a similar launch vehicle. 

Decreasing the cost of delivering payloads to LEO has been a vision for the National Aeronautics and 

Space Administration (NASA) and the private sector for decades, and aligns well with national interests. 

Most recently, this national goal has been reiterated within NASA’s integrated set of fourteen space 

technology roadmaps, each of which recommends the overall technology investment strategies and 

prioritization of NASA’s space technology activities. The first of these fourteen technology areas is the 

Launch Propulsion Systems Roadmap, which emphasizes that “reliable and cost-effective access to space 

is a fundamental capability required for all of NASA’s in-space missions.” Repeatedly within this 

roadmap, the desire to reduce launch costs is highlighted as a figure of merit, and several technology 

investment areas are proposed to achieve this overarching goal, one of which is the development of 

advanced nozzle concepts. This roadmap recognizes and outlines that “design, modeling, and 
3

demonstration of advanced nozzle concepts” are required milestones within the path to advance these

technologies to a TRL of 6 (ref. 5).

Another negative consequence of a CB nozzle design is caused by opposing design requirements,

between the requirement to optimize nozzle performance and the requirement to mitigate lateral structural 

loads. During engine start-up and shut-down transients, asymmetric detached-flow within the nozzle can 

occur, causing significant side loads that the engine must withstand and greater structural loads on the 

nozzle shell (ref. 6). The requirement to mitigate these structural loads leads to a CB nozzle design that 

mitigates nozzle flow overexpansion during launch, which is therefore, not optimized for nozzle 

performance over the rocket’s integrated trajectory.

ACN and Dual-Bell Nozzle Background

To improve the efficiency of a rocket when it is at any other altitude than its design altitude, a type of 

ACN is required, allowing the nozzle-flow pressure at the nozzle exit plane to be better matched with the 

ambient pressure. Several types of ACN designs have been proposed and analyzed with this goal in mind. 

Figure 3 shows a schematic for four of these ACN designs. Unfortunately, other than the aerospike 

nozzle, only minimal advances in the TRL of these designs have been achieved since their conception. 

Several full scale aerospike engines have been ground tested, and to our knowledge, only the aerospike 

nozzle has been tested in a relevant flight environment; during the NASA Dryden Flight Research Center 

(DFRC) (Edwards, California) aerospike nozzle flight research effort (ref. 7). The goal of the current 

effort is to advance the TRL of the dual-bell rocket nozzle through flight research.

A dual-bell nozzle has a distinct shape of two overlapped bells, introducing a geometric inflection 

point along the nozzle’s inner contour. Figure 4 shows two views of a dual-bell nozzle. The inflection 

point in the contour presents a discontinuity in the nozzle plume flow field along the nozzle wall, 

facilitating a more symmetric and predictable nozzle flow field during engine transients at launch. In 

addition, the dual-bell nozzle is also expected to achieve a higher performance over a CB nozzle at lower 

altitudes (with higher ambient pressures), since the plume will not be significantly over-expanded, and it 

will be better matched to the ambient pressure. At higher altitudes (with lower ambient pressures), the 

dual-bell nozzle takes advantage of utilizing the second bell, expanding the plume further. Once again, the 

plume is allowed to be better matched to the ambient pressure, now at higher altitudes. Figures 5 and 6 

show computational fluid dynamics (CFD) analysis conducted by the NASA Marshall Space Flight 

Center (MSFC) (Huntsville, Alabama), with the dual-bell nozzle operating with different back pressures. 

When considering a rocket’s performance over its entire trajectory, the dual-bell nozzle has been 

predicted to achieve a higher integrated nozzle performance efficiency, which is expected to result in a 

greater capability of payload mass to LEO.

