Introduction to Rocket Design
3. Nosecone Design ConsiderationsThe primary purpose of a nosecone is to serve an an aerodynamic fairing that gently parts the air during the forward motion and channels the air around the sides of the body tube with a minimum of disturbance of the affected air. A rocket will fly without a nosecone, however, a blunt end will greatly disturb the air flow, which requires energy to do such. Energy added to the surrounding air is kinetic energy taken away from the rocket. A secondary purpose of the nosecone is to serve as additional payload space. While this may not seem important at first glance, in fact, the nosecone payload space is an ideal location for certain payloads such as a GPS transmitter. This is particularly true if the rocket body is metallic, as a transmitter must be housed in a compartment that is transparent to radio waves. Nosecones are typically made of a non-metallic material such as plastic or wood.
Nosecone ProfileFrom an aerodynamic perspective, the shape, or profile, of a nosecone is not that important for a typical amateur rocket that flies subsonically (LoPER class). A hemispherical nosecone or a straight cone will work just as admirably as a gently curved profile (ever notice that wings on commercial jetliners have round, not sharp, leading edges?). At speeds well below Mach 1, air behaves as an incompressible fluid (i.e. density is constant) and behaves as water flowing around the bow of a boat. Pressure drag for a subsonic nosecone is essentially zero for all shapes. Solely friction drag is generated and is dependant upon the wetted area, surface smoothness and geometrical discontinuities. For model rockets, which are exceedingly lightweight in comparison to amateur rockets (and thus have a very low ballistic coefficient) and travel at low-subsonic speeds, nosecone selection gains some importance, whereby even a fraction of a gram-force drag can make a difference. Wind-tunnel testing (Resource 7) indicates that the optimum shape is "long elliptical" (other reference sources suggest likewise). Nosecone shape becomes more important for rockets that achieve a velocity approaching Mach 1 (MiPER class), refered to as the transonic flight regime, as compressibility effects of the airflow around the rocket begins to take hold. Some of the energy of the rocket goes into compressing the air and locally changing the density of the air. The resulting rise in drag, known as wave drag, acting on the nosecone results from air building up in front of it. At lower speeds this air has time to "get out of the way", guided by the air in front of it that is in contact with the nosecone, but at the speed of sound this can no longer happen, and the air which was previously following the streamline around the nosecone now hits it directly. At Mach 1, the amount of power needed to overcome this effect is considerably greater than at subsonic velocity. In supersonic flow, a shockwave is generated in front of the nose cone. Shock waves are a very thin region across which severe changes in flow properties take place. The gas molecules that collide with the tip of the nose cone set up a disturbance in the flow which then propagates, by means of weak pressure waves, to other regions of the flow away from the nose cone, at the local speed of sound. In subsonic flow, this pressure wave works its way upstream and is felt by all other regions upstream. However, when the flow is supersonic, the speed of the flow is greater than the pressure waves, which propagate at local speed of sound. Thus, these pressure waves can’t work their way upstream. Instead, these disturbances merge at a finite distance from the tip to form a shock wave. The flow upstream of the shock waves are unaffected by pressure disturbance while the downstream flow is affected by this pressure disturbance. Figure 1 illustrates three nosecone shapes suitable for a subsonic LoPER class rocket. No.1 is a straight cone, and is the easiest to make, and has useable volume. No.2 is a hemisphere, which does not look very pretty, but will work fine when volume is not needed. No.3 is a "classical" nosecone shape that offers the best volume and aesthetics. But it can be more challenging to fabricate.
For a subsonic rocket, the majority of drag is due to friction on the rocket body and fins. Drag force due to the nosecone represents a small percentage of overall rocket aerodynamic drag. The situation changes for MiPER class rockets that fly in the transonic range and for HiPER class rockets that fly in the low-supersonic range. This is illustrated by Figure 2 which shows a graph of nosecone pressure drag coefficient for various shapes. This particular graph is for a nosecone with a length to diameter (fineness) ratio of 4:1, which is a practical choice. Note that the total nosecone drag coefficient is somewhat higher, as nosecone frictional drag is not included. Also note that hemispherical profile is a subset of ellipsoidal.
(Ref. AeroLab & Resource 9) As a general guideline, there are three factors that should be considered by the amateur rocket designer to minimize drag losses for a supersonic flight regime:
* Aerodynamic heating of the tip may be a design factor for high Mach number flight Figure 3 shows an example of a supersonic nosecone detailing these three parameters.
For a ViPER class rocket that is expected to achieve a velocity greater than mach two, in particular if a rocket is designed for hypersonic velocity (> Mach 5) the optimum choice for minimum drag is a power series with n = 0.66 with a fineness ratio of at least 5:1 (see Resource 8), as illustrated in Figure 4, where R is the base radius, and L is the nosecone length.
