Interview guides · Hermeus
Hermeus Software Engineer Interview Guide
Hermeus Software Engineer interview guide: reported rounds, questions and how candidates describe the process.
The 60-second brief
As a Software Engineer at Hermeus, you will build the foundational software platforms driving hypersonic flight. Hermeus is developing aircraft capable of traveling at Mach 5+, fundamentally changing high-speed transportation and defense capabilities. Software engineers sit at the intersection of embedded controls, flight dynamics, and real-time hardware testing. Your code directly interacts with physical systems that experience extreme aerodynamic environments, high thermal loads, and rigorous control constraints. The scope of this role spans multiple critical domain areas across the organization. You will develop embedded real-time software for flight computers, create high-fidelity flight simulation and modeling environments, build robust Hardware-in-the-Loop (HIL) test suites, or architect intuitive Human-Machine Interfaces (HMI) for real-time mission monitoring and control.
This guide is scoped to a Software Engineer candidate at Hermeus.
Focus areas
- GD&T (Geometric Dimensioning and Tolerancing)
- Project communication & decision-making rationale
- Solid mechanics (stress, fracture)
The Hermeus interview process
Hermeus candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.
Recruiter Call
Initial conversation with a recruiter to align on background, motivations, and fit.
What to demonstrate
- Initial conversation with a recruiter to align on background, motivations, and fit
- Depth in GD&T (Geometric Dimensioning and Tolerancing)
How to prepare
- Be able to walk your CV end to end in two minutes, and say why this company specifically.
- Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.
Technical Screens
Technical interviews with engineering leads focusing on past projects and domain-specific knowledge.
What to demonstrate
- Technical interviews with engineering leads focusing on past projects and domain-specific knowledge
- Depth in GD&T (Geometric Dimensioning and Tolerancing)
How to prepare
- Answer aloud and timed: Draw, explain, and derive the equation for a Wheatstone bridge and explain its use in strain gauges or sensor telemetry.
- Answer aloud and timed: What strategies do you use to guarantee deterministic execution timing in flight control loops?
Project Presentation
Deliver a detailed presentation on a past engineering project to a panel of peers.
What to demonstrate
- Deliver a detailed presentation on a past engineering project to a panel of peers
- Depth in GD&T (Geometric Dimensioning and Tolerancing)
How to prepare
- Answer aloud and timed: How do you handle signal noise or dynamic sensor drift in real-time embedded environments?
- Answer aloud and timed: Compare different numerical integrators (e.g., Euler, Runge-Kutta 4) and explain why you would choose one over another for real-time physics simulations.
Final Round
Comprehensive final round featuring deep-dive technical discussions and behavioral evaluations.
What to demonstrate
- Comprehensive final round featuring deep-dive technical discussions and behavioral evaluations
- Depth in GD&T (Geometric Dimensioning and Tolerancing)
How to prepare
- Answer aloud and timed: How do you structure a Hardware-in-the-Loop (HIL) setup to test flight software against simulated sensor feeds?
- Answer aloud and timed: Explain how you would model continuous fluid dynamic or aerodynamic dynamics inside a discrete-time simulation.
Where candidates lose points
Review Your Engineering Fundamentals
Brush up on dynamic equations, signal processing, Wheatstone bridges, and basic stress-strain relationships. Interviewers frequently ask fundamental engineering questions to gauge your baseline intuition.
Structure Your Presentation Clearly
When delivering your project presentation, clearly define the project problem statement, your specific individual contribution, the technical challenges faced, and how you validated the final design on physical systems.
Demonstrate First-Principles Thinking
If you encounter an unfamiliar scenario during a whiteboard or technical discussion, state your assumptions clearly and walk through the problem using basic math and physics rules rather than guessing a higher-level answer.
Questions & practice
How do you mitigate heat transfer into critical electronics or structural enclosures operating in high-Mach re
Medium · First-Principles Engineering & Applied M
How do you mitigate heat transfer into critical electronics or structural enclosures operating in high-Mach regimes?
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
Describe how you would improve a sluggish build pipeline or automated test framework for high-concurrency flig
Medium · System Design, Architecture & Problem So
Describe how you would improve a sluggish build pipeline or automated test framework for high-concurrency flight code.
Approach
- Say what the runtime actually does before reasoning about the code.
- Name what is shared across threads and what owns each piece of state.