In 1949, the dual-bell nozzle first appeared in literature within a study by the Jet Propulsion 

Laboratory (Pasadena, California) (ref. 8). At the time, this conceptual rocket nozzle shape was believed 

to offer performance advantages over the CB nozzle. Since that time, several organizations around the 

world have studied the dual-bell nozzle analytically, and echoed the belief that this nozzle promises 

greater performance. Considering the performance advantages suggested, the TRL of the dual-bell nozzle 

has not made the advancements expected. The first static-test experiments published, over four decades 

after the dual-bell nozzle was first conceived, were conducted by the Rocketdyne Division of Rockwell 

International (now part of Aerojet Rocketdyne, Sacramento, California) with cold-flow through the nozzle

(ref. 9). Since then, analytical studies of dual-bell nozzles have continued but only a few organizations 

around the world have complemented their analytical effort with static test data to verify their 

performance predictions. Of these few static tests, to date, most have been conducted with non-reacting 

flow exhausting through the nozzle into a quiescent and non-transient environment. Very few 

organizations have conducted dual-bell nozzle testing with reacting flow. To the authors’ knowledge, no 
4

organization has tested the dual-bell nozzle in a relevant flight environment, and it is our opinion that 

dual-bell nozzle technology is approximately at a NASA TRL of 3 (where a TRL of 9 corresponds to a 

flight-proven technology, through successful mission operations) (ref. 10).

NASA MSFC is one of the organizations that have complemented their analytical efforts on the 

dual-bell nozzle with static tests. MSFC conducted static testing with the dual-bell nozzle in the MSFC 

Nozzle Test Facility (NTF), utilizing a non-reacting flow while varying the nozzle pressure ratio (NPR). 

During this research, the performance of the dual-bell nozzle was also quantitatively compared to a 

similar CB nozzle. This testing and research helped verify the prediction that the dual-bell nozzle had a 

greater nozzle efficiency and thrust than the comparable CB nozzle at several NPR conditions, 

particularly at the lower NPR conditions. Figure 7 shows a photo of the test setup at the MSFC NTF with 

a dual-bell nozzle. Figure 8 shows a photo with the comparable CB nozzle at the NTF. As can be seen in 

these figures, both tests included pressure ports along the nozzle wall to measure how the flow responds 

to varying NPR.

The dual-bell nozzle is predicted to provide an increase in performance over the CB nozzle, 

warranting further investigation of this technology in a relevant flight environment. Combined with the 

research conducted at NASA MSFC, NASA DFRC has the capability to advance the TRL of the dual-bell 

nozzle through flight research. The following section will provide more detail on advancing the TRL of 

the dual-bell nozzle through the proposed flight-research campaign, including the objectives and rationale 

of the proposed flight effort.

Objectives, Flight-Research Campaign, and Rationale

The overarching objective of this project is to advance the TRL of the dual-bell nozzle. The dual-bell 

rocket nozzle was selected as the ACN concept for flight research since it has much of the 

altitude-compensating capabilities of an aerospike nozzle, and is expected to have less complexity in the 

design and manufacturing. In addition, the dual-bell nozzle is also expected to be relatively easy to 

integrate within the architecture of existing rocket engines and launch systems. 

The TRL advancements for the dual-bell nozzle will be achieved through parallel efforts of analysis, 

static testing, and flight research. There are four specific technical objectives: (1) develop methods to 

reliably control dual-bell internal flow behavior, and demonstrate those methods in a relevant 

environment; (2) develop and validate the design and analysis tools required for dual-bell nozzles; 

(3) develop the F-15 flight testbed and the flight-test techniques required for advanced rocket nozzles; and 

(4) develop dual-bell performance databases, and databases of flight research with advanced nozzles.

The flight-research campaign is designed to include three phases, which will all require tests at 

various flight conditions. These three flight-research phases are as follows: (Phase I) initial flight research 

with a surrogate, non-operating nozzle to quantify the experiment (local) flow-field conditions near the 

nozzle exit plane; (Phase II) flights while operating cold flow through various nozzles; and (Phase III) 

flights while operating reacting flow through various nozzles. Phase II and Phase III flight research will 

require tests at similar flight conditions with a CB nozzle, to produce a quantitative comparison of the 

nozzle performance.

The rationale for the first technical objective (develop methods to reliably control dual-bell internal 

flow behavior, and demonstrate those methods in a relevant environment) are: (1) reliable control of the 

mode transition will enable optimizing dual-bell mission integrated performance; and (2) reliable control 

of the mode transition will reduce the likelihood of internal flow asymmetries developing near mode 

transition, thereby keeping nozzle off-axis loads manageable. The project plans to conduct trade studies 
5

on feasible methods to control the mode transition within the dual-bell nozzle during flight. One or more 

of the leading methods will be incorporated into the design of the system, and then demonstrated in flight.

