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enter image description here Hypersonic Technology Vehicle 2 (HTV-2) is a crewless, experimental hypersonic glide vehicle rocket glider developed as part of the DARPA Falcon Project capable of flying at 13,000 mph (Mach 17.53, 21,000 km/h) https://en.wikipedia.org/wiki/DARPA_Falcon_Project

This video shoes the HTV-2 reentering. Said to have reached Mach 20 before failure.

I keep hearing about reentry vehicle having to be a blunt nose but this reentry glider has a sharp profile. Could interplanetary reentry be better controlled if the angle of attack was shallower with more aerodynamic profile rather than blunt?

My next question will be if having this profile could increase the speed of reentry by rotating the heat away from the nose using this wing particular cross-section in a 2 part disk form.

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  • $\begingroup$ @Uwe I'm sure it was going that fast on reentry but it is the altitude in which it was doing Mach 20 which I will add. $\endgroup$
    – Muze
    Mar 20, 2019 at 23:04
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    $\begingroup$ That shape would be blunt if it was pitched up. As I understand the sharp profile is more important for slower flight in thicker atmosphere (e.g. mach 5, rather than mach 25) and so the overall shape permits flight in both regimes - though I'd be interested to be corrected by a well reasoned answer. $\endgroup$
    – Puffin
    Mar 21, 2019 at 0:11
  • $\begingroup$ A reentry speed of 8 km/s is 28.8 Mm/h or about Mach 26.7 $\endgroup$
    – Uwe
    Mar 21, 2019 at 10:47
  • $\begingroup$ @Uwe I know this but this vehicle could hit the atmosphere at higher speeds than Mach 26 for interplanetary reentry. $\endgroup$
    – Muze
    Mar 21, 2019 at 16:32
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    $\begingroup$ I'm not sure if I follow your reasoning but it seems unlikely to be intended for interplanetary approach given the funding agency. A very rough way of estimating altitude would be to first estimate the angular rate as seen in the video (though this can be subjective) and then use the supposed horizontal speed to determine altitude. $\endgroup$
    – Puffin
    Mar 22, 2019 at 17:53

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I keep hearing about reentry vehicle having to be a blunt nose but this reentry glider has a sharp profile.

This is because the vehicles have very different use cases. The HTV-2 is looking for a low drag profile to maintain its speed and energy for a long range, fast strike capability. A more traditional civilian entry vehicle (i.e., for spaceflight) is looking to slow down (at an acceptable rate) and not melt.

The vehicles' ballistic coefficient and lift-to-drag ratio are key parameters that define these behaviours (trajectory independent). Here is a comparison table of the HTV-2 to some blunt body vehicles you may be familiar with:

Vehicle: $L/D$ : $\beta$ ($\frac{kg}{m^2}$): Reference:
HTV-2 2.6 13,000 [1]
Apollo Command Module ~0.4 ~400 [2]
MSL/Mars 2020 Entry Vehicle ~0.3 ~120 [3]

The vehicle's ballistic coefficient is its ability to resist slowing down from drag, and its lift-to-drag ratio is representative of its maneuverability. You can see the order of magnitude (or more) differences between the blunt civilian spaceflight entry vehicles and the weapons platform HTV-2.

Could interplanetary reentry be better controlled if the angle of attack was shallower with more aerodynamic profile rather than blunt?

Assuming this configuration offered more maneuverability (I think this is what is meant), then the entry vehicle would certainly have more control authority, but not necessarily be "better controlled" (i.e., more accuracy in landing).

Using Mars 2020 as the current state of the art/practice, specifically use of a range trigger for parachute deployment, it has been shown (and now demonstrated) that virtually all of the "landing error" (landing ellipse size) can be eliminated in the hypersonic maneuvering phase ([4], see the very cool figures showing importance of each contributor to uncertainty/ellipse size with velocity VS range triggers). This proves that even the modest $L/D$ of an MSL/M2020 class entry vehicles has a sufficient amount of control.

My next question will be if having this profile could increase the speed of reentry by rotating the heat away from the nose using this wing particular cross-section in a 2 part disk form.

I'm not sure how you "rotate heat away" or what "a 2 part disk form" is, but the configuration of the HTV-2 vehicle is more "at risk" (for lack of a better description) from entry heating. The shock created by the vehicle moving supersonically through the atmosphere massively compresses the air, causing it to heat up. This popular schlieren image shows the position of the shock for "pointy" and blunt bodies:

blunt body schlieren photos

(Wikipedia Commons)

The shock from the pointy vehicle (analogous to the HTV-2) is attached to its nose, unlike the blunt bodies where the shock is detached. The closer the vehicle surface is to this shock, the higher the heat transfer from the shock to the vehicle because there is a smaller volume of air behind the shock to serve as a kind of heat sink to dissipate that energy. Both flight tests of HTV-2 reportedly experienced aerothermal failure.

References:

  1. James M. Acton, "Hypersonic Boost-Glide Weapons," Science & Global Security 23, no. 3 (2015): 191-219
  2. Planetary Mission Entry Vehicles Quick Reference Guide. Version 3.0 (NTRS link)
  3. Noyes, C. "Robust Optimal Entry Guidance for Future Mars Landers DISSERTATION," (2021) (archived link)
  4. S. Dutta and D. W. Way, “Comparison of the Effects of Velocity and Range Triggers on Trajectory Dispersions for the Mars 2020 Mission,” AIAA Atmospheric Flight Mechanics Conference, no. 2017-0245, 2017. (NTRS direct link)
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