- Identify the window where an invariant is briefly untrue.
- Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
- What happens if two callers reach this at the same time?
- Where could this allocate more than you expect?
How do you feel about working in an aggressive, fast-paced aerospace startup environment with ambiguous or cha
Medium · Behavioral & Team Execution
How do you feel about working in an aggressive, fast-paced aerospace startup environment with ambiguous or changing requirements?
Approach
- Restate the input: its shape, its size, and what is guaranteed about it.
- Name the brute-force solution and its complexity before improving on it.
- Choose the data structure from the access pattern, not from familiarity.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
Replace offset paging on the resource feed with keyset
Easy · keyset pagination · composite index · sort order
resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.
Approach
- Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
- Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
- Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
- Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.
Follow-up
- The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
- Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?
Denormalise tenant onto revisions and backfill it live
Hard · denormalisation · online migration · backfill
resource_revision (revision_id, resource_id, version, actor_user_id, change_kind, patch, request_id, created_at) has 400M rows and no tenant column; tenant_id lives only on resource. Two reads need it: a tenant-scoped audit feed ordered by created_at DESC, and an offboarding purge. Both join back to resource today. Justify adding tenant_id to resource_revision against those two reads, name the anomaly the copy introduces and the constraint that prevents it, then give the ordered migration for a live table taking 1.2k writes/second — the lock each step takes, how the backfill is batched, and where each step stops being reversible. PostgreSQL 16.
Approach
- Justify from the access path rather than from taste. Without the column, the audit feed either scans resource_revision by created_at and discards other tenants' rows, or resolves the tenant's resource_ids first and probes with them — both proportional to the tenant's whole history rather than to one page. With (tenant_id, created_at DESC, revision_id DESC) it is a seek that stops at 50 rows, and the purge becomes a ranged delete instead of a join.
- Name the cost exactly: a second copy of a fact can disagree with the first. Make the disagreement unwritable rather than documented — add UNIQUE (resource_id, tenant_id) on resource so it can serve as a foreign-key target, then FOREIGN KEY (resource_id, tenant_id) REFERENCES resource (resource_id, tenant_id) on the revision table. A revision can then only ever carry its parent's tenant.
- Step one, expand: ALTER TABLE resource_revision ADD COLUMN tenant_id BIGINT NULL, with no default, so it is a catalogue change and no rewrite. It still needs ACCESS EXCLUSIVE for an instant, and that instant queues behind the longest open transaction on the table while every later query queues behind it — set lock_timeout to 2s and retry rather than wait.
- Step two, dual-write: deploy the writer that populates tenant_id on every new revision while reads still use the join. Reversible by redeploying the previous build, because nothing reads the column yet.
Follow-up
- The backfill is half finished and a rollback is required. What state is the table in, and what does the previous build do with a half-populated column?
- How do you verify the backfill actually finished, given rows are still being inserted while it runs?
Describe how you would build a circuit or embedded routine to read a binary message without using a high-level
Medium · Flight Software & Real-Time Systems
Describe how you would build a circuit or embedded routine to read a binary message without using a high-level controller microcontroller unit.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Explain how you would implement low-latency data parsing for telemetry streams running over hardware buses.
Medium · Flight Software & Real-Time Systems
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Draw, explain, and derive the equation for a Wheatstone bridge and explain its use in strain gauges or sensor
Medium · Flight Software & Real-Time Systems
Draw, explain, and derive the equation for a Wheatstone bridge and explain its use in strain gauges or sensor telemetry.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
What strategies do you use to guarantee deterministic execution timing in flight control loops?
Medium · Flight Software & Real-Time Systems
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Compare different numerical integrators (e.g., Euler, Runge-Kutta 4) and explain why you would choose one over
Medium · Modeling, Simulation & Hardware-in-the-L
Compare different numerical integrators (e.g., Euler, Runge-Kutta 4) and explain why you would choose one over another for real-time physics simulations.
Approach
- Say who the caller is and what they do when the call fails halfway.
- Define the identity of a request so a retry cannot double-apply it.
- Separate accepted, pending, failed and confirmed; they are different facts.
- Design the error taxonomy before the success shape; callers branch on it.
Follow-up
- What happens if the caller retries after a timeout?
- How does a client discover it is on an old version of this contract?