The rationale behind the second technical objective (to develop and validate the design and analysis 

tools required for dual-bell nozzles) is focused on improving the current designs of the dual-bell nozzle, 

as well as improving our understanding of dual-bell nozzle flow behavior. Design and analysis of 

dual-bell nozzles is relatively immature, and a greater number of ground and flight tests will enable 

improvement and validation of the existing analytical tools.

The rationale behind the third technical objective (to develop the F-15 flight testbed and the flight-test

techniques required for advanced rocket nozzles) is focused on improving the current capabilities at 

DFRC to support development of advanced rocket nozzles. It is the authors’ opinion that the potential 

payoff of ACN technology warrants investment, and expanding on DFRC’s current flight testbed 

capabilities will facilitate the rapid flight research of other advanced rocket nozzles, even beyond the 

dual-bell rocket nozzle.

The rationale behind the fourth technical objective (to develop dual-bell performance databases, and 

databases of flight research with advanced nozzles) is similar to the rationale behind the second technical 

objective. Static ground test and flight research data for ACN designs are either rare or nonexistent, hence 

the need to conduct the current research and expand on the existing nozzle performance database. As 

noted earlier, most of the static tests that have been completed have been conducted with non-reacting 

flow into a quiescent environment. In many publications, the limitations of the static test data were often 

followed by the researcher’s admission that the dual-bell nozzle flow field and performance should be 

investigated in a relevant flight environment, to better understand the performance gains that are possible. 

Finally, the rationale behind conducting each of the three phases of the flight-research campaign will 

be highlighted here, in order of an increasing level of TRL and system complexity. The rationale behind 

Phase I is to fully understand the local flow-field conditions prior to obtaining data with an operating 

nozzle, since the nozzle flow behavior is predicted to be highly dependent on the external flow field. 

Rationale behind Phase II of the flight-research campaign is two-fold: (1) to allow existing cold-flow data 

from MSFC testing to be leveraged as much as possible, and compared against flight-research data; and 

(2) to permit an intermediate or build-up approach in system complexity (prior to reacting flow), for risk 

mitigation purposes. The rationale behind Phase III of the flight-research campaign is obvious; that 

operating a dual-bell rocket nozzle with reacting flow into a high velocity flow field will be the most 

relevant flight-test environment possible, enabling the TRL to be advanced.

The F-15/PFTF Flight Testbed Capability

The dual-bell rocket nozzle should be flight-tested in a relevant flight environment prior to integration 

and utilization within the space launch architectures of tomorrow. With a focus on flight testing and 

research, NASA DFRC has a unique capability with its fleet of F-15 airplanes, as well as with the 

Propulsion Flight Test Fixture (PFTF). This section will provide an overview of the F-15/PFTF, and 

top-level details on the nozzle placement and sizing criteria. The F-15/PFTF limitations will also be 

detailed, including the PFTF internal capacity available, PFTF in-flight thrust limitations, and the 

F-15/PFTF flight envelope. 

Overview of the NASA F-15/PFTF

NASA DFRC has a long history of rocket propulsion flight research, including an equally significant 

background in captive carry flight research. This expertise has been utilized since the 1940s to help 

advance the TRL of several types of systems, allowing a more detailed inspection of physical 
6

phenomenon in a relevant flight environment. When conducted in concert with analytical efforts and 

ground testing, the ability to conduct captive carry flight research almost always proves valuable, and can 

often be used for independent verification and validation of pre-existing data.

Among DFRC’s fleet of airplanes is the F-15B airplane and three F-15D airplanes. The F-15B 

airplane was acquired in 1993 from the Hawaii Air National Guard, and then converted into a research 

testbed airplane. DFRC’s three F-15D airplanes were acquired in 2010 from the United States Air Force 

(USAF) Tyndall Air Force Base (Florida), and are being converted from their role as air-superiority 

fighters into future flight research and operational support aircraft at DFRC. Figure 9 shows a photo of the 

NASA DFRC F-15B airplane. Figure 10 shows a photo of the NASA DFRC F-15D airplanes.

