Spiked Blunt Bodies for Hypersonic Flights

Figure 1: Structured multiblock grid for a blunt body with a spike in hypersonic flights.

1950 words / 10 minutes read

Intro

In the design of aerospace vehicles such as missiles, rockets and reentry vehicles which typically travel at hypersonic velocities, reducing pressure drag and aerodynamic heating are the two most critical design requirements. On one hand, high drag is needed to decelerate a reentry capsule, on the other, for vehicles escaping the earth’s atmosphere, drag reduction is a primary requirement.

Drag reduction in hypersonic vehicles is essential because it aids in economizing the fuel usage, maximizing the range, simplification of the propulsion system and maximizing the ratio of payload to take-off gross weight. Also, an increase in drag means an increase in surface temperature. Excessive heating can cause ablation of the vehicle surface material resulting in fluctuations in vehicle performance and in some fatal cases complete material failure. Further, the generation of ionized gases at elevated temperatures will result in communication black-outs also.

One common and effective way to reduce the heat load is by providing a blunt body shape for the hypersonic vehicle. Blunt shapes are preferred because they are better at heat distribution and also during the descent phase they are very effective in generating the desired vehicle deceleration. However, there will be a dramatic increase in drag due to the formation of a detached bow shock in front of the blunt nose during the ascent phase also, which is something designers want to avoid at all cost.

If on the other hand, pointed slender bodies are used, they generate lower drag but they are less effective in dissipating the heat away from the body. Hence it is a tough design challenge to minimize the pressure drag and aerodynamic heating at the same time. As a consequence, there will be a trade-off between these two critical requirements.

**Figure 2:** *Trident II (D5) on the left, and the Trident I (C4) on the right with aerodisk spikes. Photo courtesy of Lockheed-Martin Missile and Space.*

Since the introduction of hypersonic vehicles in the late 1940s, there is ongoing research to conquer these problems. The understanding is that the two requirements of lower aerodynamic heating for re-entry and lower drag for atmospheric flights can be achieved by altering the flowfield pattern around the blunt-body which results in eliminating the strong detached bow shock. A large array of techniques have been explored and implemented. Some of them are, usage of spikes, forward-facing cavity and aero discs, energy deposition using plasma torch, focussed gas jet, projection of laser or microwave beams upstream of the nose stagnation point, arc discharge, DC corona discharge, non-ablative thermal protection, etc.

Out of these wide arrays of approaches, the spiked blunt body is considered the most promising because of its simplicity, efficiency and effectiveness. Occupying only a small space, spikes transform the strong bow shock in front of the blunt nose to weak oblique shock waves. The flow phenomena which develop due to spikes are quite complex and have many fascinating characteristics. Before we delve more into the flow around spiked bodies, let’s understand the unspiked flow physics first.

**Figure 3:** Features of flowfield around a blunt body with and without a spike during hypersonic flights. Image source Ref [10].

Blunt Body in Hypersonic Flights

A blunt body moving at high speeds generates a detached bow shock wave in front of it. Fluid particles experience a sudden deceleration as it crosses the bow shock. Moving further downstream, the flow loses most of its kinetic energy but simultaneously gains an increase in local pressure and temperature. This results in an increase in pressure drag and aerodynamic heat transfer to the body.

When we consider aerodynamic heating and distribution, a larger blunt nose radius is preferred since wall heat flux is inversely proportional to the square root of nose radius. The larger the nose radius, the lesser is the heat transfer to the wall and vice-versa. So in a way, a larger radius scales back the heat from the body and dissipates the warmth to the outside air stream. In this way, a blunt nose protects the vehicle from the severe mechanics of heating.

