James A. Jolly

jamesajolly@gmail.com

Scramjets: From Supersonic to Hypersonic

Abstract: Scramjets represent an important next step in the future of air and space travel.

But there are still many kinks that need to be worked out before we see scramjets in widespread

use. This article will cover the basics of what a scramjet is, and the potential they have in air and

space travel. Additionally, it will provide analysis on already existing scramjet designs and

concepts like the X-43 and X-51; in both their strengths and weaknesses. Scramjets are a quickly

developing and evolving field, and there are still many unknowns. Overall, however, it is clear

there are numerous possibilities of scramjet applications for both military and commercial use

and that there will undoubtedly be a great deal of future research and development in the. This

article attempts to dive not only in to the past, present, and future of scramjets.

Bio: James Jolly is a Bachelors Student at USC studying Astronautical Engineering and on track

to graduate in the spring of 2015. He is interested space travel, specifically in regard to orbit

insertion from Earth.

Introduction

In 1948, the X-1 became the first supersonic aircraft in the world, traveling faster than the

speed of sound. A decade and a half later, in response to the U-2 spy plane being shot down

during the Cold War, the SR-71 Blackbird was developed to fly higher and faster than any other

aircraft before it, pushing the limit to over Mach 3, or 3 times the speed of sound [3]. In today’s

times, aerospace companies and researchers are looking to push aircraft speeds even further, to

what we call hypersonic speeds; or Mach 5 and above; and the application of such speeds goes

far beyond just spy planes. Being able to fly faster has obvious applications in both commercial

and military areas, including a lucrative market for faster commuter air travel, and increased

lethality in air combat vehicles. But how do we make aircraft travel this fast? To achieve these

hypersonic speeds, we must look to a specific type of engine, called a scramjet.

What is a scramjet?

All of the planes that you see flying around today use a form of a turbojet. A turbojet is a

jet engine that uses a turbine to suck in and compress the needed oxygen for combustion in the

engines. A scramjet is a specific type of more general type of engine called a ramjet, which

operates slightly differently than a turbojet. To understand the difference between a turbine

engine and a scramjet, it is important to take a step back and understand basically what any

engine does. For simplicity we can think of aircraft engines in two categories; air breathing and

non-air breathing. A non-air breathing engine would be a rocket, where everything needed for

combustion to produce thrust is carried on the aircraft itself. For an air-breathing engine, oxygen

is required from the surrounding air for combustion of the fuel in the engine to take place,

meaning the aircraft does not have to carry the oxygen itself [1]. In this regard both turbojets and

ramjets are air breathing engines, but beyond this they are very different.

In order to produce thrust, a jet engine must combust far more oxygen than would

naturally flow through an engine at a standstill or at low speeds. In order to maintain this

necessary flow of oxygen into the engine, a turbojet

uses turbofans to suck in the outside air into the

engine to be combusted. At higher velocities, these

turbofans become unnecessary because the velocity

of the aircraft itself is high enough that the

surrounding air is automatically compressed into the

engine [1]. This is the concept of a ramjet; where air

is literally compressed by being “rammed” into the

engine rather than from a compressor like a turbofan.

This is where the term ramjet comes from.

A scramjet is a specific type of ramjet known as a

supersonic combusting ramjet. At high velocities, the

air must be slowed down in order for it to be

combusted and then reaccelerated out the back. A ramjet engine slows the incoming air to

subsonic speeds before it is ignited with the fuel. A scramjet, however, is designed so that the

flow of oxygen through the engine, while still being decelerated, never becomes subsonic [2].

This makes a scramjet capable of higher velocities than a ramjet.

When talking about aircraft and aircraft engines it is far too simple to talk just about

maximum speed capabilities. It is critical to talk about efficiency. While a scramjet is capable of

greater speeds than both a turbojet and an ordinary ramjet, it is not superior in every way, and

certainly not at every speed. At lower speeds a turbojet is far more efficient in terms of fuel

Figure 1: Three types of engines; (a) Turbojet, (b) Ramjet, (c) Scramjet

http://en.wikipedia.org/wiki/File:Turbo_ram_scramjet_comparative_diagram.svg

consumption, and even in terms of capability. This is because ramjets (and thus scramjets), by

their design, cannot even operate at lower speeds. Also, at higher speeds however, the turbofan

compressors get in the way and become a hindrance to higher flight speeds using a turbojet.