DFRC’s expertise with captive-carried flight research led to the creation of the PFTF. DFRC led the 

design and development of the PFTF, and then integrated the PFTF with the centerline pylon of the 

F-15B airplane. The F-15B/PFTF initial expansion flights were completed in 2001 and 2002. Figure 11 

shows a photo of the PFTF mated with the F-15B airplane, with the Cone Drag Experiment (CDE). The 

CDE was mated underneath the PFTF, and was utilized in the envelope expansion flights to spatially and 

inertially simulate a large propulsion test experiment that could be carried by the PFTF (ref. 11). Since 

these initial flights, the F-15B/PFTF has been utilized for a variety of propulsion-focused captive-carried 

flight research experiments. Combined with the flight-qualified PFTF, one of DFRC’s F-15 airplanes can 

be utilized as a testbed, presenting a unique capability to advance the TRL of the dual-bell rocket nozzle.

Flight research helps a researcher identify more of the unknown unknowns, before a technology can 

be reliably utilized in a fully operational vehicle. Flight testing and flight research are the crucial phases 

within the advancement of any flight technology, and are often the barrier to technology utilization by the 

private sector. There is no question that flight research with a free-flying rocket will be required in the 

future, but this research should be preceded by captive-carried flight research. Within flight research,

there are several reasons to conduct captive-carried flight research with the dual-bell nozzle, as opposed 

to only conducting sounding-rocket research, five of which will be addressed: (1) captive-carried flight 

can utilize cold-flow propellant, allowing the existing MSFC NTF test data to be leveraged; 

(2) captive-carried flight permits a closer examination into the plume behavior and flow physics, with 

more control of the flight-test conditions; (3) captive-carried flight enables an isolated performance 

assessment of the nozzle, as opposed to the combined performance assessment of the integrated rocket 

vehicle; (4) captive-carried flight permits the propulsion assets to be better protected for future testing; 

and (5) captive-carried flight permits a rapid flight turn around, and assessment of nozzle performance 

with differing nozzles at the same flight-test conditions. 

Nozzle Placement and Sizing Criteria

Figure 12 shows the original design of the CDE mated with the PFTF (ref. 11), with a possible nozzle 

location identified. The observant reader will note the slight nose-up attitude of the CDE, as is visible in

this figure. The actual nose-up attitude was originally designed to be 2.5°, which was used to align the 

forward end of the CDE with the local flow field under the F-15B/PFTF in that region (ref. 11). Since the 

initial F-15B/PFTF envelope expansion flights, flight measurements have been taken of the local flow 

field near a simulated propulsion experiment inlet (near the front of the CDE) at various flight conditions

(refs. 12 and 13). Although several measurements in this region have been taken, measurements near the 

aft end of a simulated rocket forebody (such as the CDE) are still required during Phase I, since this is 

near the placement of the nozzle exit plane for the proposed experiments. As noted earlier, the external 

flow field of the dual-bell nozzle is expected to have a significant effect on the nozzle plume behavior, 

including the dual-bell nozzle plume mode transition. As of the date of this publication, only qualitative 

local flow-field measurements in this region on the F-15B/PFTF have been taken, through flow 

visualization techniques (ref. 11).
7

PFTF Internal Capacity

The PFTF has an internal capacity that can be used for the dual-bell rocket nozzle propellant feed 

system. Figure 13 shows an expanded view of the PFTF, with the PFTF main structure, experiment racks, 

and force balance (ref. 14). The PFTF main structure was fabricated from a solid billet of 6061-T6 

aluminum, with an overall length of 107 inches, an overall height of 19 inches, and an overall width of 

10 inches. The PFTF main structure is divided into three bays (forward, mid, and aft), that are separated 

by bulkheads and the force-balance system. The forward rack, mid rack, and aft rack were designed for 

installation into these three bays, and are designed to contain components such as propellant tanks, 

control valves, propellant feed system plumbing, and instrumentation (ref. 14).

