DEFENCE Hypersonic glide vehicles

Hypersonic come weapons of age​

Over 70 years after it was first envisaged – has Eugen Sänger’s vision of an unstoppable suborbital glide bomber returned with Russia’s Avangard hypersonic glide vehicle? TIM ROBINSON assesses the new high-speed arms race.

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Today, in hindsight, it is clear that the Nazi regime in WW2 was pushing the boundaries of aerospace in the quest to develop ‘wonder weapons’ designed to alter the outcome of the war. Jet fighters and ballistic missiles came as a surprise to the Allies but were rapidly adopted by the superpowers throughout the Cold War as key weapon systems.

Nazi suborbital bomber

However, one particular vision of future warfare has waited over 70 years for technology to catch up – that of a long-range hypersonic glide vehicle. Originally conceived by Eugen Sänger, the Silbervogel (Silver Bird) was designed as an unstoppable ultra-long-range suborbital bomber, able to strike the US from Europe by ‘skipping’ in and out of the upper atmosphere. The Silbervogel glider would have been launched from the ground by rocket-powered sled, before engaging its own rockets to climb to an altitude of 90miles and then using the upper atmosphere in a series of ‘skips’ to extend its range – reaching a top speed of Mach 17 during its flight.

Once over its target, it would have dropped an 8,800 lb bomb, before continuing on to land in Japanese Axis-held territory. Immune to countermeasures or interception, the Silbervogel would have had a staggering range of 1924,000km. Fortunately for the Allies, this design for a spaceplane bomber was constrained by the technology of the day, although its concept did go on to inform the US X-20 Dynasoar and Space Shuttle. (There is also speculation in some quarters that the USAF’s secretive X-37B spaceplane also uses atmosphere ‘skipping’ to alter its trajectory and confuse those attempting to track it).

Hypersonic projects today

Fast forward 70 plus years to 2019 and a mantra in many defence research activity projects is ‘speed is the new stealth’ as a way to penetrate ever more highly defended airspace and strike time-critical targets from long range. Hypersonic projects are now being fast-tracked around the world. Even the UK, faced with defence budget crises and post-Brexit uncertainty has realised the need to join this race and has committed £10m to investigate advanced hypersonic projects.

Missiles or powered vehicles (either scramjets or rockets from Mach 5+ flight) are one strand of development – but another is hypersonic glide vehicles (HGV) – where the initial energy from a powered ballistic launch is converted into highspeed cruise glide by the vehicle pitching back into the atmosphere.

In a RAeS HQ lecture on 15 October, Dr Malcolm Claus, Course Director – Astronautics and Space Technology, Kingston University, gave a fascinating overview of the technology itself and the various programmes underway around the world in the US, Russia and China.

The US, for example, had two main HGV projects running from 2003-2014 – the Mach 20 Falcon HTV-2 and the US Army’s Advanced Hypersonic Weapon (AHW). Three out of four flight tests were unsuccessful, with both flight tests of the HTV-2 failing at the nine-minute mark. Last year saw the US redouble its hypersonic efforts – with Lockheed Martin selected to develop a new Mach 20 HGV – the ARRW (Air-launched Rapid Response Weapon) or AGM-183A. Other powered hypersonic missiles are also in development.

Meanwhile, in early 2018, Russia sent shockwaves around the world when President Putin unveiled to the world its strategic Avangard HGV. Previously known as Project 4202 or Yu-71, the arrow-like lifting body Avangard is launched by a SS-19 ICBM, before gliding at Mach 20-27. Designed to evade the latest US ABM defences, Avangard is speculated to use a ‘skip’ mission profile, similar to Sänger’s Silbervogel, to increase its range.

Russia had conducted a number of test launches with Avangard – with the latest in December 2018. Dr Claus, describing Avangard as a “ferocious piece of equipment,” speculated that the glider would likely also carry countermeasures and, when carried by the new Sarmat (SS-X30) ICBM, could be launched in packs of four, given the estimated dimensions of the air vehicle. Moscow now claims that Avangard is operational – and plans to deploy two regiments worth of the weapons by 2025.

China, meanwhile, has followed its own opaque path in HGVs, with Western analysts attempting to track progress of projects such as Starry Sky 2 and the DF-ZF. The latter broke cover early in October when HGVs prominently marked with ‘DF17’ appeared on launcher vehicles which took part in China’s 70th anniversary military parade. These HGVs appear to be non-nuclear conventional weapons (unlike Avangard) and are launched by medium range ballistic missiles. They also travel at half the speed (Mach 10) of US/Russian HGVs – possibly to ease heating requirements.

One interesting piece of research published in a Chinese journal in 2019 saw a hypersonic shape very much like the US HTV-2 investigated via CFD – with the Chinese using US failures to advance their own HGV technology. That said, Claus explained that China boasts a robust infrastructure to support hypersonic research.

Hypersonics – the advantages

Some might well ask – in a world where superpowers already have access to long-range cruise missiles and ICBMs – what is driving this new interest in HGVs?

HGVs can use three mission profiles – a purely ballistic one, a glide or a skip profile. Ballistic – (as used by missiles since the V2) gives least time within the atmosphere – but also has the highest thermal heating (albeit for the shortest period of time). A glide profile, meanwhile, sees a long period of time in the atmosphere but a lower comparative level of heating for a longer, sustained flight period of time. It also allows the vehicle to manoeuvre. Finally, the skip profile sees flight within and outside the atmosphere. This allows for extended range and by ‘skipping’ out of the atmosphere the vehicle can also temporarily reduce heating – making it more difficult to track by IR sensors. A skip flight profile can also manoeuvre.

