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QuarterhorseHermeus—QuarterhorseHermeus—
Quarterhorse
Photo: Hermeus · CC BY 2.0 (opens commons.wikimedia.org)

Quarterhorse

by HermeusUS

ProtoStage 2 of 5

Mk 2.1 went supersonic in May 2026; next steps push to higher speeds.

Updated 9 Sep 2026Checked 25 Sep0 updates this week

Milestones

Next · Faster flights and engine-switch tests
  1. Quarterhorse Mk 1 first flightMay 2025Complete.
  2. Quarterhorse Mk 2.1 first flight2 Mar 2026Complete.
  3. Mk 2.1 reaches Mach 1.2126 May 2026Complete.
  4. Faster flights and engine-switch testsNowCurrent milestone.

Most important updates

  • 9 Sep 2026
  • 26 May 2026
  • May 2025
  • 23 Aug 2024
  • 17 Nov 2022

Current obstacles

  • Switching from jet to ramjetThe engine must hand over smoothly from jet to ramjet around Mach 3 while handling shock waves and heat.

Physics limits

  • Fast air gets too hot for a jet engine's turbineRamming air slows it and heats it: at Mach 3 the air reaches about 330°C before any fuel burns, at Mach 5 over 1,000°C. The turbine can't add heat on top, so jets stop working near Mach 3.
  • Ramjets give no thrust standing stillA ramjet needs speed to compress its air and works well only from about Mach 2–3. Something else must get it there, and the handover between engines is where thrust can dip.
  • The airframe itself heats upAir hitting leading edges near Mach 5 can exceed 1,000°C. Aluminium softens above about 150°C, so hypersonic planes need titanium, nickel alloys or ceramics, which are heavier and costlier.

How it works

3 parts
Hermeus's Chimera engine firing on a test stand: an off-the-shelf jet engine wrapped in a ramjet, the combination the Quarterhorse steps build toward
Hermeus's Chimera engine firing on a test stand: an off-the-shelf jet engine wrapped in a ramjet, the combination the Quarterhorse steps build towardPhoto: Hermeus Corp · CC BY 2.0 (opens flickr.com)
Step

One new aircraft per step

Each Quarterhorse flies faster than the last, so problems in heat, control and engines are met and fixed one speed band at a time.

Engine

A jet engine with a ramjet around it

Chimera starts as a normal jet engine with a pre-cooler; around Mach 3 the air is routed around the jet core into a ramjet.

Ramjet

Speed does the squeezing

A ramjet has no fan: at high speed the incoming air is compressed just by being rammed into a narrowing inlet, then fuel is burned in it.

Update log

6 updates

Wed 9 Sep

  • Minor: PressHardware

Tue 26 May

  • Major: PressTest

May 2025

  • Minor: PressTest

Fri 23 Aug 2024

  • Minor: PressTest

Thu 28 Mar 2024

  • Minor: PressHardware

Thu 17 Nov 2022

  • Major: PressTest

About Hermeus

The team behind Quarterhorse

Hermeus

Hypersonic aircraft, Quarterhorse test jets

Atlanta-based Hermeus builds a series of pilotless fast test jets, called Quarterhorse, on the way to a reusable hypersonic plane with an engine that switches from jet to ramjet mode.

  • Quarterhorse Mk 1 first flew in May 2025.
  • Quarterhorse Mk 2.1, about the size of an F-16, flew faster than sound (Mach 1.21) on 26 May 2026.
  • Builds a new aircraft for each step instead of one long program.

hermeus.com (opens hermeus.com)Company profile →

Founded
20188 yrs
Headquarters
United States
Status
Private
Valuation
private
Works in
AviationSupersonic & hypersonic
Coverage
1 program · 6 updateslatest 9 Sep 2026checked 25 Sep
People
AJ PiplicaCo-founder and CEO
Primer