How do you structure a Hardware-in-the-Loop (HIL) setup to test flight software against simulated sensor feeds
Medium · Modeling, Simulation & Hardware-in-the-L
How do you structure a Hardware-in-the-Loop (HIL) setup to test flight software against simulated sensor feeds?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Explain how you would model continuous fluid dynamic or aerodynamic dynamics inside a discrete-time simulation
Medium · Modeling, Simulation & Hardware-in-the-L
Explain how you would model continuous fluid dynamic or aerodynamic dynamics inside a discrete-time simulation.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
How would you scope and structure a wind tunnel test or simulation run when facing severe time and budget cons
Medium · Modeling, Simulation & Hardware-in-the-L
How would you scope and structure a wind tunnel test or simulation run when facing severe time and budget constraints?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Describe the architectural balance between real-time performance and dynamic model fidelity in continuous cont
Medium · Modeling, Simulation & Hardware-in-the-L
Describe the architectural balance between real-time performance and dynamic model fidelity in continuous control simulation.
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
Explain the stress-strain curve and define key structural regions like yield strength, ultimate tensile streng
Medium · First-Principles Engineering & Applied M
Explain the stress-strain curve and define key structural regions like yield strength, ultimate tensile strength, and necking.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Discuss the material property differences between aluminum, steel, and Inconel, and why you would select each
Medium · First-Principles Engineering & Applied M
Discuss the material property differences between aluminum, steel, and Inconel, and why you would select each for extreme pressure or high-temperature environments.
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
Where does the highest pressure occur inside a jet engine, and how do fluid dynamic equations model this?
Medium · First-Principles Engineering & Applied M
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
If you applied a point load to a beam, where would it fracture, and how do you calculate the internal bending
Medium · First-Principles Engineering & Applied M
If you applied a point load to a beam, where would it fracture, and how do you calculate the internal bending moments?
Approach
- Clarify what is being asked and what a complete answer contains.
- State your assumptions explicitly before working the problem.
- Say what you would check first and why it is the highest-information step.
- Work from the requirement backwards to the design.
Follow-up
- What assumption would you test first?
- How would you know your answer was wrong?
How would you design a cart to move heavy flight components safely through a testing facility?
Medium · System Design, Architecture & Problem So
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
Draw and explain various GD&T (Geometric Dimensioning and Tolerancing) symbols and describe their practical im
Medium · System Design, Architecture & Problem So
Draw and explain various GD&T (Geometric Dimensioning and Tolerancing) symbols and describe their practical impact on mechanical hardware assembly.
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
How would you determine what size bolts to use for a structural interface subjected to cyclic vibration?
Medium · System Design, Architecture & Problem So
Approach
- Fix the scope first: who calls this, how often, and what they do when it fails.
- Name the read and write paths separately; they rarely have the same bottleneck.
- Choose a partition key and say what query it makes expensive.
- State the consistency you need, and where you are willing to be stale.
Follow-up
- What breaks first when traffic grows ten times?
- How does this behave when that dependency is down for an hour?
Edge instances grow 400 MB per hour until the nightly restart
Hard · memory leak · heap analysis · metric cardinality
Edge API instances start at 700 MB resident and grow about 400 MB/hour; a nightly rolling restart has hidden it for weeks. Growth continues unchanged when request rate halves overnight, p99 degrades in the last hours before an instance is recycled, and heap used immediately after a forced full GC rises monotonically. The service holds no product state. Name the discriminating measurement that separates the plausible causes, give the most likely cause, and give the fix and how you would verify it.
Approach
- Separate resident memory from live heap first, because they fail differently. Resident size can grow from fragmentation, native buffers or thread stacks while the heap is flat; heap used after a full GC rising monotonically is the measurement that says objects are reachable and not being released. You already have it, so this is retention, not fragmentation, and that closes off half the candidate list.
- Use the rate's independence from traffic as the discriminator. Growth that continues at half the request rate rules out per-request objects that are merely slow to collect and points at a structure that grows with distinct values observed rather than with call volume. Write the candidates that have that property: a metrics registry keyed on a high-cardinality label, an unevicted cache, an interner, a per-key lock map.
- Take two heap snapshots an hour apart and diff by retained size, reading the dominator tree, not by allocation count or instance count. Expect one root holding a map with millions of entries, then follow the reference chain to the code that inserts and never removes. Allocation profilers point at churn, which is the wrong signal here.