Fineness RatioThe fineness ratio, or aspect ratio, has a more profound effect on nosecone drag than does the profile. Figure 5 illustrates the effect of fineness ratio on pressure drag coefficient for a conical profile and 1/2 power profile. As is the case with profile, fineness ratio is of significance only in the transonic velocity range and higher, as compressibility effects come into play. Greater aspect ratio results in lower drag for both profiles, however, the returns are diminishing as greater surface area results in greater frictional drag. The minimum drag fineness ratio is ultimately going to be a trade-off between the decreasing wave drag and increasing friction drag.The best take from this is to avoid a low aspect ratio nosecone such as 3:1 or less for transonic or higher velocity regimes. Ref.AeroLab
Nosecone Shape and Rocket StabilityIt is not solely the fins that determine a rocket's stability margin, or more specifically, the location of the rocket's Centre of Pressure (C.P.). The nosecone likewise directly affects the C.P. location and therefore influences the static stability margin of the rocket. As explained in the Fin Sizing and Static Stability webpage, the C.P. location of the rocket as a whole is based upon the C.P. location of the individual rocket parts (e.g. fins, nosecone, body transitions), as given by equation 1:
The nosecone terms are circled. The subscripts refer to the following:
As such, both the profile of the nosecone and the fineness ratio will influence the stability of a rocket. Figure 6 illustrates the C.P. location for some of the more common nosecone shapes (for a rocket that flies subsonically). A conical profile is seen to offer the most positive influence on rocket stability, as the C.P. is furthest aft of all the profiles. Conversely, the ellipsoidal is seen to have the most negative influence of rocket stability. It is also clear that increasing fineness ratio has a destabilizing effect. Increasing fineness ratio (or nosecone length) has a minor destabilizing effect.
One interesting take from this discussion is the ability to fine tune your rocket's stability margin simply by changing the nosecone to a different profile. For example, if a rocket fitted with a ellipsoidal nosecone has a stability margin that is a tad lower than desired, changing to a conical nosecone will increase the rocket's stability margin (choosing a shorter nosecone length will further help).
A rocket that flies supersonic (or even near supersonic) speed experiences compressibility effects on the nosecone, with the result being a change in both the magnitude of the Normal Force coefficient (CN α)n and the nosecone C.P. location (Xn). The effect of a higher magnitude Normal Force coefficient is destabilizing, as an examination of Equation 1 reveals (the denominator is more greatly affected by (CN α)n, and thus an increase in (CN α)n leads to a reduction in Xcp). As seen in Figure 7, which plots CN α l for the nosecone as a function of Mach number, the coefficient increases dramatically as Mach one is approached. A further increase happens as higher Mach values are approached. This particular set of curves is for a tangent-ogive nosecone and may be considered valid for small (≤10°) angle-of-attack.
Note that the subscript l indicates that boundary-layer effects are not included, for simplicity of presentation. Boundary-layer effects include a contribution arising from an apparent enlargement
of the body cross-section as a result of boundary-layer displacement as well as skin-friction contribution. To include these two terms requires knowledge of the flight Reynolds Number which complicates matters. The boundary-layer terms are generally small in relation to CN α l .
The effect on nosecone C.P. location is an aftward movement with increasing Mach number as illustrated in Figure 8.
This set of curves is for a tangent-ogive nosecone at near-zero angle-of-attack. From equation 1 it can be seen that an increase in Xn results in a larger value of XCP.which tends to enhance stability. The net effect, however, is a tendency of reduced stability with Mach number, as (CN α)n has a more dominant influence.
For completeness, the effect of varying angle of attack should be taken into consideration for stability (see Resource 10).
There is one additional effect the nosecone has on rocket stability, namely mass. A rocket's stability margin is not only dependant upon the location of the C.P., but also upon the centre of gravity (C.G.) of the rocket as a whole. The nosecone, being at the extreme forward location, potentially has great influence on the location of the rocket's C.G. In model rocketry, we are familiar with the practice of putting a small weight in the nosecone to improve stability. Not only is the nosecone a convenient location for such a weight, but just as importantly, this location has the most influence. As such, it is important to keep in mind that making a nosecone very lightweight is helpful in minimizing the rocket's mass and therefore improving altitude, but likewise has a destabilizing affect. On the other hand, too heavy a nosecone could result in an overly-stable rocket.
With regard to aerodynamic loading, strength is rarely an issue with typical AE nosecones, as:
where the various terms are defined in the Rocket Body webpage.
Note that Equation [2] is essentially the same as Equation [1] given previously in the Rocket Body webpage. The only difference is with the drag coefficient. For Equation [1], CD is the drag coefficient of the complete rocket. In Equation [2], CDn is the drag coefficient of the nosecone.