PFTF Thrust Limitations

The PFTF includes a force balance system, with a six-degree-of-freedom in-flight force measurement 

capability. Figure 13 shows an expanded view of the PFTF, with the force balance system which is 

located in the mid-bay (ref. 14). The maximum design loads are noted in table 1, along with the original 

predicted accuracy. The system was designed to meet ultimate loads of three times the maximum design 

loads for each of the six component cases noted (ref. 14).

Table 1. PFTF force balance specifications.

Component Design

Predicted accuracy 

(percent error of 

measured value)

Axial force ±2,000 lbf < 0.5

Side force ±500 lbf < 1.0

Vertical force ±1,500 lbf < 2.0

Roll moment ±8,520 in-lbf < 5.0

Yaw moment ±10,080 in-lbf < 5.0

Pitch moment ±55,080 in-lbf < 5.0

The force balance was utilized to take measurements of the aerodynamic and inertial forces, and the 

moments generated by the CDE. Data from these early flights have shown the PFTF force balance system 

to have repeatable axial force measurements at various flight conditions (ref. 11). The PFTF force balance 

system was also designed with an isolation capability, where the system could be protected with lock-out 

pins in the case where an in-flight measurement capability is not required (ref. 14).

PFTF Flight Envelope

The PFTF flight envelope is bounded by the F-15 free-stream altitude limit of 60,000 ft, the Mach 

limit of 2.0, and the dynamic pressure limit of 1,100 psf. This flight envelope is conservative, since the 

PFTF was designed to withstand much higher aerodynamic loads in flight (ref. 14). Although the PFTF is 
8

capable of flights up to 60,000 ft, flights higher than 50,000 ft with the F-15 airplane will require 

additional life support systems. Figure 14 shows the PFTF flight envelope (ref. 13), which is a subset of 

the F-15 flight envelope. This flight envelope provides an ample range capability in which to test the 

dual-bell nozzle in a relevant flight environment at numerous flight-test conditions of interest. Preliminary 

thoughts on flight research within this flight envelope are discussed in the following paragraphs.

Phase II of the flight-research campaign will be to operate a dual-bell nozzle with cold flow at various 

test conditions. Key flight test points will be complemented with cold-flow operation of a CB nozzle to 

enable a quantitative comparison of nozzle performance. For the dual-bell nozzle, two key altitudes will 

be identified: (1) a lower altitude flight for nozzle plume expansion that is optimally expanded in the first 

bell; and (2) a higher altitude flight for nozzle plume expansion that is optimally expanded out to the end 

of the second bell, at the nozzle exit plane. The high altitude flights are envisioned to be at a relatively 

high altitude within the F-15/PFTF range capability, since increasing altitude will enable lower nozzle

combustion chamber pressures to be utilized (due to the lower ambient pressure). Also, Phase II flight 

research is envisioned to require several tests within the F-15/PFTF altitude range capability, since tests at 

various altitudes between the two ideal altitudes (noted above) will facilitate a better understanding of 

nozzle flow behavior and performance as a function of NPR. Flights between these two ideal altitudes 

will facilitate detailed measurements of the dual-bell flow mode transition.

Phase III of the flight-research campaign will also require several tests at various altitudes of interest, 

as noted for Phase II. In addition to these altitude-range tests, Phase III of the flight-research campaign 

will likely require testing with reacting flow up to the dynamic-pressure limit of the F-15/PFTF envelope. 

These tests will be beneficial along this boundary of the envelope since research conducted at these test 

points will be closer to a relevant flight environment of a free-flying rocket. Nozzle operation may also be 

conducted for a prolonged duration while the F-15 airplane is rapidly climbing in altitude along the high 

dynamic pressure boundary, simulating a rocket’s trajectory.

Phase I of the flight-research campaign will be to conduct a local flow field survey of the flight-test 

conditions envisioned during Phase II and Phase III. Once all of the appropriate flight-test conditions for 

Phase II and Phase III have been identified, these conditions will form the basis for flight activity within 

Phase I.

Conclusion

Although the dual-bell rocket nozzle is predicted to have greater performance over the CB rocket 

nozzle, the dual-bell nozzle has still not been tested in a relevant flight environment. If the predictions are 

accurate, this performance advantage would be a valuable benefit for the architecture of future launch 

systems, resulting in a cost reduction of delivering payloads to LEO.