- The candidate that fits this service is an observability label carrying an identifier, such as a request path recorded before templating so that /v1/resources/48213 becomes its own metric series. That grows with distinct ids seen, is independent of rate, and explains the late p99 degradation, since GC cost rises with the size of the live set.
Follow-up
- Post-GC heap is now flat but resident size still creeps. What are you looking at, and does it matter?
- How would you have detected this before an OOM, given the nightly restart masked the trend?
Suggested practice checklist
Built from the rounds and topics Hermeus candidates report.
Day 1 — Map the Hermeus loop
- Write out the reported sequence: Recruiter Call, Technical Screens, Project Presentation, Final Round.
- For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.
Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.
Day 2 — Work GD&T (Geometric Dimensioning and Tolerancing)
- Spend the session on GD&T (Geometric Dimensioning and Tolerancing), which Hermeus candidates report being tested on.
- Write one worked example in GD&T (Geometric Dimensioning and Tolerancing) and time yourself on it.
Deliverable: One timed worked example in GD&T (Geometric Dimensioning and Tolerancing).
Day 3 — Work Project communication & decision-making rationale
- Spend the session on Project communication & decision-making rationale, which Hermeus candidates report being tested on.
- Write one worked example in Project communication & decision-making rationale and time yourself on it.
Deliverable: One timed worked example in Project communication & decision-making rationale.
Day 4 — Work Solid mechanics (stress, fracture)
- Spend the session on Solid mechanics (stress, fracture), which Hermeus candidates report being tested on.
- Write one worked example in Solid mechanics (stress, fracture) and time yourself on it.
Deliverable: One timed worked example in Solid mechanics (stress, fracture).
Day 5 — Answer out loud: Flight Software & Real-Time Systems
- Answer aloud, timed: Describe how you would build a circuit or embedded routine to read a binary message without using a high-level controller microcontroller unit.
- Answer aloud, timed: Explain how you would implement low-latency data parsing for telemetry streams running over hardware buses.
Deliverable: Spoken answers to 2 reported Flight Software & Real-Time Systems question(s), under time.
Day 6 — Answer out loud: Modeling, Simulation & Hardware-in-the-Loop (HIL)
- Answer aloud, timed: Compare different numerical integrators (e.g., Euler, Runge-Kutta 4) and explain why you would choose one over another for real-time physics simulations.
- Answer aloud, timed: How do you structure a Hardware-in-the-Loop (HIL) setup to test flight software against simulated sensor feeds?
Deliverable: Spoken answers to 2 reported Modeling, Simulation & Hardware-in-the-Loop (HIL) question(s), under time.
Day 7 — Answer out loud: First-Principles Engineering & Applied Mechanics
- Answer aloud, timed: Explain the stress-strain curve and define key structural regions like yield strength, ultimate tensile strength, and necking.
- Answer aloud, timed: Discuss the material property differences between aluminum, steel, and Inconel, and why you would select each for extreme pressure or high-temperature environments.
Deliverable: Spoken answers to 2 reported First-Principles Engineering & Applied Mechanics question(s), under time.
Behavioural preparation
Behavioural rounds judge the decision you made and what it cost.
How do you handle signal noise or dynamic sensor drift in real-time embedded environments?
Medium · Flight Software & Real-Time Systems
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Tell me about a complicated system you designed from scratch. What were the core architectural trade-offs?
Medium · System Design, Architecture & Problem So
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Tell me about a time when you completely owned the entire design, build, test, and operational cycle of a hard
Medium · Behavioral & Team Execution
Tell me about a time when you completely owned the entire design, build, test, and operational cycle of a hardware or software project.
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Describe a time you worked in a multidisciplinary team. What challenges arose, and what was your specific cont
Medium · Behavioral & Team Execution
Describe a time you worked in a multidisciplinary team. What challenges arose, and what was your specific contribution?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Describe a time you learned from a critical technical failure. How did you diagnose the issue, and what change
Medium · Behavioral & Team Execution
Describe a time you learned from a critical technical failure. How did you diagnose the issue, and what changes did you institute?
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Why do you want to work for Hermeus specifically over traditional defense primes or commercial tech companies?
Medium · Behavioral & Team Execution
Approach
- Pick a story where you made the decision, not one where you watched it.
- State the situation in two sentences and spend the rest on the reasoning.
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?