Supersonic Tailcones in SImulators

CCotner said:

I'm starting this based on discussions with Casey Barker and Kurt Von Delius, and also brief interactions with Ken Biba. I want to know about people's flights with extreme tailcones and boat-tails (more than 10 degrees of total included angle and more than a 25% reduction in base area or somewhere around those numbers) at supersonic (>M1.2 for >1sec) speeds. I want quantitative comparisons between simulator data and recorded flight data.

From looking at both data comparisons and underlying physics assumptions, it seems apparent to me that RASAero is basically the only hobby-accessible rocket simulator that has any hope of accurately predicting supersonic flight information. OpenRocket seems to do slightly better than Rocksim. I do not consider Rocksim Pro to be hobby-accessible for it's $1000 pricetag, although I have heard mixed things about it's accuracy.

The allegation is that while RASAero accurately predicts supersonic CD and CP-shift in most cases, it exaggerates the contributions of extreme tailcones (as defined above), leading to inflated altitude performance (such as designes I have made for a J510 flying case rocket reaching 32,000', or more topically, the predictions of my and cjl's N5800CS MD rockets reaching 120,000' plus compared to the ~60,000' expectations of Bandman444 and others).

So, as the internet was once misquoted,

SHUT UP AND GIVE ME YOUR DATA!

Click to expand...

Design of Aerodynamically Stabilized Free Rockets said:

The purpose of the boattail is to decrease the drag of a body that has a squared off base. This squared off
base has local pressure on its surface much less than the free-stream pressure and, therefore, produces a
large drag force. When the exit diameter of the rocket nozzle is smaller than the body-cylinder diameter, the
afterbody maybe tapered to form a boattail to reduce the base area and thus reduce the base drag. However,
for a given boattail length, decreasing the base diameter increases the boat tail wave drag. Therefore, an
optimum configuration can be strongly influenced by the boattail design. This is clearly illustrated in Fig.
5-120 (Ref. 80) which shows the variation in the total zero-lift drag of a 10.5-cal body having a 3.0-cal
tangent-ogive nose, a 6.0-cal cylindrical afterbody, and a 1.5-cal conical boattail with various base
diameters. In subsonic flow, decreasing the base diameter continues to decrease the total drag. In transonic
and supersonic flows, however, decreasing the base diameter to a ratio of dbt/d = 0.4 or lower results in an
increase in the total drag. In supersonic flow, as the Mach number increases, the influence of the base drag
decreases rapidly and thus allows larger base diameters to achieve a minimum drag boattail design.
Fig. 5-121 presents theoretical subsonic and transonic boattail wave drag for various boattail angles and
diameter ratios (Ref. 81). Figs. 5-120 and 5-121 show that subsonic drag is relatively unimportant when
compared to transonic and low supersonic drag.
The supersonic wave drag of conical and parabolic boattails is presented in Figs. 5-122 and 5-123,
respectively. No analytical method or suitable parametric experimental data exist for the accurate
prediction of boattail wave drag at subsonic and transonic speeds. If experimental data for a particular
configuration cannot be found, it is suggested that supersonic data be extrapolated to peak value at a Mach
with a sharp reduction to a lower value at subsonic speeds.
The lift of a boattail is in the negative direction, and although the usual boattail is relatively very short
and thus decreases the lift very little, it does have a strong destabilizing effect—i.e., the boattail tends to
move the overall body center of pressure forward quite markedly. This means that a larger stabilizing
device is required to stabilize the rocket. Thus the increase in drag of the larger stabilizing device may
negate the saving in drag realized by the boattail.
Accurate determination of boattail drag is rather difficult because of the dependence on the geometry of
the fore and aft portions of the body and the real fluid effects. However, for preliminary design analysis,
results from the correlation of both theoretical and experimental data should be used when available.

Click to expand...

Chuck Rogers said:

<< SHUT UP AND GIVE ME YOUR DATA! >>

Well, here it is!


Chuck Rogers
Rogers Aeroscience

Click to expand...

Reinhard said:

I can't contribute flight data, but an illustrative example about RASAero and extreme tailcones. A minimum diameter rocket launched on M2020 simulated to an altitude of 29751ft. Changing the parameters "Boattail Length" and "Boattail Base Diameter" from 0.0000 to 0.0001 increases the altitude to 38194ft - an increase by 28% for a practically negligibly change in airframe geometry.

RASAero apparently changes between mathematical models for different configurations. I guess it is safe to assume that the model without boattail is reasonable accurate. Most rockets don't have a boattail and RASAero wouldn't have its reputation for accuracy otherwise. Therefore, one might conclude that RASAero's model for boattails starts to diverge from reality as the boattail becomes more extreme.

Reinhard

Click to expand...