A good design practice would be to take varying angle-of-attack into account when designing the nosecone. A force acting normal (perpendicular) to the rocket longitudinal axis is applied at the nosecone C.P. This normal force is given by Equation [2] of the Rocket Body webpage:
NNOSE = q A α (CN α)n
where the various terms are defined in the Rocket Body webpage and (CN α)n is obtained from curves such as those presented in Figure 7.
Handling force is perhaps the most significant loading condition that a nosecone will experience. Handling loads can be a result of transporting the rocket, positioning the rocket onto the pad, or simply "bumping" the nosecone. This condition can stress the nosecone as well as its attachments to the rocket body. To be conservative, a design load of 5× the loaded weight of the rocket should be applied perpedicular to the nosecone tip. Based on my experience, this magnitude of loading will ensure a robust design.
Drag coefficient for all nosecone profiles is a low value until approximately 80% of the speed of sound is approached. As discussed earlier, this is where compressibility effects kick in. For a rocket designed to fly subsonic and well below the speed of sound (LoPer class), any profile will work equally well with regard to drag.
For a rocket designed to approach, but not exceed, the speed of sound (MiPer class)), the best choice would appear to be either ¾ Power or Parabolic (Ref. Figure 2). Two profiles to be avoided are ellipsoidal and conical.
Figure 9, excerpted from Resource 11, rates various nosecone shapes in the transonic and supersonic flight regimes. As can be seen, no single nosecone profile is "ideal" over this range. For a HiPer class rocket, that flies in the Mach 1 to Mach 2 range, Von Karman, Parabola or ½ Power may be the best choice profiles. As mentioned earlier, for a ViPer class rocket that is expected to fly at velocities greater than Mach 2, a 0.66 Power nosecone profile is aerodynamically optimum. An aerospike may be an interesting option for hypersonic rockets (>Mach 5) (see Res.12 & 13). Figure 10 illustrates such a nosecone design.
A rocket may soar through a wide range of velocities during flight, from subsonic, to transonic, to supersonic, continually accelerating (or decelerating). To complicate matters even more, as a rocket climbs skyward, the atmospheric properties continually change, the air gets less dense and the temperature drops. Even if a nosecone is selected for optimal drag resistance at supersonic speed at a certain altitude, drag tradeoffs will occur at other velocities and height along its trajectory. Furthermore, if a rocket is relaunched later for a different mission, where it now experiences a different flight velocity profile, the previously used nosecone may not be optimal over any of the velocities that the rocket will now fly at.
If one wanted to optimize the nosecone shape for a high performance rocket (ViPer class) that is expected to fly to hypersonic velocity (> mach 5), an energy study would be the best approach, to ascertain which shape provides the minimum energy loss due to drag over the full flight trajectory.
To summarize, there are multiple considerations to take into consideration when choosing a nosecone profile, regardless of what velocity range a rocket is intended for:
Res.8
Design and CFD Analysis of Hypersonic Nose Cone
Naveen Kumar G., Dr. Velliangiri M., Kamesh Adithya S.B., Kirubakaran A., Kishore M.,
International Research Journal of Engineering and Technology (IRJET),
Volume: 07 Issue: 07 | July 2020
Res.9 ESDU 89008 Normal-force-curve and pitching-moment-curve slopes of forebody-cylinder combinations at zero
angle of attack for Mach numbers up to 5
Res.10
ESDU 89014 Normal force, pitching moment and side
force of forebody-cylinder combinations
for angles of attack up to 90 degrees and
Mach numbers up to 5
Res.11 Missile Configuration Design,
Chin, S. S. (1961), McGraw-Hill.
Res.12
Performance of Nose Cone Geometries on Sounding Rockets,
Nathan Robbins,
260774713,
McGill University, Montréal, QC, Canada
Res.13
Experimental Results on the Feasibility of an Aerospike for Hypersonic Missiles, AIAA 95-0737, L.D. Huebner, NASA Langley Research Center
Res.14 Transonic Drag Measurements of Eight Body-Noseshape (NACA RM L53K17), William E.Stoney,Jr.,
Langley Aeronautical Laboratory, 1954
Res.15 Simplified Aerodynamic Heating of Rockets
, Hans Olaf Toft,
Dansk Amatør Raket Klub
Res.16 Investigation of the Drag of Various Axially Symmetric Nose Shapes of Fineness Ratio 3 for Mach Numbers 1.24 to 7.4 , NACA RM A52H28, E.W.Perkins & L.H.Jorgensen
Res.17
Nose Cone Design (Wikipedia)
Res.18 The Descriptive Geometry of Nose Cones , Gary A.Crowell Sr.
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