Because of this, at around Mach 2 or 3 ramjets become far more effective and efficient. Because

they slow the flow of oxygen to subsonic speeds, a ramjet will only remain effective up to

around Mach 6. Anything above this speed requires a scramjet. The limits of scramjet speed is

uncertain, but has been estimated as high as Mach 20 to 25 [6]!

Modern Scramjets

There has been a great deal of research and development into the field of scramjets on the

part of aerospace companies and aerospace engineers throughout the United States and all over

the world. But what kind of scramjets exist today, and how well do they work? To answer this

question we will look at two particular and recent scramjet

developments; the X-43 and X-51. These were not the first

scramjets to be developed, nor do they encompass the entire

field of scramjets research, but they are two good examples of

scramjet capabilities and drawbacks.

The X-43 was part of NASA’s Hyper X program, and was

an attempt by engineers at NASA to develop a hypersonic

aerial vehicle. The X-43 itself was attached to a rocket booster

that was dropped from a B-52 as shown in Figure 2, and

which accelerated the X-43 to high velocities before the

scramjet engines engaged and the X-43 detached and

flew on its own. In March of 2004 the X-43 flew at Mach

Figure 2: The X-43 shown by its self (bottom), and attached to Pegasus booster rocket and B-52 (top)

https://www.fas.org/spp/guide/usa/launch/HyperX1.jpg

http://upload.wikimedia.org/wikipedia/commons/4/4a/X43a2_nasa_scramjet.jpg

6.8, and in its last flight in November of 2004 it reached a speed of Mach 9.6. In each case the

engines operated and recorded data for 10 seconds. In the Mach 6.8 flight the scramjet engines

were able to accelerate the X-43 during their operation, and in the Mach 9.6 flight the engines

were able to allow the X-43 to maintain its velocity [2]. The X-43 currently holds the speed

record for an air breathing engine, and is an impressive display of the speed capabilities of a

scramjet.

The X-51 was developed by Boeing and was aimed at more extended hypersonic flight.

Like the X-43, the X-51 was accelerated by a rocket

after being dropped from a B-52 as shown in Figure 3.

In May of 2010, the X-51 was released at Mach 4.5,

from which the X-53’s scramjet engines took over and

accelerated it to Mach 5 for 143 seconds [7]. The X-

51’s performance was a huge step forwards in

displaying hypersonic capabilities for a more extended

period of time.

While both the X-42 and X-51 display critical advancements in Scramjet technology,

there are still drawbacks and problems that need to be fixed before more relevant applications of

Scramjet engines become available. The first problem is the limited duration for these engines.

This will most likely be solved with further improvements to Scramjet vehicle design. The

second critical drawback is the assisted launch requirement. Both the X-43 and X-51 required

assisted takeoff from both a B-52 and a rocket. This has cruciual implications and concerns for

Scramjet applications in regard to how they will actually take off and fly.

Assisted Takeoff: ignore or solve?

Figure 3: X-51 attached to a B-52

http://en.wikipedia.org/wiki/File:X-51A_Waverider_on_B-52_2009.jpg

The question remains on what to do with assisted takeoff for Scramjets. It is clear that

scramjets (or any form of ramjet) will not be able to operate at lower speeds. One possible choice

is to simply use assisted takeoff for any future Scramjet applications, like the use of an external

aircraft and a rocket to build up altitude and speed before the scramjet engine is engaged. It has

also been proposed to use a rail gun to give a scramjet vehicle its necessary velocity boost

required to start its scramjet engines. The concept is highly theoretical; but rather than using

rockets it proposes using electricity and electromagnetic forces to accelerate a vehicle [5].

Another possibility is the use of a hybrid vehicle, which makes use of both turbojets and

scramjets. One of the best ways to look at this

idea is to examine the work done by Lockheed

Martin in the design and development of “hybrid”

aircraft and concepts. The SR-71 Blackbird,

developed by Lockheed, was able to fly at speeds

over Mach 3. An interesting thing about the SR-

71 was that it made use of both Turbojet and

Ramjet technology. The Blackbird needed to be

designed so that it could operate by itself all the

way from takeoff to over Mach 3 [3]. It needed turbojets to operate at lower speeds, and ramjets

in order to achieve higher speeds. To solve this problem Lockheed’s “skunk works” team

developed an engine that could act as both a turbojet and a scramjet. Essentially, the way it

worked was that at higher velocities inlets would open up that would bypass the turbofans and

travel directly to the ignition chamber and would be ignited and reaccelerated to supersonic

speeds and flow out the back. This would bypass the turbofans and prove the large amounts of