NASA DFRC and NASA MSFC have formed a collaborative effort to advance the TRL of ACN 

concepts, starting with the dual-bell rocket nozzle. This report provides highlights of a proposed effort to 

conduct testing and research with a dual-bell rocket nozzle in a relevant flight environment. The NASA 

F-15/PFTF is offered as the flight testbed, an asset that was specifically developed and flight-proven for 

the purpose of advancing propulsion-focused technologies through captive-carried flight research. Results 

from this proposed flight research effort provide evidence that the F-15/PFTF will be an excellent testbed 

for advancing the TRL of the dual-bell nozzle. The DFRC and MSFC collaboration will leverage the 

capabilities and expertise of two NASA centers that have focused on ACN technologies, and combine 

their resources to contribute to a better space-launch capability for the nation.
9

References

1. Smil, Vaclav, Creating the Twentieth Century: Technical Innovations of 1867 to 1914 and their 

Lasting Impact, Oxford University Press, Inc., New York, 2005.

2. Anderson, John D., Modern Compressible Flow, with Historical Perspective, Third Edition,

McGraw-Hill, New York, 2003.

3. National Aeronautics and Space Administration Goddard Space Flight Center, Robert Goddard and 

His Rockets, URL: http://www-istp.gsfc.nasa.gov/stargaze/Sgoddard.htm. Accessed August 5, 2013.

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Power Technology – Nozzle Design, Pratt & Whitney Rocketdyne, Spring 1992, URL:

http://www.pwrengineering.com/articles/nozzledesign.htm. Accessed July 31, 2013.

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Ryan, and Steven A. Taylor, “Launch Propulsion Systems Roadmap: Technology Area 01,”

National Aeronautics and Space Administration, April 2012, URL: 

http://www.nasa.gov/pdf/500393main_TA01-ID_rev6-NRC-wTASR.pdf. Accessed July 31, 2013.

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Analysis at Marshall Space Flight Center,” AIAA 2009-4856, August 2009.

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Dennett, “Flight Research of an Aerospike Nozzle Using High Power Solid Rockets,” 

AIAA 2005-3797, July 2005. 

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Propulsion Flight Test Fixture,” AIAA 2002-4131, July 2002.

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Propulsion Experiments on the NASA F-15B Propulsion Flight Test Fixture,” 

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Research,” AIAA 2001-3303, July 2001.
10

Figures

 Figure courtesy: Aerojet Rocketdyne

Figure 1. CB nozzle exhaust plume conditions.
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Figure 2(a). Plot of nozzle thrust coefficient versus altitude.

Figure 2(b). Plot of nozzle efficiency versus altitude.

Figure 2. Plot of nozzle performance versus altitude, with three different CB nozzles, with area ratios of 

50, 70, and 100.
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Figure 3. Examples of ACN designs.

Figure 4. Front view and isometric view of a typical dual-bell nozzle.
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Figure 5. Mach contours from a CFD analysis for the dual-bell nozzle at low altitude (in mode 1), with 

nozzle flow into a quiescent environment.

Figure 6. Mach contours from a CFD analysis for the dual-bell nozzle at high altitude (in mode 2), with 

nozzle flow into a quiescent environment.
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 Photo courtesy: NASA/MSFC

Figure 7. Dual-bell nozzle during testing at the NASA MSFC NTF.

 Photo courtesy: NASA/MSFC

Figure 8. CB nozzle during testing at the NASA MSFC NTF.
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 EC96-43546-01 

Figure 9. The NASA DFRC F-15B airplane in flight.

 ED10-0283-10

Figure 10. The NASA DFRC F-15D airplanes, during arrival from the USAF.
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 EC01-0324-3

Figure 11. Photo of the NASA DFRC F-15B/PFTF, with the CDE.

Figure 12. Design of the CDE mated with the PFTF.
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Figure 13. Expanded view of the PFTF, with the PFTF main structure, experiment racks, and 

force balance.

Figure 14. PFTF flight envelope.