Typical features of the flowfield associated with (a). Unspiked blunt bodies (b). Disk-spiked blunt bodies with separation shock. (c) Disk-spiked blunt bodies without separation shocks. — **Figure 4:** Typical features of the hypersonic flights flowfield associated with (a). Unspiked blunt bodies (b). Disk-spiked blunt bodies with separation shock. (c) Disk-spiked blunt bodies without *separation shocks. Image source Ref [1].*

Spike Flow Physics

Now if we attach a slender rod at the stagnation point of a blunt body, it brings in two major modifications to the flow field. Firstly, it replaces the strong detached bow shock with a system of weaker oblique shock waves. Secondly, it acts as a flow-separator, i.e., it enforces boundary layer flow separation and creation of a shear layer. Figure 4 b-c, illustrates the main features of a spiked blunt body flow field.

Because of the spike, the strong detached bow shock is replaced by an oblique shock wave called foreshock which significantly reduces the rise in levels of pressure and temperature. The presence of adverse pressure gradient ahead of the body and friction on the spike surface triggers boundary layer detachment downstream of the oblique shock, leading to the creation of a shear layer. Simultaneously, at the point of flow separation, a shock called flow separation shock is formed which turns the flow parallel to the shear layer. Sometimes due to geometric effects, the separation shock may coincide with the foreshock formed ahead of the spike.

The detached shear layer propagates downstream and reattaches onto the blunt nose surface, creating an enclosed zone of recirculating air at low pressure and temperature. This dead zone screens a large part of the blunt forebody, resulting in a significant drop in drag and aerodynamic heating.

However, at the location where the shear layer reattaches to the blunt-body, there is a shoot-up in surface pressure and heating rates. Further, to align the flow outside the shear layer parallel to the body surface, a shock is formed at the point of reattachment called the reattachment shock. The strength of this shock depends on the flow turning angle which in turn is governed by the location of the reattachment point on the nose surface. Furthermore, whether the reattachment point can be moved backward or forward or completely removed depends on the nose radius and spike length.

To summarise, the weak oblique shock wave creation and establishment of the recirculation zone, both contribute to the dramatic reduction in wave drag and aerodynamic heating relative to the case without a spike.

**Figure 5:** Hypersonic flights, CFD mesh: Structured grid for a disk spike-tipped hemispherical nose body. Image source Ref [1].

Unsteady Flow

For certain flow conditions with certain nose shapes and spike dimensions, flow instability may set in characterized by a continuously changing flow pattern in a self-exited, self-sustained manner. Depending on the level of instability, the flow can be categorized into a pulsatory or oscillatory mode.

If the flow is pulsatory, there will be a drastic change in the foreshock shape, varying from conical to hemispherical shape, along with variations in the recirculation zone, high oscillation frequency, and high local pressure amplitudes on the body surface.

On the other hand, if the flow turns out to be oscillatory in nature, there will be small lateral changes to the foreshock and shear layer shape, regularly shifting from concave to a convex shape, with lower oscillation frequency and lower local pressure amplitudes on the nose face.

Flow instabilities will result in fluctuations in surface pressure and heat flux, increased acoustic and structural dynamic loads, spike bending, flight perturbations, and difficulties in flight control. These issues can become worse due to hysteresis phenomena setting in when there are extensions and retractions of the telescopic spikes.

Studies have shown that flow stability is mainly dependent on the nose contour and spike geometry parameters. Therefore, for each specific flight condition there exists an optimal nose profile and spike length which ensures good aerothermodynamic performances and structural stability.

Figure 6: Possible geometric variants for forebody used in hypersonic flights. Image source Ref [8].

Geometric Effects

The diminution of drag and aero heating is governed by design parameters like nose shape, spike length and tip geometric shapes. Multiple research studies have consistently shown that spike length controls the drag response while forebody shape dictates the aerodynamic heating levels.

Figure 7: Hypersonic flights flowfield: Flow field variation with change in spike length. Image source Ref [11].

Effect of Spike Length

It is observed that irrespective of the aerodisk shape, an increase in spike length up to a certain length reduces both pressure and heat flux. What happens is that, when we increase the spike length, the reattachment point moves towards the shoulder of the blunt nose and the recirculation region expands covering a larger area of the nose. As this happens, the strength of the reattachment shock reduces due to a reduction in angle required to deflect the flow outside the shear layer to make the flow parallel to the body surface. The net effect is reduced levels of pressure increase on the nose surface. However, if we increase the spike length beyond a critical value, the recirculation zone is split into two bubbles, with a shock sitting in between them, resulting in an increase in overall drag.