Figure 4: The SR-71 Blackbird in flight.

http://en.wikipedia.org/wiki/File:Lockheed_SR-71_Blackbird.jpg

thrust that the Blackbird would need to travel at such high speeds, and that the turbojets

themselves would be incapable of providing. The Blackbird still holds all of the speed and

altitude records for an aircraft that could take off and fly on its own, and is a great example of the

possibility from combining turbojet and ramjet technology [3].

One main consideration and drawback when using both turbojets and scramjets, is that

having both takes up more space. More space means more drag, and more drag means less

efficiency and lower speed capabilities. The solution

according to Lockheed has been to combine the two

into a single operational engine. The SR-71 did this to

some degree; but the goal is to push an aircraft to

speeds beyond even that of the SR-71. For Lockheed,

this concept has evolved into the SR-72. This plane

would make use of a multi-operational engine that has

a common inlet and nozzle, but two pathways for

incoming flow to go through: a turbine engine and a

dual-mode ramjet as shown in Figure 5. The turbine

engine would take the plane up close to Mach 3, and

then the dual-mode ramjet, which includes a scramjet,

would accelerate to aircraft to hypersonic speeds of

around Mach 6 [8]. The plane is still in early

development, but nonetheless goes to show the potential of combining Turbojets with Scramjets

for future aviation applications.

Looking to the Future

Figure 5: The SR-72 with artist rendering (top) and engine concept (bottom)

http://www.lockheedmartin.com/us/news/features/2013/sr-72.html

There are many potential applications of scramjet technology. One of the primary

applications is military technology. Being able to travel at hypersonic speeds would provide a

huge advantage on striking a target with little or no warning. This is one of the main objectives

of planes like the SR-72, and other current projects like the DARPA (Defense Advanced

Research Projects Agency) and United States Air Force Falcon Project, aimed at developing a

supersonic military vehicle capable of striking anywhere in the world faster than any country

would have time to react to. Travelling at such high velocities also has huge commercial

possibilities, as people would be able to get anywhere in the world much faster than is currently

possible. One of the biggest and most relevant potential applications of scramjets is cheaper

space entry. A vehicle using scramjets could provide the speed and altitude conditions of near

orbit, from which it would much easier to put a vehicle and any payload into orbit. A study done

by DARPA has suggested that airborne assist can decreases space launch costs, and increase

payload efficiency, or how much payload can be sent into space for a given launch and cost [4].

We are of course still far off from many of these applications; nevertheless, what we see is that

scramjets have huge possibilities in carrying us into the future of air and space travel.

Works Cited

[1] B. Dunbar. (2004) What’s a Scramjet? [Online]. Available FTP: nasa.gov Directory:

missions/research File: f_scramjets.html

[2] B. Dunbar. (2014) NASA Armstrong Fact Sheet: Hyper-X Program [Online]. Available FTP:

nasa.gov Directory: centers/dryden/news/FactSheets File: FS-040-DFRC.html#.Ux-

ThPldUlI

[3] Creating the Blackbird. Available FTP lockheedmartin.com Directory: us/100years/stories

File: blackbird.html

[4] M. B. Clapp. (2012) Airborne Launch Assist Space Access (ALASA). Available FTP

DARPA.mil Directory: WorkArea File: DownladAsset.aspc?id=2147485147

[5] R. M. Pacella. (2010) NASA Engineers Propose Combining a Rail Gun and a Scramjet to

Fire Spacecraft Into Orbit [Online]. Available FTP: popsci.com Directory:

technology/article/2010-11 File: nasa-engineers-propose-combining-rail-gun-and-

scramjet-fire-spacecraft-orbit

[6] Scramjets [Online]. Available FTP: orbitalvector.com Directory:

Orbital%20Travel/Scramjets File: Scramjets.htm

[7] (2012) X-51A WaveRider [Online]. Available FTP: Boeing.com Directory:

assets/pdf/defense-space/military/waverider/docs File: X-51A_overview.pdf

[8] (2013) Meet the SR-72 [Online]. Available FTP lockheedmartin.com Directory:

us/news/features/2013 File: sr-72.html