As well as long-range and ultra-high-speed (Mach 20) another key advantage for military planners is the ability of the HGV to manoeuvre. This means its flight path to the target can be unpredictable, avoiding ABM defences which rely on careful placement to intercept incoming ballistic missiles. Ballistic missiles, which launch their warheads on a predictable trajectory are thus increasingly vulnerable to interception by the latest anti-ballistic missile (ABM) systems – made more capable today by 21st-century computers, sensors and radars.

Another advantage, noted Claus, is that HGVs are difficult to track – with current over-the-horizon early warning radars being optimised for incoming ballistic missiles. Meanwhile, while infra-red missile warning satellites in GEO are optimised to detect the large plume of rocket launches and missiles during the boost phase, they have more difficulty tracking HGVs once in the glide phase where they are lower in the atmosphere.

Finally, although the heating of the vehicle is still a major challenge, it is still lower than ballistic (or orbital re-entry).

Challenges of HGVs

​As might be expected – HGVs bring their own challenges – including structural, thermodynamics, control and communications. Three out of four HGV test flights (at least in the ‘white’ unclassified world) by the US have ended in failure. It remains unknown how many failures Russian and China have encountered along the way.

An obvious one is how to manage the thermodynamic structural challenges of extended high-speed flight in the atmosphere. This may require exotic materials such as ceramic matrix composites (CMC), metal matrix composites (MMC) or ultra-high temperature (up to 3,000°C) ceramics. (However, it is important to note that given the high speeds of HGVs, these are still short periods compared to even supersonic flight. A Mach 20 HGV launched from Moscow to Washington DC, for example, would be airborne for just 19mins.)

Another challenge, noted Claus, was in control of the HGV, which could be via control surfaces (flaps or tabs), rocket/reaction control motors or even magnetic hydrodynamics (MDI) manipulating the high-speed plasma field to control the flight path. Control at these speeds though has its own additional challenges – as even tiny imbalances could cause aerodynamic instability and destruction of the vehicle. Flaps or tabs, for example, will have gaps that could disrupt the flow, while jet/rocket thrusters (used in some SAMs for terminal thrust control) might also interact with airflow in unanticipated ways.

Finally, there is the challenge of controlling or commanding this vehicle. At Mach 20, a HGV travelling through the Earth’s atmosphere will create its own ‘plasma’ shield – creating a barrier to transmissions similar to the ‘radio silence’ encountered by astronauts re-entering the Earth’s atmosphere. This means the selection of radio frequencies will need to be carefully optimised.

Counter hypersonics – a new SDI?

The interest in the potential of these weapons has thus kick-started an equal effort in ways to defend against them – with a number of alternatives being proposed. In 2017 the US MDA (Missile Defense Agency) launched its hypersonic defence programme, focused on adapting ABMs for point defence against hypersonic threats. This year also saw the US Space Development Agency (SDA) stand up, which focuses on new and emerging space threats that include HGVs.

However, part of the challenge will be detecting and tracking these high-speed targets that fly inside Earth’s atmosphere. This, predicts Claus, will require a large number (perhaps even up to 1,000), of satellites in LEO to detect and track HGVs – a critical task given the short warning that hypersonics give. Given the need for the number of satellites, could the new generation of commercial LEO mega-constellations such as OneWeb and SpaceXs Starlink be co-opted by the US military to provide this sort of hypersonic threat sensor layer?

Once detected, the next challenge will be to intercept and destroy the incoming HGVs. This challenge, put forward by DARPA in its Glide Breaker counter-hypersonics programme and MDA efforts has already sparked a wide range of potential solutions, including interceptor missiles (SkyFire – Raytheon, Valkyrie – Lockheed Martin), hypervelocity projectiles (General Atomics/Boeing/BAE Systems), laser guns (Boeing), EW and electronic attack (Northrop Grumman/L3).

Indeed, the effort now put into counter hypersonic research prompted Claus to ask whether this was a ‘return to SDI’ – the Reagan-era Strategic Defence Initiative that aimed to provide a missile shield against ballistic missiles.

Command and control at Mach 20

Finally, there is one other consideration to take into account for warfare in the hypersonic age – that of the compressed reaction times that are ever so gradually moving armed forces in the direction of embracing AI for missile defence. Flying at Mach 20, an HGV could cross the entire US coast to coast in just 10 minutes – reducing warning and response times of defenders to frighteningly short times.

Just as a close-in weapon system (CIWS) onboard a warship is entirely computer-controlled because no human has the nano-second reflexes to be able to track and destroy incoming antiship missiles – will counter-hypersonic defences also need to be automated? If so, what does that mean for deterrence or ambiguous launch warnings – particularly for conventionally-armed HGV launched by ICBMs? In 1983 Russian air defence Colonel Stanislav Petrov ignored what an early warning system had identified as a US ICBM heading for the USSR – thereby preventing a retaliatory response and nuclear Armageddon. For strategic hypersonic defence, what safeguards will need to be in place to allow humans to make the final call – even when they may have just minutes to decide?

Summary​

Some 70+ years on after it was originally conceived, a concept for a sub-orbital longrange glide bomber has returned from the past – with modern materials and technologies. Hypersonic glide vehicles, which keep an ICBM’s speed but duck into the atmosphere to evade detection, extend their range and manoeuvre to outwit existing defences, represent a new era of hypersonic warfare that is accelerating a new arms race around the globe.

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