Heat Flux Reduction

Researchers have observed that the aero heating levels at the reattachment region are mainly determined by the reattachment angle. The smaller the reattachment angle, the larger is the peak value of heat flux. So when we increase the spike length, the point of reattachment shifts towards the nose shoulders, resulting in increased reattachment angle, which in turn decreases the peak value of heat flux.

Also, the total heat transfer depends on the type of flow. Under fully turbulent shear layer conditions, the total heat transfer rate doubles while it reduces under laminar separated flow conditions. Further, the spike’s drag reduction performance is also determined by the surface temperature. An increase in wall temperature increases recirculation area temperature which results in the outward movement of the reattachment point.

In this way longer spikes improve the vehicle performance both w.r.t reduced pressure drag and aerodynamic heating levels.

Figure 8: Possible geometric variants of aerodisks for hypersonic flights. Image source Ref [8].

Effect of Spike Tip Geometry

Over the years, various spike tip geometric shapes have been experimented ranging from sharp-pointed, to flat head aerodisk to hemispherical tip to biconical shapes. Further, other unique profiles like a double hemisphere, double disks, etc have also been tested.

As per general consensus, it is observed that aerodisks yield better performance for thermal protection and drag reduction compared with the pointed spike. Drag reductions from 30 percent to as high as 75 percent have been claimed by various researchers. Also, double-headed profiles are observed to show better drag reduction compared to single profile spikes.

Furthermore, it is observed that, for a fixed aerospike length, flat aerodisks generate the least drag compared to biconical, hemispherical, or pointed spikes.

Figure 9: Hypersonic flights CFD: Mach flowfield around: (a) Single flat-faced disc aerospike. (b). Double flat-faced disc aerospike. Image source Ref [11].

Disk Diameter Effect

If on the other hand, we keep the length fixed and keep increasing the tip disk diameter, the reattachment point starts to shift towards the nose shoulder. Expansion of the recirculation zone reduces the surface pressure. An increase in disk diameter enhances the bowed foreshock, but its effect on the drag is local. However, once the disc diameter exceeds a critical value, the stronger bowed foreshock neutralizes the benefit gained from the expansion of the recirculation zone to the drag reduction.

Studies have shown that an optimum disk diameter is related to the spike length and is found to be inversely proportional to the spike length. This implies spike length should be greater than disk diameter for stable flow, reduced drag, and aeroheating.

Effect of the spike on shock wave structure at Mach 2, alpha = 20 degs. Body with and without spikes. — **Figure 10:** Effect of the spike on shock wave structure at Mach 2, alpha = 20 degs. **(a)**. Body without spike. **(b).** Body with a conventional fixed spike. **(c).** Body with an aligned spike of the same length. Image source DLR.

Additionally, irrespective of the spike head shape, drag increases with the angle of attack for a large range of Mach number. As a solution, some researchers proposed the concept of ‘pivoting spike’ in which the spike is kept aligned to the freestream direction while the rest of the body is aligned to the flight path. This arrangement helps in maintaining the drag at a similar value as that at level flight.

Hypersonic flights CFD: Streamlines on symmetry plane at 8 degs alpha. (a) 0 deg pivoting. (b) 8 deg pivoting — **Figure 11:** Hypersonic flights CFD: Streamlines on symmetry plane at 8 degs alpha. **(a)** 0 deg pivoting. **(b)** 8 deg pivoting. Image source Ref [11].

Parting Thoughts

Blunt profile approach for hypersonic vehicle design has been in existence since the late 1950s. Though it has its inherent limitations, it has been in wide usage in all most all reentry vehicles for decades as it was considered to be the most practical workable solution. Aerospike with its simplicity and efficiency has presented itself as an effective solution to the limitations in blunt bodies aerothermal performances. It will come as no surprise if we see more often of these aerospikes in hypersonic vehicles in the coming years.